- Haskell-Cafe - Haskell.org

GADTs and Exponentiated Functors
by Daniel Filonik 12 Mar '16

12 Mar '16

Hi, I have been recently playing around with GADTs, using them to keep track of how many times a functor has been applied. This provides a solution to what seems to be a long standing problem, summarized here: https://byorgey.wordpress.com/2007/08/16/mapping-over-a-nested-functor/ If the nesting depth is part of the type, it is possible to apply fmap automatically as needed. This allows you to write fairly elegant expressions like: data Person = Person { name :: String, age :: Integer, gender :: String, status :: String } deriving Show let persons = fromList' [Person {name="Alice", age=20, gender="F", status="Good"}, Person {name="Bob", age=18, gender="M", status="Good"}, Person {name="Chuck", age=16, gender="M", status="Bad"}] :: NList N1 Person persons `select` age -- [20,18,16] persons `groupby` gender `select` age -- [[20],[18,16]] persons `groupby` gender `groupby` status `select` age -- [[[20]],[[18],[16]]] Note that "`select` age" works regardless of nesting depth. You can also append or remove nesting levels: persons `groupby` gender `select` age `produce` (\x -> [0..x]) -- [[[0..20]],[[0..18],[0..16]]] persons `groupby` gender `reduce` (sumof age) -- [20, 34] Would this kind of thing be of interest? The code is here: https://github.com/filonik/nlists Please feel free to suggest improvements. Cheers, Daniel

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GHC API arguments via cabal/stack?
by David Turner 12 Mar '16

12 Mar '16

Hi all, I'm working on a little program that uses GHC as a library to do some static analysis on a project I'm working on. It runs things as far as the typechecker (so there's no Template Haskell remaining) and then does the analysis on the resulting AST. Here is how I'm calling GHC at the moment: runAnalyser :: FilePath -> [String] -> [String] -> IO [(Module, [Fact], [Assertion])] runAnalyser srcDir args modules = do defaultErrorHandler defaultFatalMessager defaultFlushOut $ do runGhc (Just libdir) $ do dflags <- getSessionDynFlags (dflags', leftover, warnings) <- parseDynamicFlagsCmdLine dflags (map noLoc $ args ++ modules) setSessionDynFlags dflags' setTargets =<< forM modules (\exampleModule -> guessTarget (exampleModuleFile srcDir exampleModule) Nothing) load LoadAllTargets execWriterT $ forM_ modules $ \exampleModule -> do modSum <- lift $ getModSummary $ mkModuleName exampleModule p <- lift $ parseModule modSum t <- lift $ typecheckModule p case tm_renamed_source t of Nothing -> return () Just (hsGroup, _, _, _) -> do assertions <- liftIO $ loadAssertions $ exampleModuleFile srcDir exampleModule let mod = ms_mod $ pm_mod_summary $ tm_parsed_module t tell [( mod , runFactM (moduleName mod) (facts hsGroup) , assertions)] The problem I'm currently facing is that this requires me to pass in the arguments to GHC, including where all the package databases are and all the package ids that stack has decided to use. So far, I've just copy-pasted this from the stack log and hard-coded it, but that's clearly not a good long-term solution. I've half-heartedly tried to fool stack into running my analyser as the compiler, but stack calls ghc more times than just the one call that I need the arguments from. I could make it pass through to the real ghc but this feels like piling hacks on top of hacks. I've also briefly contemplated using the Cabal library to read my .cabal file and work out what to do, but I'm unsure that this would work nicely under stack. At least, I'm not sure quite what to do with all the package databases and other stuff that stack does for you. Is there a sensible and robust way to get these args as stack would make them? Many thanks, David

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Can't get any memory profile data
by Cody Goodman 12 Mar '16

12 Mar '16

Here is the command line log of my attempt following real world haskell's memory profiling example: src λ stack exec -- ghc --version The Glorious Glasgow Haskell Compilation System, version 7.10.3 src λ cat Main.hs module Main where main :: IO () main = do let x = {-# SCC sum #-}sum [1..500] print x {-# SCC helloworld #-} putStrLn "hello world" src λ stack exec -- ghc -O2 --make Main.hs -prof -auto-all -caf-all -fforce-recomp [1 of 1] Compiling Main ( Main.hs, Main.o ) Linking Main ... src λ ./Main +RTS -hc -p 125250 hello world src λ cat Main.hp JOB "Main +RTS -hc -p" DATE "Fri Mar 11 22:51 2016" SAMPLE_UNIT "seconds" VALUE_UNIT "bytes" BEGIN_SAMPLE 0.00 END_SAMPLE 0.00 BEGIN_SAMPLE 0.00 END_SAMPLE 0.00

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MPTC vs. constructor class
by David Feuer 11 Mar '16

11 Mar '16

I've been messing around with type-aligned sequences for a while, and I'm currently exploring the design space of classes for them. The type-aligned package uses this: class TASequence s where tempty :: s c x x Source tsingleton :: c x y -> s c x y -- can default to tsingleton = ( <| tempty ) -- but doesn't for some reason (><) :: s c x y -> s c y z -> s c x z tviewl :: s c x y -> TAViewL s c x y tviewr :: s c x y -> TAViewR s c x y (|>) :: s c x y -> c y z -> s c x z (<|) :: c x y -> s c y z -> s c x z tmap :: (forall x y. c x y -> d x y) -> s c x y -> s d x y An obvious (to me) improvement is to add TAFunctor, TAFoldable, and TATraversable classes, make TASequence a subclass of TATraversable, and remove tmap from TASequence. That's one pleasant direction. Another class expressing some of these ideas: class Category s => TASequence' s c | s -> c where tviewl :: s x y -> TAViewL s c x y tviewr :: s x y -> TAViewR s c x y (|>) :: s x y -> c y z -> s x z (<|) :: c x y -> s y z -> s x z This variant actually offers all the functionality of the original except tmap, which it has no way to talk about whatsoever. The Category s constraint allows id to replace tempty and (>>>) to replace (><). I really like this aspect. Unfortunately, losing tmap et al sucks. It's possible to glue that on to the original TASequence using Data.Constraint: class ForallF Category s => TASequence s where ... but then accessing the Category instances requires mucking about with (\\) and instF and throwing a bunch of explicit type signatures around. Is there any good "design pattern" to resolve this conflict? David Feuer

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Materials on Compiling Functional Languages.
by Maksymilian Owsianny 11 Mar '16

11 Mar '16

Hi Cafe, I'm looking for materials on compiling functional languages. The materials that I was able to gather from the web were kind of all over the place, so I'm hoping someone could help me out with something more in order for a novice. Starting from something specific and simple so I could have something small and concrete to implement, play and ponder about. And then going forward with more advanced materials about different optimization strategies for eager and lazy languages or what could be achieved when limiting yourself only to a total language, etc. Cheers, Max.

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Use cases of empty type classes
by Tomas Tauber 10 Mar '16

10 Mar '16

Hi everyone, I have one question. What are current use cases of type classes with no methods? I saw early uses in type-level programming (e.g. HList [1]). In the OO world, interfaces with no methods are called marker interfaces -- their use cases range from things that could be done with datatype generic programming in Haskell (e.g. serialization) to metadata annotations (e.g. RandomAccess [2]). Regards, Tomas Tauber [1] http://okmij.org/ftp/Haskell/HList-ext.pdf [2] https://docs.oracle.com/javase/8/docs/api/java/util/RandomAccess.html

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Re: [Haskell-cafe] Materials on Compiling Functional Languages. (Gil Mizrahi)
by Kei Davis 10 Mar '16

10 Mar '16

>> Hi Cafe, >> >> I'm looking for materials on compiling functional languages. The >> materials that I was able to gather from the web were kind of all over >> the place, so I'm hoping someone could help me out with something more >> in order for a novice. >> >> Starting from something specific and simple so I could have something >> small and concrete to implement, play and ponder about. And then >> going forward with more advanced materials about different >> optimization strategies for eager and lazy languages or what could be >> achieved when limiting yourself only to a total language, etc. >> >> Cheers, >> Max. >> >> >> _______________________________________________ >> Haskell-Cafe mailing list >> Haskell-Cafe <at> haskell.org >> http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe >> > > Hi Maksymilian, > > Here you'll find a (perhaps not so) short list I've accumulated: > > http://pastemarkdown.com/XUEvs > > > and here it is in markdown format: > > - [Implementing a JIT Compiled Language with Haskell and > LLVM](http://www.stephendiehl.com/llvm/) > - [Write You a Haskell](http://dev.stephendiehl.com/fun/index.html) > - [Theory and Implementation of a Functional Programming > Language](http://www.rose-hulman.edu/mathjournal/archives/2000/vol1-n2/pape… > - [Implementing Functional Programming > Languages](http://research.microsoft.com/en-us/um/people/simonpj/papers/slp… > - SPJ > - [Implementing Functional Langauges: a > tutorial](http://research.microsoft.com/en-us/um/people/simonpj/Papers/pj-l… > - SPJ > - [Practical Implementation of a Dependently Typed Functional Programming > Language | Brady](http://compsoc.dur.ac.uk/~ecb/proofread/thesis.pdf) > - > [PLAI](http://cs.brown.edu/~sk/Publications/Books/ProgLangs/2007-04-26/plai-… > - [STG > Machine](http://www.dcc.fc.up.pt/~pbv/aulas/linguagens/peytonjones92implementing.pdf) > - [Typing Haskell in > Haskell](http://web.cecs.pdx.edu/~mpj/thih/TypingHaskellInHaskell.html > - [Practical type inference for arbitrary-rank types / SPJ et > al.](http://research.microsoft.com/en-us/um/people/simonpj/papers/higher-ra… > - Compiling with Continuations - Appel > - [PureScript Compiler](https://github.com/purescript/purescript) > > > Cheers and good luck, > Gil > Nice list Gil. We've recently implemented a pure FL compiler. We started with the "stock hardware" paper ([STG Machine] in Gil's list), but it's somewhat dated (e.g. they're still using a trampoline). Our main reference was the "fast curry" paper: http://community.haskell.org/~simonmar/papers/evalapplyjfp06.pdf Bernie Pope's MiniSTG is a useful learning tool based on that paper: https://wiki.haskell.org/Ministg Our type inferencer currently handles ADTs including unboxed types. I strongly recommend http://soft.vub.ac.be/~cfscholl/Capita-Selecta-2015/papers/2002%20Heeren.pdf supplemented by https://galois.com/wp-content/uploads/2014/08/pub_JL_UnboxedValuesAsFirstCl… Then if you get serious [Typing Haskell in Haskell] in Gil's list. For compiling pattern matching we started with one of the [Implementing Functional ...] in Gil's list, then found some other references online (I don't know exactly which--I didn't implement that part). For a parallel implementation we started with http://community.haskell.org/~simonmar/papers/multiproc.pdf Then there are subsequent papers that can be found on SPJ's publication page. Hope this helps, it helped us! Kei -- Kei Davis Applied Computer Science Group CCS-7, Mail Stop B287 Los Alamos National Laboratory Los Alamos, NM 87545, U.S.A.

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Type equality with "forall"
by Tom Murphy 09 Mar '16

09 Mar '16

So I've written a library I'm pretty excited about, and there's just one type issue keeping it from all working! I've got a graph that represents functions and function composition, and I've written a class (pretty directly from Oleg's "TypeEq" work [0]) that tells whether two types are equal. The problem is it doesn't always say they're equal. Here's a simplified version that has the same issue: [pragmas [1]] data Fs a c where F :: (a -> c) -> Fs a c Compose :: forall a b c. Fs b c -> Fs a b -> Fs a c -- Type equality: class TEq x0 x1 y0 y1 (b :: Bool) | x0 x1 y0 y1 -> b where isEq :: Fs x0 x1 -> (y0 -> y1) -> String -- The case where the types are equal: instance {-# OVERLAPS #-} TEq x y x y 'True where isEq _ _ = "yes!" -- The case where they're unequal: instance (b ~ 'False) => TEq x0 x1 y0 y1 b where isEq (F _) = "no" isEq (Compose a b) f = "compose("++isEq a f++", "++isEq b f++")" When I try this on example programs, I see weird results: > isEq (F not) not "yes!" -- expected > isEq (Compose (F (\_->"")) (F not)) not "compose(no, no)" -- weird! I would expect the result of the second one to be "compose(no, yes!)". It seems to think that the (F not) in the Compose has a different type than (F not) not in a Compose. Anyone spot my problem? Thanks! Tom [0] http://okmij.org/ftp/Haskell/typeEQ.html [1] {-# LANGUAGE DataKinds #-} {-# LANGUAGE FlexibleInstances #-} {-# LANGUAGE FunctionalDependencies #-} {-# LANGUAGE GADTs, NoMonoLocalBinds #-} {-# LANGUAGE KindSignatures #-} {-# LANGUAGE MultiParamTypeClasses #-} {-# LANGUAGE Rank2Types #-} {-# LANGUAGE UndecidableInstances #-}

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ICFP 2016 Call for Sponsorships
by Lindsey Kuper 09 Mar '16

09 Mar '16

ICFP 2016 The 21st ACM SIGPLAN International Conference on Functional Programming http://conf.researchr.org/home/icfp-2016 Call for Sponsorships Web version of this call for sponsorships: http://conf.researchr.org/info/icfp-2016/call-for-sponsorships ## Why Sponsor ICFP 2016? ICFP is the premier conference on functional programming languages, covering all aspects of theory, implementation, and application. Every year, we bring over 400 world-leading researchers, practitioners, and students together to discuss the latest findings, collaborate on new ideas, and meet new people. By sponsoring ICFP, your organization can demonstrate its commitment to supporting high quality research and to developing the next generation of functional programming experts. Most of our sponsorship funds are used to help students from around the world afford to attend the conference and get the most out of their experience. We believe that this commitment will pay dividends for our students, our sponsors, and the public good for years to come. ## Sponsorship Opportunities and Benefits Bronze: $500: Logo on website, poster at industrial reception, listed in proceedings. Silver: $2500: As above, plus: logo in proceedings, logo on publicity materials (e.g., posters, etc.) Gold: $5000: As above, plus: named supporter of industrial reception with opportunity to speak to the audience, and opportunity to include branded merchandise in participants’ swag bag. Platinum: $10000: As above, plus: named supporter of whole event, logo on lanyards, badge ribbon, table/booth-like space available (in coffee break areas), other negotiated benefits (subject to ACM restrictions on commercial involvement). ## How to Become a Sponsor If you’re interested in becoming a sponsor, we’d love to hear from you. You can get in touch with our Industrial Relations Chair at ryan.trinkle(a)gmail.com.

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extensible effects + classy lenses
by Mitchell Rosen 09 Mar '16

09 Mar '16

Hi all, I'm trying to combine an extensible effects style state with classy lenses. That is, instead of pinning the type of my state down, I'd like to only require the pieces I need. For example, {-# language FlexibleContexts #-} import Control.Monad.Freer import Control.Monad.Freer.State class HasInt s where getInt :: s -> Int foo :: (Member (State s) effs, HasInt s) => Eff effs Int foo = get However, this fails to typecheck: Overlapping instances for Member (State s0) effs Matching givens (or their superclasses): (Member (State s) effs) bound by the type signature for foo :: (Member (State s) effs, HasInt s) => Eff effs Int at example.hs:9:8-56 Matching instances: instance Data.Open.Union.Member' t r (Data.Open.Union.FindElem t r) => Member t r -- Defined in ‘Data.Open.Union’ (The choice depends on the instantiation of ‘effs, s0’) In the ambiguity check for the type signature for ‘foo’: foo :: forall (effs :: [* -> *]) s. (Member (State s) effs, HasInt s) => Eff effs Int To defer the ambiguity check to use sites, enable AllowAmbiguousTypes In the type signature for ‘foo’: foo :: (Member (State s) effs, HasInt s) => Eff effs Int Is this a weakness of extensible effects, or is there another way to express this function? Thanks, Mitchell

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implicit call stacks and calling function
by Evan Laforge 08 Mar '16

08 Mar '16

I noticed this when I started using the new implicit call stacks feature. I didn't bring it up because I figure it probably had a good reason and was too late to change anyway, but the recent talk about HasCallStack reminded me and I'm curious. When you do GHC.Stack.getCallStack you get a [(String, SrcPos)]. The SrcPos is the position of the calling function, but the String is the callee function. So you can't get the name of the calling function. Instead, you get the name of the function with the call stack annotation. That's not so useful because in say a logging function, I'm interested in the caller's name. I don't need the name of the logging function, it's just something boring like "info" or "warn"! When I switched from a custom preprocessor that sort of implemented SRCLOC_ANNOTATE, it was definitely nice to lose the custom local hackery, but not so nice to lose the caller's name. For tests I used an unsafe mutable global via unsafePerformIO, otherwise failed tests can't report the name of the failing test, but the hack doesn't work for logging. Is there a reason it was done the way it was, or a way to get the name of the calling function?

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1-to-many preprocessor in stack/Cabal builds
by Andrey Sverdlichenko 07 Mar '16

07 Mar '16

Hi, I'm writing a package which uses custom preprocessor for creating some of the source files. It takes one IDL file as input and writes several .hs modules, depending on IDL contents. I need to compile this modules with a few normal ones and link them as a library, preferably with generated modules exposed. It doesn't look like a complex task and never was in with make/etc, but I run into some problems now. Obviously, cabal supports 1-to-1 preprocessors to create one Haskell module from one source file (alex, happy, etc), but I can't find anything for 1-to-many preprocessors. So, I decided to use hookedPrograms and hooks and do it myself. The call to preprocessor in the postConf hook worked just fine and created source files and module list. Cabal pre-* hooks seems to be right place to read this module list and use it, but are presenting two problems: can only modify other-modules, not exposed-modules, which is tolerable, and there is no access to build configuration. The latter is fatal, I need to know build dir to read module list. So I have to change action hooks. As the additional annoyance, there is absolutely no way to modify configuration once and for all, I have to override every single hook with the same "read module list and change config" wrapper. So, I have buildHook, copyHook and regHook replaced with wrappers which change PackageDescription parameter and call original function. Library is built and installed, but somewhere between build and copy actions there is still a message "Warning: modules not listed in foo.cabal for library component (add to other-modules)". I can't identify where it comes from. I'm using stack 1.0.4 to build this library, but all the problems seems to be cabal-related. Here is minimal example of what I ended up with: https://github.com/rblaze/cabal-hooks-sample Now I wonder if I missed an easier way to handle this code generation. If not, at least how to get rid of warning about missing modules between build and copy actions?

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Re: [Haskell-cafe] HasCallStack - runtime costs?
by Johannes Waldmann 07 Mar '16

07 Mar '16

Eric wrote earlier: > If you import a function with a HasCallStack constraint > there's no way to disable the overhead ... > For non-recursive functions like head and tail, > the overhead is probably not large enough to be noticeable. Now I'm confused: should this mean that head and tail are "functions with a HasCallStack constraint"? If I ask ghci for their type, it just says head :: [a] -> a and that's also what's in the source https://downloads.haskell.org/~ghc/8.0.1-rc2/docs/html/libraries/base-4.9.0… but the Callstack explanation in the user manual does show a different version of head https://downloads.haskell.org/~ghc/8.0.1-rc2/docs/html/users_guide/glasgow_… I am not particularly concerned about head and tail here but generally about how to find out whether my code is taking a performance penalty. I think the documentation (user manual Section 9.31) is rather brief about the semantics. It speaks about solving "HasCallStack" constraints - but they could also be propagated? There is secondary documentation like https://ghc.haskell.org/trac/ghc/wiki/ExplicitCallStack but it does not tell what is actually implemented, or I don't see it. - J.W.

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Tagged instances for Aeson overlapped
by Dmitry Olshansky 07 Mar '16

07 Mar '16

Hello, I tried to compile my project with GHC 8 (from HEAD). And I got > • Overlapping instances for > aeson-0.11.1.0:Data.Aeson.Types.Class.ToJSON > ... > arising from a use of ‘serve’ > Matching instances: > two instances involving out-of-scope types > instance aeson-0.11.1.0:Data.Aeson.Types.Class.ToJSON b => > aeson-0.11.1.0:Data.Aeson.Types.Class.ToJSON > (Data.Tagged.Tagged a b) > -- Defined in ‘aeson-0.11.1.0:Data.Aeson.Types.Instances’ > instance forall k x (r :: k). > aeson-0.11.1.0:Data.Aeson.Types.Class.ToJSON x => > aeson-0.11.1.0:Data.Aeson.Types.Class.ToJSON > (Data.Tagged.Tagged r x) One of instances should be removed probably. I suppose that instance for removing should be instance in Data.Tagged. But version in Aeson should be PolyKinded. What do you think? Best regards, Dmitry

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Trouble on Travis CI with recent cabal versions
by Sven Panne 07 Mar '16

07 Mar '16

For my OpenGLRaw project, I'm using a .travis.yml generated by Herbert's https://github.com/hvr/multi-ghc-travis and the corresponding https://launchpad.net/~hvr/+archive/ubuntu/ghc PPA. Recently things broke on Travis CI, and there are a few strange things: * The cabal-install-head package seems to be based on cabal 1.23, which is older than cabal-install-1.24. Is this intended? * The OpenGLRaw package has no test suite, and this seems to be OK for cabal-install-head ( https://travis-ci.org/haskell-opengl/OpenGLRaw/jobs/114109605#L929) while cabal-install-1.24 fails in the "cabal test" step ( https://travis-ci.org/haskell-opengl/OpenGLRaw/jobs/114109606#L1022) This is a regression: I think that "cabal test" for a package without a test suite should be a no-op, at least that used to be the case. * Why does "cabal test" say "Re-configuring with test suites enabled. If this fails, please run configure manually." when "cabal configure" has already been run (including --enable-tests)? This looks like a bug... Cheers, S.

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Type-level list "elem" inference
by amindfv＠gmail.com 06 Mar '16

06 Mar '16

So I've got some code that looks like: {-# LANGUAGE DataKinds, UndecidableInstances, TypeFamilies, KindSignatures, TypeOperators #-} import Data.Proxy import GHC.TypeLits type family IsSubset (as :: [Symbol]) (bs :: [Symbol]) where IsSubset as bs = IsSubsetPrime as bs bs type family IsSubsetPrime (as :: [Symbol]) bs bs' where IsSubsetPrime as '[] bs' = 'False IsSubsetPrime '[] bs bs' = 'True IsSubsetPrime (a ': as) (a ': bs) bs' = IsSubsetPrime as bs' bs' IsSubsetPrime (a ': as) (b ': bs) bs' = IsSubsetPrime (a ': as) bs bs' This lets me write functions like: foo :: (IsSubset '["foo", "bar"] args ~ 'True) => Proxy args -> Int foo args = undefined I've also got a type family: type family IsElem (a :: Symbol) (bs :: [Symbol]) where IsElem a (a ': bs) = 'True IsElem a (b ': bs) = IsElem a bs IsElem a '[] = 'False This lets me write functions like: bar :: (IsElem "foo" args ~ 'True) => Proxy args -> Int bar args = undefined The problem comes when I want to use "bar args" in the definition of "foo args" -- even though it's clear to me that ["foo","bar"] being a subset of args implies that "foo" is an element of args, I haven't written "IsElem" or "IsSubset" in a way that it's clear to the compiler. Is there a way to write IsElem and IsSubset so they "compose"? Thanks! Tom

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PolyKind for Fixed and HasResolution from Data.Fixed
by Никита Тимофеев 06 Mar '16

06 Mar '16

Hi all! If we define HasResolution and Fixed like that: class HasResolution (a :: k) where resolution :: p a -> Integer newtype Fixed (a :: k) = Fixed Integer We can do something like that: instance KnownNat a => HasResolution a where resolution = natVal 2.3 :: Fixed 1 -- Timofeev N.D.

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Accuracy of Data.Number.Fixed
by Lyle Kopnicky 06 Mar '16

06 Mar '16

This does not seem right to me: Prelude Data.Number.Fixed> read "4.00" :: Fixed (EpsDiv10 (EpsDiv10 Eps1)) 4.00 Prelude Data.Number.Fixed> read "4.01" :: Fixed (EpsDiv10 (EpsDiv10 Eps1)) 4.00 Prelude Data.Number.Fixed> read "4.02" :: Fixed (EpsDiv10 (EpsDiv10 Eps1)) 4.03 Prelude Data.Number.Fixed> read "4.03" :: Fixed (EpsDiv10 (EpsDiv10 Eps1)) 4.04 Prelude Data.Number.Fixed> read "4.04" :: Fixed (EpsDiv10 (EpsDiv10 Eps1)) 4.05 Maybe I am misunderstanding the purpose of this library. I guess that "two digits" of precision means that it is accurate within +/- 0.01? I think that this library is meant for doing arbitrary-precision transcendental operations. But it would not be good as a model for something like currency. It's using ratios as an internal representation, and I'm not sure what ratios it's choosing, but I think I'd choose, say, 401/100 for the "4.01" representation. I guess it's picking a slightly smaller factor than that. The non-reversibility of read/show is somewhat disconcerting. I think I prefer the Decimal library for currency. Regards, Lyle

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HasCallStack - runtime costs?
by Johannes Waldmann 05 Mar '16

05 Mar '16

Dear Cafe, the new (8.*) call stack feature https://downloads.haskell.org/~ghc/8.0.1-rc2/docs/html/users_guide/glasgow_… is certainly nice for debugging during development. But how costly is it at runtime? I notice a 5 percent slowdown. That's not a problem if there's an easy way to switch this off for production (without changing the code). Related: how to make code that uses it, compile with older ghcs that don't have it. I made this hack: do not import GHC.Stack.Types, but instead {-# language CPP, MultiParamTypeClasses #-} #if (__GLASGOW_HASKELL__ < 710) {-# language NullaryTypeClasses #-} #endif module Stack ( HasCallStack ) where #if (__GLASGOW_HASKELL__ >= 800) import GHC.Stack.Types #else class HasCallStack instance HasCallStack #endif When I compile with 8.rc2, and change ">= 800" to ">= 900", I am getting the 5 percent speedup mentioned above. But does it really do what I hope it does (remove all runtime overhead that call stacks may have)? When I compile with 7.10.3, I am getting 5 .. 10 percent faster again. My code does nothing fancy (w.r.t. types and libraries), it just uses Data.IntMap heavily. And it has some class Semiring s where zero :: s one :: s plus :: HasCallStack => s -> s -> s times :: HasCallStack => s -> s -> s - J.W.

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Re: [Haskell-cafe] HasCallStack - runtime costs?
by Johannes Waldmann 05 Mar '16

05 Mar '16

Thanks, Eric. > If you import a function with a HasCallStack constraint > there's no way to disable the overhead But then this means that each library should avoid these constraints? Or not, because this rightly scares away users from calling nontotal functions? > ... with a couple special rules for > building dictionaries in GHC's constraint solver. then activation of these special rules could be a compiler switch? Though it'd probably create a mess with separate compilation/linking. But switching via CPP will do the same. > class Semiring s where > plus :: HasCallStack => s -> s -> s > times :: HasCallStack => s -> s -> s > I'm curious, why do plus and times take a CallStack? because I want to (be able to) debug implementations. simplified example: {-# language ConstrainedClassMethods #-} import GHC.Stack.Types class C a where p :: a -> Int q :: HasCallStack => a -> Int instance C () where p x = error "huh" q x = error "huh" Calling q gives more information: *Main> p () *** Exception: huh CallStack (from HasCallStack): error, called at CS.hs:10:9 in main:Main *Main> q () *** Exception: huh CallStack (from HasCallStack): error, called at CS.hs:11:9 in main:Main q, called at <interactive>:44:1 in interactive:Ghci1 it, called at <interactive>:44:1 in interactive:Ghci1 I did not see another way than to change the type of the method in the class. (Which looks terrible of course.) - J.

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Reducing boilerplate
by Sylvain Henry 05 Mar '16

05 Mar '16

Hi, To write FFI bindings, I use c-storable-deriving [1] to automatically derive CStorable instances for many data types (the only difference between Storable and CStorable is that CStorable provides default methods for types that have Generic instances): {-# LANGUAGE DeriveGeneric #-} {-# LANGUAGE DeriveAnyClass #-} ... data X = X { fieldStatus :: Vector 24 Word8 , fieldPadding :: Word8 } deriving (Generic, CStorable) However I also need a Storable instance, hence I have to write (the "c*" methods are provided by CStorable): instance Storable X where peek = cPeek poke = cPoke alignment = cAlignment sizeOf = cSizeOf Is there a way to automatically generate this instance for every data that has an instance of CStorable? Ideally, I would like to say once and for all: instance CStorable a => Storable a where peek = cPeek poke = cPoke alignment = cAlignment sizeOf = cSizeOf As I don't think it is currently possible, would it be sensible to add the support for automatically derived instances attached to classes (as a GHC extension)? Regards, Sylvain [1] https://hackage.haskell.org/package/c-storable-deriving

1 0

Digraphs With Text: More flexible than graphs, eager to hear from you
by Jeffrey Brown 05 Mar '16

05 Mar '16

This is a demonstration of DWT, a data structure built from and more flexible than graphs. It is powered by the *wonderful* Functional Graph Library. We beseech your involvement! DWT is open-source and actively seeking co-developers. I would happily trade any and all the privilege of determining tasks, teaching haskell or math, learning anything, and receiving obligations. We surely have much knowledge to trade! Even just a "hey why don't you do [this]!" could be precious to me. This transcript makes tiny digressions about what monads make possible. > let g = empty :: Mindmap -- g is now an empty mindmap (which is a kind of graph). > g <- pure $ insStr "frog" g -- This inserts a string into g. > -- "g <- pure $" because we use the IO monad to redefine g. > -- Now g has one thing, the word frog: > v g $ nodes g -- This views every expression in g. (0,"0: frog") > -- The 0 before the comma is a count of how many expressions refer to that expression. None do, because none yet exist. Such contexts offer many possibilities for tracking where you are (the data) and how you got there (also kind of the data, and perhaps enabling a desirable kind of removal of self|redaction). > -- The second 0 is an FGL Node, the address of the string "frog". > -- Let's add more words: > g <- pure $ insStr "moist" g > g <- pure $ insStr "springy" g > -- Now g has three words: > v g $ nodes g (0,"0: frog") (0,"1: moist") (0,"2: springy") > -- Let's indicate that moist and springy are both qualities of frogs: > -- First, we'll create a template for relationships about qualities. > g <- pure $ insTplt "_ is a quality of _" g > v g $ nodes g (0,"0: frog") (0,"1: moist") (0,"2: springy") (0,":3 _ #(is a quality of) _") > -- The template has been inserted at node 3. Its address is preceded by a : to indicate that it is a template, as opposed to the string "_ is a quality of _". (That will seem more natural in the context of how relationships are displayed.) > -- Now we'll use the template at 3 to insert two relationships among the other nodes: > -- We could have added them in a single stage: > g <- pure $ fromRight $ insRel 3 [2,0] g >>= insRel 3 [1,0] > -- insRel 3 [1,0] inserts "moist is a quality of frog". > -- insRel 3 [2,0] inserts "springy is a quality of frog". > -- Both relationships use the template at node 3, and in both the second member is frog, at node 0. > -- Since the template at 3 relates 2 expressions, each list has length two. Templates can have any arity -- for instance, "_ does _ to _" has arity 3. > -- I used the monadic bind operator (>>=) because that way I only need to escape one Either monad (using fromRight) rather than two of them, one containing the other. > -- Inserting those two relationships worked: > v g $ nodes g (2,"0: frog") (1,"1: moist") (1,"2: springy") (2,":3 _ #(is a quality of) _") (0,"4:3 2: springy ##(is a quality of) 0: frog") (0,"5:3 1: moist ##(is a quality of) 0: frog") > -- When showing a relationship, the node (integer) listed first, before the :, is it's address, and the node listed after the : is the address of its template. > -- After the first two, any other node in a relationship is the address of the symbol(s) it appears just left of. > -- Notice that now the frog is in two relationships. One of those relationships involves springy, the other involves moist. > -- The # symbols are better explained later. > -- DWT can encode meta-statements, statements about other statements. As an example, next let's encode that maybe frogs are springy because they use rubber bands. > -- First we should insert a new string and a couple new templates for relationships. > g <- pure $ insTplt "maybe _ because _" g > g <- pure $ insStr "rubber bands" g > g <- pure $ insTplt "_ uses _" g > v g $ nodes g (2,"0: frog") (1,"1: moist") (1,"2: springy") (2,":3 _ #(is a quality of) _") (0,"4:3 2: springy ##(is a quality of) 0: frog") (0,"5:3 1: moist ##(is a quality of) 0: frog") (0,":6 #maybe _ #because _") (0,"7: rubber bands") (0,":8 _ #uses _") > -- Now we can encode the subexpression that frog uses rubber bands. > g <- pure $ fr $ insRel 8 [0,7] g > v g $ nodes g (3,"0: frog") (1,"1: moist") (1,"2: springy") (2,":3 _ #(is a quality of) _") (0,"4:3 2: springy ##(is a quality of) 0: frog") (0,"5:3 1: moist ##(is a quality of) 0: frog") (0,":6 #maybe _ #because _") (1,"7: rubber bands") (1,":8 _ #uses _") (0,"9:8 0: frog ##uses 7: rubber bands") > -- From those elements, we can encode that maybe frogs are springy because they use rubber bands. > g <- pure $ fr $ insRel 6 [4,9] g > v g $ nodes g (3,"0: frog") (1,"1: moist") (1,"2: springy") (2,":3 _ #(is a quality of) _") (1,"4:3 2: springy ##(is a quality of) 0: frog") (0,"5:3 1: moist ##(is a quality of) 0: frog") (1,":6 #maybe _ #because _") (1,"7: rubber bands") (1,":8 _ #uses _") (1,"9:8 0: frog ##uses 7: rubber bands") (0,"10:6 ####maybe 4:3 2: springy ##(is a quality of) 0: frog ####because 9:8 0: frog ##uses 7: rubber bands") At last I can explain the # symbols: They indicate how fast an expression binds. In the last, at node 10, the highest-level|slowest-binding relationship is "maybe _ because _". In the first _ of the maybe-because relationship is another relationship, one that binds faster and hence has fewer # symbols, between springy and frog. The strings "springy", "frog" and "rubber bands" can be thought of as atomic, "binding" even before the first-level relationships bind. This illustrates one of DWT's advantages over graphs. In a graph, an edge cannot be a member of another edge. By contrast, in DWT a relationship can involve other relationships. -- Jeffrey Benjamin Brown

1 1

Month in Haskell Mode February 2016
by Gracjan Polak 04 Mar '16

04 Mar '16

Welcome Haskell Mode users, Haskell Mode progress report for February 2016. For previous issue see January 2015 <https://github.com/haskell/haskell-mode/wiki/Month-in-Haskell-Mode-January-…> . Reddit discussion <https://www.reddit.com/r/haskell/comments/48wrru/month_in_haskell_mode_febr…> . <https://github.com/haskell/haskell-mode/wiki/Month-in-Haskell-Mode-February…>What is Haskell Mode? Haskell Mode is an umbrella project for multiple Emacs tools for efficient Haskell development. Haskell Mode is an open source project developed by a group of volunteers constantly looking for contributions. For more information how to help see https://github.com/haskell/haskell-mode. <https://github.com/haskell/haskell-mode/wiki/Month-in-Haskell-Mode-February…>Important developments Fontification received improvements so that it closer follows Haskell lexical rules. It was necessary to change to syntax-propertize-function available only starting with Emacs 24. There was already an improvement to how modules are shown: We also have a Twitter account: @HaskellMode <https://twitter.com/HaskellMode>. Follow us! <https://github.com/haskell/haskell-mode/wiki/Month-in-Haskell-Mode-February…>Issues closed in February - Disable Indentation #90 <https://github.com/haskell/haskell-mode/issues/90> - Break point highlighting #127 <https://github.com/haskell/haskell-mode/issues/127> - Type query from up to date file information #185 <https://github.com/haskell/haskell-mode/issues/185> - haskell-stylish-on-save with Yasnippet + ghc-mod sometimes removes import list #256 <https://github.com/haskell/haskell-mode/issues/256> - haskell-indentation mode can't parse -XDoAndIfThenElse syntax #321 <https://github.com/haskell/haskell-mode/issues/321> - haskell-indentation mode sometimes can't parse multiline string literals #324 <https://github.com/haskell/haskell-mode/issues/324> - Compatibility with column-enforce-mode and long literate (latex-style) comments #331 <https://github.com/haskell/haskell-mode/issues/331> - haskell-debug parse error #399 <https://github.com/haskell/haskell-mode/issues/399> - Use preprocessor highlighting from cc-mode #521 <https://github.com/haskell/haskell-mode/issues/521> - haskell-mode-contextual-space called on zero length identifier #572 <https://github.com/haskell/haskell-mode/issues/572> - In 12.34e-56 (a single number) both . and - are colorized as operators #628 <https://github.com/haskell/haskell-mode/issues/628> - haskell-doc-mode is not asynchronous wrt ghci process, unlike eldoc-mode #649 <https://github.com/haskell/haskell-mode/issues/649> - Fix haskell-customize #667 <https://github.com/haskell/haskell-mode/issues/667> - Use easy-mmode-defmap to define all keymaps #669 <https://github.com/haskell/haskell-mode/issues/669> - Multi line string editing #727 <https://github.com/haskell/haskell-mode/issues/727> - Implement backward-sexp #759 <https://github.com/haskell/haskell-mode/issues/759> - haskell-stylish-on-save causes double saving #788 <https://github.com/haskell/haskell-mode/issues/788> - haskell-mode becomes unresponsive when adding a language pragma #820 <https://github.com/haskell/haskell-mode/issues/820> - Use flycheck to manage error and warning overlays #821 <https://github.com/haskell/haskell-mode/issues/821> - [Discussion] Separation of concerns #836 <https://github.com/haskell/haskell-mode/issues/836> - [Discussion] Perhaps tag a release? Monthly releases? #842 <https://github.com/haskell/haskell-mode/issues/842> - Split up gifcasts.el and connect dependency to specific gifs in Makefile #848 <https://github.com/haskell/haskell-mode/issues/848> - Can't press enter in haskell interactive mode #876 <https://github.com/haskell/haskell-mode/issues/876> - haskell-process-load-or-reload and haskell-interactive-switch-back #955 <https://github.com/haskell/haskell-mode/issues/955> - Font lock should support mixed Bird and Latex style literate Haskell sources #1038 <https://github.com/haskell/haskell-mode/issues/1038> - Use syntax-propertize-function #1039 <https://github.com/haskell/haskell-mode/issues/1039> - Highlight errors inside string literals #1040 <https://github.com/haskell/haskell-mode/issues/1040> - Default indentation mode ought to support and use Johan Tibell's style by default #1042 <https://github.com/haskell/haskell-mode/issues/1042> - Support spaces in project paths for haskell-process-load-file or haskell-process-load-or-reload with errors in source file #1107 <https://github.com/haskell/haskell-mode/issues/1107> - haskell-process-path-* now allowed to be lists #1112 <https://github.com/haskell/haskell-mode/issues/1112> - haskell-font-lock-symbols no longer works #1120 <https://github.com/haskell/haskell-mode/issues/1120> - Indenting data types seems broken and not configurable #1125 <https://github.com/haskell/haskell-mode/issues/1125> - Error: defcustom for ‘haskell-indent-spaces’ fails to specify type #1126 <https://github.com/haskell/haskell-mode/issues/1126> - Reconsider word breaks for haskell-cabal-mode #1146 <https://github.com/haskell/haskell-mode/issues/1146> - haskell-forward-sexp should signal scan-error at the end of list like forward-sexp #1148 <https://github.com/haskell/haskell-mode/issues/1148> - Syntax highlighting is broken for quasiquoted code #1155 <https://github.com/haskell/haskell-mode/issues/1155> - Font lock module qualification differently #1157 <https://github.com/haskell/haskell-mode/issues/1157> - HTML manual Retina resolution animated gifs #1160 <https://github.com/haskell/haskell-mode/issues/1160> - Completion function hangs in REPL in multi-line editing mode #1173 <https://github.com/haskell/haskell-mode/issues/1173> - [Discussion] Interactive mode improvements #1176 <https://github.com/haskell/haskell-mode/issues/1176> - Fontification is too inefficient #1177 <https://github.com/haskell/haskell-mode/issues/1177> <https://github.com/haskell/haskell-mode/wiki/Month-in-Haskell-Mode-February…>Pull requests merged in February - Use syntax-propertize-function #1050 <https://github.com/haskell/haskell-mode/pull/1050> - Require at least Emacs 24.1 #1055 <https://github.com/haskell/haskell-mode/pull/1055> - Define haskell-package struct's slots #1083 <https://github.com/haskell/haskell-mode/pull/1083> - Detect comma style #1099 <https://github.com/haskell/haskell-mode/pull/1099> - Cleanup haskell-compat.el #1103 <https://github.com/haskell/haskell-mode/pull/1103> - Ignore stack and cabal files from hasktags #1118 <https://github.com/haskell/haskell-mode/pull/1118> - Remove suggested imports completely #1121 <https://github.com/haskell/haskell-mode/pull/1121> - Support spaces in project directories #1123 <https://github.com/haskell/haskell-mode/pull/1123> - Use condition-case-unless-debug #1124 <https://github.com/haskell/haskell-mode/pull/1124> - Fontify True/False in cabal mode #1127 <https://github.com/haskell/haskell-mode/pull/1127> - Fix snapshot build #1128 <https://github.com/haskell/haskell-mode/pull/1128> - Add indentation tests for constructor and deriving #1129 <https://github.com/haskell/haskell-mode/pull/1129> - Align data declaration contructors in indentation #1130 <https://github.com/haskell/haskell-mode/pull/1130> - Remove haskell-package.el #1132 <https://github.com/haskell/haskell-mode/pull/1132> - Docstrings and documentation for haskell-process-path-*. #1135 <https://github.com/haskell/haskell-mode/pull/1135> - Add front-sticky property to read-only text in REPL #1136 <https://github.com/haskell/haskell-mode/pull/1136> - Stylish haskell after save #1137 <https://github.com/haskell/haskell-mode/pull/1137> - Make all define-key calls use kbd #1139 <https://github.com/haskell/haskell-mode/pull/1139> - Use haskell-interactive-switch in process-load #1140 <https://github.com/haskell/haskell-mode/pull/1140> - Revert "Stylish haskell after save" #1142 <https://github.com/haskell/haskell-mode/pull/1142> - Make keymap definition more consistent #1143 <https://github.com/haskell/haskell-mode/pull/1143> - Remove some outdated code #1144 <https://github.com/haskell/haskell-mode/pull/1144> - Fix #1146 by treating . as punctuation #1149 <https://github.com/haskell/haskell-mode/pull/1149> - Add test for forward-sexp-function #1151 <https://github.com/haskell/haskell-mode/pull/1151> - Better string animation #1152 <https://github.com/haskell/haskell-mode/pull/1152> - Properly find begin of a quasiquote #1156 <https://github.com/haskell/haskell-mode/pull/1156> - Implement backward case in haskell-forward-sexp #1158 <https://github.com/haskell/haskell-mode/pull/1158> - Improve font-lock #1159 <https://github.com/haskell/haskell-mode/pull/1159> - HTML manual fixup machinery #1161 <https://github.com/haskell/haskell-mode/pull/1161> - Deriving indentatation test #1162 <https://github.com/haskell/haskell-mode/pull/1162> - Make RET in REPL create new prompt with blank input #1163 <https://github.com/haskell/haskell-mode/pull/1163> - Revert "Remove suggested imports completely" #1165 <https://github.com/haskell/haskell-mode/pull/1165> - Fix haskell-hoogle to work with buffer local var #1167 <https://github.com/haskell/haskell-mode/pull/1167> - Support simple completions in non-interactive context. Support keyword completions #1172 <https://github.com/haskell/haskell-mode/pull/1172> <https://github.com/haskell/haskell-mode/wiki/Month-in-Haskell-Mode-February…>Contributors active in February Arthur Fayzrakhmanov (Артур Файзрахманов), Chris Done, Christoph Ruegge, Daniel Bergey, Daniel Selinger, Fice T, Gracjan Polak, Kamil Dworakowski, Karl Smeltzer, Mike Sperber, Sergey Vinokurov, issue-dispenser, mrBliss <https://github.com/haskell/haskell-mode/wiki/Month-in-Haskell-Mode-February…> Contributing Haskell Mode needs volunteers like any other open source project. For more information see: https://github.com/haskell/haskell-mode/wiki Also drop by our IRC channel: #haskell-emacs at irc.freenode.net. Thanks!

1 0

Function that put elements in Left or Right side of Either depending on type
by Kristoffer Føllesdal 04 Mar '16

04 Mar '16

I want a function fun :: q -> Either a b where q is of type a or b. fun shall work in the following way fun x gives Left x if x :: a fun x gives Right x if x :: b Following is a more precise description of what I want. I have function fun1 and fun2 newtype Vec k b = V [(b,k)] deriving (Eq,Ord,Show) fun1 :: (Num k) => a -> Either a (Vec k a) fun1 = Left fun2 :: (Num k) => Vec k a -> Either a (Vec k a) fun2 = Right that work in the following way {- >>> fun1 6 Left 6 >>> fun2 (V[(6,1)]) Right (V [(6,1)]) -} I want a overloaded function ‘fun' such that {- >>> fun 6 Left 6 >>> fun (V[(6,1)]) Right (V[(6,1)]) -} I have tried to use a type class to do this (see code below). But when I try >>> fun 6 I get the following error Could not deduce (Num a0) from the context (Num a, Fun a b v) bound by the inferred type for ‘it’: (Num a, Fun a b v) => Either b (v b) at <interactive>:70:1-5 The type variable ‘a0’ is ambiguous When checking that ‘it’ has the inferred type it :: forall a b (v :: * -> *). (Num a, Fun a b v) => Either b (v b) Probable cause: the inferred type is ambiguous Is the someone that have know how I can solve this? Kristoffer ——— CODE ——— {-# LANGUAGE MultiParamTypeClasses #-} {-# LANGUAGE FlexibleInstances #-} newtype Vec k b = V [(b,k)] deriving (Eq,Ord,Show) fun1 :: (Num k) => a -> Either a (Vec k a) fun1 = Left fun2 :: (Num k) => Vec k a -> Either a (Vec k a) fun2 = Right class Fun a b v where fun :: a -> Either b (v b) instance (Num k) => Fun t t (Vec k) where fun = fun1 instance (Num k) => Fun (Vec k t) t (Vec k) where fun = fun2

4 3

Order of Map.fromListWith
by Niklas Hambüchen 03 Mar '16

03 Mar '16

Does anybody know why for fromListWith, the arguments to the combining function seem flipped? > import Data.Map > fromListWith (++) [('a',[1]),('a',[2])] fromList [('a',[2,1])] I often use it to group things by some key, e.g. postsByUserId :: Map Int [Int] postsByUserId = fromListWith (++) [ (userId, [postId]) | (userId, postId) <- posts ] and regularly get tricked by the postIds being reversed in the result. This is especially unintuitive to me since: > foldl (++) [] [[1],[2]] [1,2] > foldr (++) [] [[1],[2]] [1,2] Any ideas?

3 3

Local mirrors of hackage
by Victor Miller 03 Mar '16

03 Mar '16

I have a computer that I work on which isn't directly connected to the internet, that I want to use Haskell on. In order to do that I want a local version of hackage. I was previously able to do this using mirror_hackage.py, but since hackage 2 came around this no longer works. Doing this would involve running something on a computer that *is* attached to the internet, and then transferring a bunch of files over to the other computer. Can someone point me to a recipe for doing this? Victor

5 9

ICFP 2016 Final Call for Papers
by Lindsey Kuper 03 Mar '16

03 Mar '16

ICFP 2016 The 21st ACM SIGPLAN International Conference on Functional Programming http://conf.researchr.org/home/icfp-2016 Final Call for Papers Important dates --------------- Submissions due: Wednesday, March 16 2016, 15:00 (UTC) https://icfp2016.hotcrp.com (now open) Author response: Monday, 2 May, 2016, 15:00 (UTC) - Thursday, 5 May, 2016, 15:00 (UTC) Notification: Friday, 20 May, 2016 Final copy due: TBA Early registration: TBA Conference: Monday, 19 September - Wednesday, 21 September, 2016 Please note ----------- For the sake of lightweight double-blind reviewing, the submission procedure may take a little more time than in previous ICFPs; we recommend that you register your submission as early as possible (you can update your paper until the deadline). Scope ----- ICFP 2016 seeks original papers on the art and science of functional programming. Submissions are invited on all topics from principles to practice, from foundations to features, and from abstraction to application. The scope includes all languages that encourage functional programming, including both purely applicative and imperative languages, as well as languages with objects, concurrency, or parallelism. Topics of interest include (but are not limited to): - Language Design: concurrency, parallelism, and distribution; modules; components and composition; metaprogramming; type systems; interoperability; domain-specific languages; and relations to imperative, object-oriented, or logic programming. - Implementation: abstract machines; virtual machines; interpretation; compilation; compile-time and run-time optimization; garbage collection and memory management; multi-threading; exploiting parallel hardware; interfaces to foreign functions, services, components, or low-level machine resources. - Software-Development Techniques: algorithms and data structures; design patterns; specification; verification; validation; proof assistants; debugging; testing; tracing; profiling. - Foundations: formal semantics; lambda calculus; rewriting; type theory; monads; continuations; control; state; effects; program verification; dependent types. - Analysis and Transformation: control-flow; data-flow; abstract interpretation; partial evaluation; program calculation. - Applications: symbolic computing; formal-methods tools; artificial intelligence; systems programming; distributed-systems and web programming; hardware design; databases; XML processing; scientific and numerical computing; graphical user interfaces; multimedia and 3D graphics programming; scripting; system administration; security. - Education: teaching introductory programming; parallel programming; mathematical proof; algebra. - Functional Pearls: elegant, instructive, and fun essays on functional programming. - Experience Reports: short papers that provide evidence that functional programming really works or describe obstacles that have kept it from working. If you are concerned about the appropriateness of some topic, do not hesitate to contact the program chair. Abbreviated instructions for authors ------------------------------------ - By Wednesday, March 16 2016, 15:00 (UTC), submit a full paper of at most 12 pages (6 pages for an Experience Report), in standard SIGPLAN conference format, including figures but ***excluding bibliography***. The deadlines will be strictly enforced and papers exceeding the page limits will be summarily rejected. ***ICFP 2016 will employ a lightweight double-blind reviewing process.*** To facilitate this, submitted papers must adhere to two rules: 1. ***author names and institutions must be omitted***, and 2. ***references to authors' own related work should be in the third person*** (e.g., not "We build on our previous work ..." but rather "We build on the work of ..."). The purpose of this process is to help the PC and external reviewers come to an initial judgement about the paper without bias, not to make it impossible for them to discover the authors if they were to try. Nothing should be done in the name of anonymity that weakens the submission or makes the job of reviewing the paper more difficult (e.g., important background references should not be omitted or anonymized). In addition, authors should feel free to disseminate their ideas or draft versions of their paper as they normally would. For instance, authors may post drafts of their papers on the web or give talks on their research ideas. We have put together a document answering frequently asked questions that should address many common concerns: http://conf.researchr.org/track/icfp-2016/icfp-2016-papers#Submission-and-R… (last updated February 8, 2016). - Authors have the option to attach supplementary material to a submission, on the understanding that reviewers may choose not to look at it. The material should be uploaded at submission time, as a single pdf or a tarball, not via a URL. This supplementary material may or may not be anonymized; if not anonymized, it will only be revealed to reviewers after they have submitted their review of your paper and learned your identity. - Each submission must adhere to SIGPLAN's republication policy, as explained on the web at: http://www.sigplan.org/Resources/Policies/Republication - Authors of resubmitted (but previously rejected) papers have the option to attach an annotated copy of the reviews of their previous submission(s), explaining how they have addressed these previous reviews in the present submission. If a reviewer identifies him/herself as a reviewer of this previous submission and wishes to see how his/her comments have been addressed, the program chair will communicate to this reviewer the annotated copy of his/her previous review. Otherwise, no reviewer will read the annotated copies of the previous reviews. Overall, a submission will be evaluated according to its relevance, correctness, significance, originality, and clarity. It should explain its contributions in both general and technical terms, clearly identifying what has been accomplished, explaining why it is significant, and comparing it with previous work. The technical content should be accessible to a broad audience. Functional Pearls and Experience Reports are separate categories of papers that need not report original research results and must be marked as such at the time of submission. Detailed guidelines on both categories are given below. Presentations will be videotaped and released online if the presenter consents. The proceedings will be freely available for download from the ACM Digital Library from at least one week before the start of the conference until two weeks after the conference. Formatting: Submissions must be in PDF format printable in black and white on US Letter sized paper and interpretable by Ghostscript. Papers must adhere to the standard SIGPLAN conference format: two columns, nine-point font on a ten-point baseline, with columns 20pc (3.33in) wide and 54pc (9in) tall, with a column gutter of 2pc (0.33in). A suitable document template for LaTeX is available at http://www.sigplan.org/Resources/Author/ Submission: Submissions will be accepted at https://icfp2016.hotcrp.com . Improved versions of a paper may be submitted at any point before the submission deadline using the same web interface. Author response: Authors will have a 72-hour period, starting at 15:00 UTC on Monday, 2 May, 2016, to read reviews and respond to them. ACM Author-Izer is a unique service that enables ACM authors to generate and post links on either their home page or institutional repository for visitors to download the definitive version of their articles from the ACM Digital Library at no charge. Downloads through Author-Izer links are captured in official ACM statistics, improving the accuracy of usage and impact measurements. Consistently linking the definitive version of ACM article should reduce user confusion over article versioning. After your article has been published and assigned to your ACM Author Profile page, please visit http://www.acm.org/publications/acm-author-izer-service to learn how to create your links for free downloads from the ACM DL. AUTHORS TAKE NOTE: The official publication date is the date the proceedings are made available in the ACM Digital Library. This date may be up to two weeks prior to the first day of your conference. The official publication date affects the deadline for any patent filings related to published work. Special categories of papers ---------------------------- In addition to research papers, ICFP solicits two kinds of papers that do not require original research contributions: Functional Pearls, which are full papers, and Experience Reports, which are limited to six pages. Authors submitting such papers may wish to consider the following advice. Functional Pearls ================= A Functional Pearl is an elegant essay about something related to functional programming. Examples include, but are not limited to: - a new and thought-provoking way of looking at an old idea - an instructive example of program calculation or proof - a nifty presentation of an old or new data structure - an interesting application of functional programming techniques - a novel use or exposition of functional programming in the classroom While pearls often demonstrate an idea through the development of a short program, there is no requirement or expectation that they do so. Thus, they encompass the notions of theoretical and educational pearls. Functional Pearls are valued as highly and judged as rigorously as ordinary papers, but using somewhat different criteria. In particular, a pearl is not required to report original research, but, it should be concise, instructive, and entertaining. Your pearl is likely to be rejected if your readers get bored, if the material gets too complicated, if too much specialized knowledge is needed, or if the writing is inelegant. The key to writing a good pearl is polishing. A submission you wish to have treated as a pearl must be marked as such on the submission web page, and should contain the words ``Functional Pearl'' somewhere in its title or subtitle. These steps will alert reviewers to use the appropriate evaluation criteria. Pearls will be combined with ordinary papers, however, for the purpose of computing the conference's acceptance rate. Experience Reports ================== The purpose of an Experience Report is to help create a body of published, refereed, citable evidence that functional programming really works ? or to describe what obstacles prevent it from working. Possible topics for an Experience Report include, but are not limited to: - insights gained from real-world projects using functional programming - comparison of functional programming with conventional programming in the context of an industrial project or a university curriculum - project-management, business, or legal issues encountered when using functional programming in a real-world project - curricular issues encountered when using functional programming in education - real-world constraints that created special challenges for an implementation of a functional language or for functional programming in general An Experience Report is distinguished from a normal ICFP paper by its title, by its length, and by the criteria used to evaluate it. - Both in the proceedings and in any citations, the title of each accepted Experience Report must begin with the words ``Experience Report'' followed by a colon. The acceptance rate for Experience Reports will be computed and reported separately from the rate for ordinary papers. - An Experience Report is at most six pages long. Each accepted Experience Report will be presented at the conference, but depending on the number of Experience Reports and regular papers accepted, authors of Experience reports may be asked to give shorter talks. - Because the purpose of Experience Reports is to enable our community to accumulate a body of evidence about the efficacy of functional programming, an acceptable Experience Report need not add to the body of knowledge of the functional-programming community by presenting novel results or conclusions. It is sufficient if the Report states a clear thesis and provides supporting evidence. The thesis must be relevant to ICFP, but it need not be novel. The program committee will accept or reject Experience Reports based on whether they judge the evidence to be convincing. Anecdotal evidence will be acceptable provided it is well argued and the author explains what efforts were made to gather as much evidence as possible. Typically, more convincing evidence is obtained from papers which show how functional programming was used than from papers which only say that functional programming was used. The most convincing evidence often includes comparisons of situations before and after the introduction or discontinuation of functional programming. Evidence drawn from a single person's experience may be sufficient, but more weight will be given to evidence drawn from the experience of groups of people. An Experience Report should be short and to the point: make a claim about how well functional programming worked on your project and why, and produce evidence to substantiate your claim. If functional programming worked for you in the same ways it has worked for others, you need only to summarize the results?the main part of your paper should discuss how well it worked and in what context. Most readers will not want to know all the details of your project and its implementation, but please characterize your project and its context well enough so that readers can judge to what degree your experience is relevant to their own projects. Be especially careful to highlight any unusual aspects of your project. Also keep in mind that specifics about your project are more valuable than generalities about functional programming; for example, it is more valuable to say that your team delivered its software a month ahead of schedule than it is to say that functional programming made your team more productive. If your paper not only describes experience but also presents new technical results, or if your experience refutes cherished beliefs of the functional-programming community, you may be better off submitting it as a full paper, which will be judged by the usual criteria of novelty, originality, and relevance. If you are unsure in which category to submit, the program chair will be happy to help you decide. Organizers ---------- General Co-Chairs: Jacques Garrigue (Nagoya University) Gabriele Keller (University of New South Wales) Program Chair: Eijiro Sumii (Tohoku University) Program Committee: Koen Claessen (Chalmers University of Technology) Joshua Dunfield (University of British Columbia, Canada) Matthew Fluet (Rochester Institute of Technology) Nate Foster (Cornell University) Dan Grossman (University of Washington, USA) Jurriaan Hage (Utrecht University) Roman Leshchinskiy (Standard Chartered Bank) Keisuke Nakano (The University of Electro-Communications) Aleksandar Nanevski (IMDEA Software Institute) Scott Owens (University of Kent) Sungwoo Park (Pohang University of Science and Technology) Amr Sabry (Indiana University) Tom Schrijvers (KU Leuven) Olin Shivers (Northeastern University) Walid Taha (Halmstad University) Dimitrios Vytiniotis (Microsoft Research, Cambridge) David Walker (Princeton University) Nobuko Yoshida (Imperial College London, UK) External Review Committee: See http://conf.researchr.org/committee/icfp-2016/icfp-2016-papers-external-rev… .

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Take over netlink-hs
by Markus Ongyerth 02 Mar '16

02 Mar '16

Hi, I want to take over the netlink package on hackage [1]. In accordance to [2] I want to announce that publicly on this list. I have tried to contact the auther in october 2015, and talked about it on the #haskell irc channel. I was able to reach the current maintainer, he told me that he forgot about it and doesn't know his hackage password anymore. Davean (on IRC) offerden to give me maintainer rights after checking back with Max himself. I didn't hear anything after that, neither have I made any effort in this matter since then. ongy [1] https://hackage.haskell.org/package/netlink [2] https://wiki.haskell.org/Taking_over_a_package

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How do I sort items in a wxhaskell listCtrl?
by Tilmann 02 Mar '16

02 Mar '16

Hi list, I couldn't find any clues googling and turn to you in the hope that someone might point me in the right direction: I found the function |listCtrlSortItems2| with type |ListCtrl a -> Closure b -> IO Bool| and |closureCreate| (which type is a bit involved..), but I have no idea how to use them.. In the end, I would like to have a listCtrl where clicking on column headers will sort the items and maybe even indicating sorting order with a little arrow. Any help is really appreciated, Tilmann

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Haskell 2016: Call for Papers
by Geoffrey Mainland 01 Mar '16

01 Mar '16

======================================================================== ACM SIGPLAN CALL FOR SUBMISSIONS Haskell Symposium 2016 Nara, Japan, 22-23 September 2015, directly after ICFP http://www.haskell.org/haskell-symposium/2016 ======================================================================== ** The Haskell Symposium has an early track this year ** ** See the Submission Timetable for details. ** The ACM SIGPLAN Haskell Symposium 2016 will be co-located with the International Conference on Functional Programming (ICFP 2016) in Vancouver, Canada. The Haskell Symposium aims to present original research on Haskell, discuss practical experience and future development of the language, and to promote other forms of denotative programming. Topics of interest include: * Language Design, with a focus on possible extensions and modifications of Haskell as well as critical discussions of the status quo; * Theory, such as formal semantics of the present language or future extensions, type systems, effects, metatheory, and foundations for program analysis and transformation; * Implementations, including program analysis and transformation, static and dynamic compilation for sequential, parallel, and distributed architectures, memory management, as well as foreign function and component interfaces; * Libraries, that demonstrate new ideas or techniques for functional programming in Haskell; * Tools, such as profilers, tracers, debuggers, preprocessors, and testing tools; * Applications, to scientific and symbolic computing, databases, multimedia, telecommunication, the web, and so forth; * Functional Pearls, being elegant and instructive programming examples; * Experience Reports, to document general practice and experience in education, industry, or other contexts. Papers in the latter three categories need not necessarily report original academic research results. For example, they may instead report reusable programming idioms, elegant ways to approach a problem, or practical experience that will be useful to other users, implementors, or researchers. The key criterion for such a paper is that it makes a contribution from which other Haskellers can benefit. It is not enough simply to describe a standard solution to a standard programming problem, or report on experience where you used Haskell in the standard way and achieved the result you were expecting. More advice is available via the Haskell wiki: (http://wiki.haskell.org/HaskellSymposium/ExperienceReports) Regular papers should explain their research contributions in both general and technical terms, identifying what has been accomplished, explaining why it is significant, and relating it to previous work, and to other languages where appropriate. In addition, we solicit proposals for: * System Demonstrations, based on running software rather than novel research results. These proposals should summarize the system capabilities that would be demonstrated. The proposals will be judged on whether the ensuing session is likely to be important and interesting to the Haskell community at large, whether on grounds academic or industrial, theoretical or practical, technical, social or artistic. Please contact the program chair with any questions about the relevance of a proposal. Travel Support: =============== Student attendees with accepted papers can apply for a SIGPLAN PAC grant to help cover travel expenses. PAC also offers other support, such as for child-care expenses during the meeting or for travel costs for companions of SIGPLAN members with physical disabilities, as well as for travel from locations outside of North America and Europe. For details on the PAC program, see its web page (http://pac.sigplan.org) Proceedings: ============ Accepted papers will be included in the ACM Digital Library. Authors must grant ACM publication rights upon acceptance (http://authors.acm.org/main.html) Authors are encouraged to publish auxiliary material with their paper (source code, test data, etc.); they retain copyright of auxiliary material. Accepted proposals for system demonstrations will be posted on the symposium website but not formally published in the proceedings. All accepted papers and proposals will be posted on the conference website one week before the meeting. Publication date: The official publication date of accepted papers is the date the proceedings are made available in the ACM Digital Library. This date may be up to two weeks prior to the first day of the conference. The official publication date affects the deadline for any patent filings related to published work. Submission Details: =================== Submitted papers should be in portable document format (PDF), formatted using the ACM SIGPLAN style guidelines (http://www.acm.org/sigs/sigplan/authorInformation.htm) The text should be in a 9-point font in two columns. The length is restricted to 12 pages, except for "Experience Report" papers, which are restricted to 6 pages. Papers need not fill the page limit---for example, a Functional Pearl may be much shorter than 12 pages. Each paper submission must adhere to SIGPLAN's republication policy, as explained on the web. Demo proposals are limited to 2-page abstracts, in the same ACM format as papers. "Functional Pearls", "Experience Reports", and "Demo Proposals" should be marked as such with those words in the title at time of submission. The paper submission deadline and length limitations are firm. There will be no extensions, and papers violating the length limitations will be summarily rejected. A link to the paper submission system will appear on the Haskell Symposium web site closer to the submission deadline. Submission Timetable: ===================== Early Track Regular Track System Demos ---------------- ------------------- --------------- 1st April Paper Submission 20th May Notification 6th June Abstract Submission 10th June Paper Submission 17th June Resubmission Demo Submission 8th July Notification Notification Notification 31st July Camera ready due Camera ready due Deadlines stated are valid anywhere on earth. The Haskell Symposium uses a two-track submission process so that some papers can gain early feedback. Papers can be submitted to the early track on 1st April. On 20th May, strong papers are accepted outright, and the others will be given their reviews and invited to resubmit. On 17th June, early track papers may be resubmitted and are sent back to the same reviewers. The Haskell Symposium regular track operates as in previous years. Papers accepted via the early and regular tracks are considered of equal value and will not be distinguished in the proceedings. Although all papers may be submitted to the early track, authors of functional pearls and experience reports are particularly encouraged to use this mechanism. The success of these papers depends heavily on the way they are presented, and submitting early will give the program committee a chance to provide feedback and help draw out the key ideas. Program Committee: =================== James Cheney University of Edinburgh Iavor Diatchki Galois David Duke University of Leeds Richard Eisenberg University of Pennsylvania Ken Friis Larsen University of Copenhagen Andy Gill University of Kansas Zhenjiang Hu National Institute of Informatics Ranjit Jhala UC San Diego Yukiyoshi Kameyama University of Tsukuba Geoffrey Mainland (chair) Drexel University Mary Sheeran Chalmers University of Technology David Terei Stanford Niki Vazou UC San Diego Dimitrios Vytiniotis Microsoft Research =====================================================================

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[TFP 2016] 2nd call for papers
by Peter Achten 01 Mar '16

01 Mar '16

----------------------------- C A L L F O R P A P E R S ----------------------------- ======== TFP 2016 =========== 17th Symposium on Trends in Functional Programming June 8-10, 2016 University of Maryland, College Park Near Washington, DC http://tfp2016.org/ The symposium on Trends in Functional Programming (TFP) is an international forum for researchers with interests in all aspects of functional programming, taking a broad view of current and future trends in the area. It aspires to be a lively environment for presenting the latest research results, and other contributions (see below). Authors of draft papers will be invited to submit revised papers based on the feedback receive at the symposium. A post-symposium refereeing process will then select a subset of these articles for formal publication. TFP 2016 will be the main event of a pair of functional programming events. TFP 2016 will be accompanied by the International Workshop on Trends in Functional Programming in Education (TFPIE), which will take place on June 7nd. == INVITED SPEAKERS == TFP 2016 is pleased to announce keynote talks by the following two invited speakers: * Ronald Garcia, University of British Columbia * Steve Zdancewic, University of Pennsylvania == HISTORY == The TFP symposium is the heir of the successful series of Scottish Functional Programming Workshops. Previous TFP symposia were held in * Edinburgh (Scotland) in 2003; * Munich (Germany) in 2004; * Tallinn (Estonia) in 2005; * Nottingham (UK) in 2006; * New York (USA) in 2007; * Nijmegen (The Netherlands) in 2008; * Komarno (Slovakia) in 2009; * Oklahoma (USA) in 2010; * Madrid (Spain) in 2011; * St. Andrews (UK) in 2012; * Provo (Utah, USA) in 2013; * Soesterberg (The Netherlands) in 2014; * and Inria Sophia-Antipolis (France) in 2015. For further general information about TFP please see the TFP homepage. (http://www.tifp.org/) == SCOPE == The symposium recognizes that new trends may arise through various routes. As part of the Symposium's focus on trends we therefore identify the following five article categories. High-quality articles are solicited in any of these categories: Research Articles: leading-edge, previously unpublished research work Position Articles: on what new trends should or should not be Project Articles: descriptions of recently started new projects Evaluation Articles: what lessons can be drawn from a finished project Overview Articles: summarizing work with respect to a trendy subject Articles must be original and not simultaneously submitted for publication to any other forum. They may consider any aspect of functional programming: theoretical, implementation-oriented, or experience-oriented. Applications of functional programming techniques to other languages are also within the scope of the symposium. Topics suitable for the symposium include, but are not limited to: Functional programming and multicore/manycore computing Functional programming in the cloud High performance functional computing Extra-functional (behavioural) properties of functional programs Dependently typed functional programming Validation and verification of functional programs Debugging and profiling for functional languages Functional programming in different application areas: security, mobility, telecommunications applications, embedded systems, global computing, grids, etc. Interoperability with imperative programming languages Novel memory management techniques Program analysis and transformation techniques Empirical performance studies Abstract/virtual machines and compilers for functional languages (Embedded) domain specific languages New implementation strategies Any new emerging trend in the functional programming area If you are in doubt on whether your article is within the scope of TFP, please contact the TFP 2016 program chair, David Van Horn. == BEST PAPER AWARDS == To reward excellent contributions, TFP awards a prize for the best paper accepted for the formal proceedings. TFP traditionally pays special attention to research students, acknowledging that students are almost by definition part of new subject trends. A student paper is one for which the authors state that the paper is mainly the work of students, the students are listed as first authors, and a student would present the paper. A prize for the best student paper is awarded each year. In both cases, it is the PC of TFP that awards the prize. In case the best paper happens to be a student paper, that paper will then receive both prizes. == SPONSORS == TFP is financially supported by CyberPoint, Galois, Trail of Bits, and the University of Maryland Computer Science Department. == PAPER SUBMISSIONS == Acceptance of articles for presentation at the symposium is based on a lightweight peer review process of extended abstracts (4 to 10 pages in length) or full papers (20 pages). The submission must clearly indicate which category it belongs to: research, position, project, evaluation, or overview paper. It should also indicate which authors are research students, and whether the main author(s) are students. A draft paper for which ALL authors are students will receive additional feedback by one of the PC members shortly after the symposium has taken place. We use EasyChair for the refereeing process. Papers must be submitted at: https://easychair.org/conferences/?conf=tfp2016 Papers must be written in English, and written using the LNCS style. For more information about formatting please consult the Springer LNCS web site: http://www.springer.com/computer/lncs?SGWID=0-164-6-793341-0 == IMPORTANT DATES == Submission of draft papers: April 8, 2016 Notification: April 15, 2016 Registration: May 13, 2016 TFP Symposium: June 8-10, 2016 Student papers feedback: June 14, 2016 Submission for formal review: July 14, 2016 Notification of acceptance: September 14, 2016 Camera ready paper: October 14, 2016 == PROGRAM COMMITTEE == Amal Ahmed Northeastern University (US) Nada Amin École Polytechnique Fédérale de Lausanne (CH) Kenichi Asai Ochanomizu University (JP) Malgorzata Biernacka University of Wroclaw (PL) Laura Castro University of A Coruña (ES) Ravi Chugh University of Chicago (US) Silvia Ghilezan University of Novi Sad (SR) Clemens Grelck University of Amsterdam (NL) John Hughes Chalmers University of Technology (SE) Suresh Jagannathan Purdue University (US) Pieter Koopman Radboud University Nijmegen (NL) Geoffrey Mainland Drexel University (US) Chris Martens University of California, Santa Cruz (US) Jay McCarthy University of Massachusetts, Lowell (US) Heather Miller École Polytechnique Fédérale de Lausanne (CH) Manuel Serrano INRIA, Sophia-Antipolis (FR) Scott Smith Johns Hopkins University (US) Éric Tanter University of Chile (CL) David Van Horn (Chair) University of Maryland (US) Niki Vazou University of California, San Diego (US) Stephanie Weirich University of Pennsylvania (US)

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Re: [Haskell-cafe] Compile times and separate compilation
by Johannes Waldmann 01 Mar '16

01 Mar '16

> I presume you are using the -j switch. Does it really help? I recently found that for travis builds, it is used by default (first surprise) but it's faster when it's turned off (second surprise). https://ghc.haskell.org/trac/ghc/ticket/10818#comment:15 - J.

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Haskell Summer of Code
by Edward Kmett 01 Mar '16

01 Mar '16

I'm sorry to announce that this year haskell.org was not accepted for the 2016 Google Summer of Code. There has been a lot of turnover over the last 3 years as they have rotated in and out new organizations, including many that have been in the program as long as us, so while this isn't entirely unexpected, it is disheartening. As this comes on the tail of our most successful year in the program, the news was particularly devastating to all involved. We do not expect this to be a permanent condition. Many organizations rotate back in and out of the Summer of Code each year. Operationally, this raises two main concerns: The first is that there will be a rather sharp dip in income for the next year for haskell.org. Last year's GSoC accounted for $9500 worth of income towards managing servers and the like, but we will not receive such a booster shot this year. The second is that we absolutely do not want the infrastructure we have in place around the Summer of Code to fall away. We had 50 mentors register last year! To address both of these concerns, we are exploring running our own self-funded *Haskell Summer of Code *this year*. *In December, we incorporated haskell.org as a 501(c)(3) non-profit. This now enables us to pay for work directly. We should be able to fund at least one slot out of pocket from existing haskell.org funds and fund additional slots with donations. https://wiki.haskell.org/Donate_to_Haskell.org More information will be forthcoming as we work out the details. Please feel free to contact me if you think you can help or if you have any questions or concerns. -Edward Kmett

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Infer Nat type from Integer argument
by Jake 29 Feb '16

29 Feb '16

{-# LANGUAGE DataKinds, KindSignatures, ScopedTypeVariables, GADTs, AllowAmbiguousTypes #-} import GHC.TypeLits import Data.Proxy import Data.Type.Equality data NatString (n :: Nat) = NatString String deriving Show showNS :: KnownNat n => NatString n -> (String, Integer) showNS b@(NatString s) = (s, natVal b) In this example, we use NatString like this: > showNS (NatString "hey" :: NatString 4) -- ("hey", 4) We can then dynamically make NatStrings from Integers. For example, we can read an Integer from stdin and use it to create a NatString main :: IO () main = do i <- readLn case someNatVal i of Just (SomeNat (p :: Proxy n)) -> let ns :: NatString n ns = NatString "hello!" in print $ showNS NatString However, if I have trouble when I try to refactor out the middle part to create a function with type forall n. KnownNat n => Integer -> Bar n. I use sameNat to convince the compiler that the Nat we get from the Integer is the same type as the the type n of the output. fromN :: forall n. KnownNat n => Integer -> NatString n fromN i = case someNatVal i of Just (SomeNat p) -> case sameNat p (Proxy :: Proxy n) of Just (Refl) -> let ns :: NatString n ns = NatString "hello!" in ns This compiles, but if you try and use it the compiler complains. > showNS (fromN 3) -- No instance for (KnownNat n0) arising from a use of ‘showNS’ If we manually add the type annotation, everything is fine, but that kind of defeats the entire purpose. > showNS (fromN 3 :: NatString 3) -- ("hello!",3) Strangely enough, when the NatString is immediately consumed instead of being returned, the compiler does its job and infers the correct type. showFromN :: Integer -> (String, Integer) showFromN i = case someNatVal i of Just (SomeNat (p :: Proxy n)) -> let ns :: NatString n ns = NatString "hello!" in showNS ns > showFromN 3 -- ("hello!", 3) Clearly the intermediate value of ns inside of showFromN has a type NatString 3 because showNS is using that fact. How can I make the compiler infer that fromN 3 :: NatString 3 separately?

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Functional dependencies and overloading of operation
by Kristoffer Føllesdal 29 Feb '16

29 Feb '16

I am trying to use Functional dependencies to overload a operation on Vector space and its basis (Use the Vector spaces module Math.Algebras.VectorSpace <https://hackage.haskell.org/package/HaskellForMaths-0.4.8/docs/Math-Algebra…>). I have tried to mimic the example for matrices and vectors from https://wiki.haskell.org/Functional_dependencies <https://wiki.haskell.org/Functional_dependencies>. I have tried different ways of defining classes and instances, but I do not get it to work. What I want is to have the «same» function for these cases: operation :: a -> a -> Vect k a operation :: a -> Vect k a -> Vect k a operation :: Vect k a -> a -> Vect k a operation :: Vect k a -> Vect k a -> Vect k a Her are som sample code to illustrate what I want. Do anybody have an idea to how to solve it? {-# LANGUAGE MultiParamTypeClasses #-} {-# LANGUAGE FlexibleInstances #-} {-# LANGUAGE FunctionalDependencies #-} import Math.Algebras.VectorSpace linearExtension :: (Eq k, Num k, Ord a) => (a -> a -> Vect k a) -> Vect k a -> Vect k a -> Vect k a linearExtension f xs ys = linear (\x -> linear (f x) ys) xs data Tree t = Root t [Tree t] deriving(Eq, Show, Ord) op :: (Eq k, Num k, Ord t) => Tree t -> Tree t -> Vect k (Tree t) op x y = return x <+> return y opA :: (Eq k, Num k, Ord t) => Vect k (Tree t) -> Vect k (Tree t) -> Vect k (Tree t) opA = linearExtension op class Operation a b c | a b -> c where (<.>) :: a -> b -> c instance (Ord t) => Operation (Tree t) (Tree t) (Vect k (Tree t)) where (<.>)= op instance (Ord t) => Operation (Vect k (Tree t)) (Vect k (Tree t)) (Vect k (Tree t)) where (<.>) = opA instance (Ord t) => Operation (Tree t) (Vect k (Tree t)) (Vect k (Tree t)) where (<.>) x = opA (return x) instance (Ord t) => Operation (Vect k (Tree t)) (Tree t) (Vect k (Tree t)) where (<.>) x y = opA x (return y)

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Re: [Haskell-cafe] Functional dependencies and overloading of operation
by Kristoffer Føllesdal 29 Feb '16

29 Feb '16

Internally in programs I will use the specific functions for each case. But I want some «syntactic sugar» for this functions when doing direct calculations in for example ghci. It will be more user friendly for direct interaction not to have four different functions that in practice do the same thing. Kristoffer > Date: Sun, 28 Feb 2016 21:11:20 +0000 > From: Tom Ellis <tom-lists-haskell-cafe-2013(a)jaguarpaw.co.uk <mailto:tom-lists-haskell-cafe-2013@jaguarpaw.co.uk>> > To: haskell-cafe(a)haskell.org <mailto:haskell-cafe@haskell.org> > Subject: Re: [Haskell-cafe] Functional dependencies and overloading of > operation > Message-ID: <20160228211120.GB23479@weber> > Content-Type: text/plain; charset=iso-8859-1 > > On Sun, Feb 28, 2016 at 10:00:42PM +0100, Kristoffer F?llesdal wrote: >> I am trying to use Functional dependencies to overload a operation on >> Vector space and its basis > [...] >> operation :: a -> a -> Vect k a >> operation :: a -> Vect k a -> Vect k a >> operation :: Vect k a -> a -> Vect k a >> operation :: Vect k a -> Vect k a -> Vect k a > > This is unlikely to be a good idea in practice. Is there a good reason you > can't use four different names for these operations? > > Date: Sun, 28 Feb 2016 22:00:42 +0100 > From: Kristoffer F?llesdal <kfollesdal(a)gmail.com <mailto:kfollesdal@gmail.com>> > To: haskell-cafe(a)haskell.org <mailto:haskell-cafe@haskell.org> > Subject: [Haskell-cafe] Functional dependencies and overloading of > operation > Message-ID: <84F1908D-05B0-4EAE-9CB7-50847BD65344(a)gmail.com <mailto:84F1908D-05B0-4EAE-9CB7-50847BD65344@gmail.com>> > Content-Type: text/plain; charset="utf-8" > > I am trying to use Functional dependencies to overload a operation on Vector space and its basis (Use the Vector spaces module Math.Algebras.VectorSpace <https://hackage.haskell.org/package/HaskellForMaths-0.4.8/docs/Math-Algebra… <https://hackage.haskell.org/package/HaskellForMaths-0.4.8/docs/Math-Algebra…>>). I have tried to mimic the example for matrices and vectors from https://wiki.haskell.org/Functional_dependencies <https://wiki.haskell.org/Functional_dependencies> <https://wiki.haskell.org/Functional_dependencies <https://wiki.haskell.org/Functional_dependencies>>. > I have tried different ways of defining classes and instances, but I do not get it to work. > > What I want is to have the ?same? function for these cases: > > operation :: a -> a -> Vect k a > operation :: a -> Vect k a -> Vect k a > operation :: Vect k a -> a -> Vect k a > operation :: Vect k a -> Vect k a -> Vect k a > > Her are som sample code to illustrate what I want. Do anybody have an idea to how to solve it? > > {-# LANGUAGE MultiParamTypeClasses #-} > {-# LANGUAGE FlexibleInstances #-} > {-# LANGUAGE FunctionalDependencies #-} > > import Math.Algebras.VectorSpace > > linearExtension :: (Eq k, Num k, Ord a) > => (a -> a -> Vect k a) -> Vect k a -> Vect k a -> Vect k a > linearExtension f xs ys = linear (\x -> linear (f x) ys) xs > > data Tree t = Root t [Tree t] deriving(Eq, Show, Ord) > > op :: (Eq k, Num k, Ord t) => Tree t -> Tree t -> Vect k (Tree t) > op x y = return x <+> return y > > opA :: (Eq k, Num k, Ord t) => Vect k (Tree t) -> Vect k (Tree t) -> Vect k (Tree t) > opA = linearExtension op > > class Operation a b c | a b -> c where > (<.>) :: a -> b -> c > > instance (Ord t) => Operation (Tree t) (Tree t) (Vect k (Tree t)) where > (<.>)= op > > instance (Ord t) => Operation (Vect k (Tree t)) (Vect k (Tree t)) (Vect k (Tree t)) where > (<.>) = opA > > instance (Ord t) => Operation (Tree t) (Vect k (Tree t)) (Vect k (Tree t)) where > (<.>) x = opA (return x) > > instance (Ord t) => Operation (Vect k (Tree t)) (Tree t) (Vect k (Tree t)) where > (<.>) x y = opA x (return y) >

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Question: Do block precedence
by Jonne Ransijn 29 Feb '16

29 Feb '16

Dear Haskell-Cafe mailing list people (?) I've been writing parenthesis around do blocks since forever now, but I don't get why they are necessary. I can't seem to come up with a program where they are necessary. Am I missing something or are parenthesis around do blocks nececairy for no reason? Since parsing 'do' blocks as if they have parenthesis around them doesn't seem to break any code, why not do so? when (doBlocksNeedParenthesis) do putStrLn "This code is invalid." when (doBlocksNeedParenthesis) $ do putStrLn "This code is valid." when (doBlocksHaveInvisibleParenthesis) do putStrLn "These are equal v" when (doBlocksHaveInvisibleParenthesis) (do putStrLn "These are equal ^")

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FLTKHS - GHCi help
by aditya siram 28 Feb '16

28 Feb '16

Hi all, I am the author of FLTKHS (http://github.com/deech/fltkhs) which aims to make it easy to install, write and deploy a native GUI application in pure Haskell. It is already able to build static executables on Linux, *BSD and OSX (Yosemite & El Capitan). However a smooth GHCi experience across platforms is still lacking due to some outstanding issues that I am unable to figure it out. I have written up the issues in the documentation [1]. I could use some help specifically with running a REPL with a C++ shared library in GHC 7.10.3. It works fine in 7.8.4. This is documented in the section titiled "GHCi (Linux, *BSD & OSX Yosemite)" in the link below. There is also a mysterious GHCi error on Windows that I can't seem to figure out. It is documented in "GHCi (Windows only)" section. Any help or pointers are appreciated. Thanks! -deech [1] http://hackage.haskell.org/package/fltkhs/docs/Graphics-UI-FLTK-LowLevel-FL…

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Playing with OverloadedLabels in GHC 8 RC2, how to do this?
by Daniel Díaz 28 Feb '16

28 Feb '16

Hi all, I'm playing with the OverloadedLabels extension in GHC 8 RC2. I have been able to define simple record accessors, like in this gist: https://gist.github.com/danidiaz/3b9a6865686c777f328c After realizing than with OverloadedLabels a single symbol can be used to extract two different types from the same record, I tried to define an instance that says: "if a symbol can be used to extract an string from my record, then it can also be used to extract that a Text value". Here's my attempt (using a dummy Text type): {-# LANGUAGE OverloadedLabels #-} {-# LANGUAGE DataKinds #-} {-# LANGUAGE FlexibleInstances #-} {-# LANGUAGE FlexibleContexts #-} {-# LANGUAGE UndecidableInstances #-} {-# LANGUAGE MultiParamTypeClasses #-} {-# LANGUAGE MagicHash #-} module Main where import GHC.OverloadedLabels import GHC.Prim newtype Text = Text { getText :: String } deriving Show data Person = Person { _id :: Int , _name :: String } instance IsLabel "name" (Person -> String) where fromLabel _ = _name instance IsLabel symbol (Person -> String) => IsLabel symbol (Person -> Text) where fromLabel _ = Text . fromLabel (proxy# :: (Proxy# (Person -> String))) person :: Person person = Person 123 "Horace" main :: IO () main = do print (#name person :: String) print (#name person :: Text) Bu this doesn't work. The error I get is puzzling: • Expected kind ‘Proxy# ((->) Person String)’, but ‘proxy# :: Proxy# (Person -> String)’ has kind ‘Proxy# (Person -> String)’ • In the first argument of ‘fromLabel’, namely ‘(proxy# :: Proxy# (Person -> String))’ Is this a bug? What is going on here?

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[ANN] sparkle: native Apache Spark applications in Haskell
by Alp Mestanogullari 28 Feb '16

28 Feb '16

Hello -cafe! Recently at Tweag I/O we've been working on sparkle, a library for writing (distributed) Apache Spark applications directly in Haskell! We have published a blog post introducing the project (and some of its challenges) here: http://www.tweag.io/blog/haskell-meets-large-scale-distributed-analytics The corresponding repository lives at https://github.com/tweag/sparkle While this is still early stage work, we can already write non-trivial Spark applications in Haskell and have them run accross an entire cluster. We obviously do not cover the whole Spark API yet (very, very far from that) but would be glad to already get some feedback. Cheers -- Alp Mestanogullari

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Haskell Symposium 2016 CFP?
by Ivan Perez 26 Feb '16

26 Feb '16

Hi Café Is there going to be a Haskell Symposium this year? I haven't been able to find the call for papers. Best regards Ivan

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ANNOUNCE: Applied Functional Programming (AFP) Summerschool 4-15 July 2016, Utrecht, Netherlands
by Atze Dijkstra 26 Feb '16

26 Feb '16

=========== AFP Summerschool 2016 =========== Applied Functional Programming (AFP) Summerschool July 4-15, 2016 Utrecht University, Department of Information and Computing Sciences Utrecht, The Netherlands Summerschool & registration website: http://www.utrechtsummerschool.nl/courses/science/applied-functional-progra… AFP website : http://www.cs.uu.nl/wiki/USCS contact : Uscs-afp(a)lists.science.uu.nl *** The 2016 edition of the Applied Functional Programming (AFP) Summerschool in Utrecht, Netherlands will be held from 4-15 July 2016. The summerschool teaches Haskell on both beginners and advanced levels via lectures and lab exercises. More info can be found via the references above, included here is a summary from the summerschool info: ``Typed functional programming languages allow for the development of robust, concise programs in a short amount of time. The key advantages are higher-order functions as an abstraction mechanism, and an advanced type system for safety and re usability. This course explores Haskell, a state-of-the-art functional programming language, together with some of its theoretical background, such as typed lambda calculi, referential transparency, Damas-Milner type inference, type level programming, and functional design patterns. We will combine this with applications of functional programming, concentrating on topics such as language processing, building graphical user interfaces, networking, databases, and programming for the web. The goal of the course is not just to teach the programming language and underlying theory, but also to learn about the Haskell community and to get hands-on experience by doing lab exercises or a Haskell project of your own.'' The summerschool is organised and given by the Software Technology group (http://www.uu.nl/en/research/software-systems/software-technology) within the Department of Information and Computing Sciences (http://www.uu.nl/en/organisation/department-of-information-and- computing-sciences), a group which in the departmental research evaluation over the past years was singled out by ``...the functional programming activities are known to be world leading in their domain.'' *** regards, - Atze - Atze Dijkstra, Department of Information and Computing Sciences. /|\ Utrecht University, PO Box 80089, 3508 TB Utrecht, Netherlands. / | \ Tel.: +31-30-2534118/1454 | WWW : http://www.cs.uu.nl/~atze . /--| \ Fax : +31-30-2513971 .... | Email: atze(a)uu.nl ............... / |___\

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Fwd: Re: Overlapping instances
by David Feuer 25 Feb '16

25 Feb '16

Oddly, this message was rejected by the system. Trying again. ---------- Forwarded message ---------- From: "David Feuer" <david.feuer(a)gmail.com> Date: Feb 24, 2016 10:04 PM Subject: Re: [Haskell-cafe] Overlapping instances To: "Mitchell Rosen" <mitchellwrosen(a)gmail.com> Cc: "Haskell-cafe" <haskell-cafe(a)googlegroups.com> It's possible that GHC will backtrack with overlapping enabled (I can't remember, and overlapping instances give me hives) but it doesn't have here. The first instance is "more specific" than the second because the type variable x appears twice. On Feb 24, 2016 9:49 PM, "Mitchell Rosen" <mitchellwrosen(a)gmail.com> wrote: > Consider this "list membership" typeclass and two overlapping instances: > > class Elem (x :: k) (xs :: [k]) > > instance {-# OVERLAPS #-} Elem x (x ': xs) > instance {-# OVERLAPS #-} Elem x xs => Elem x (y ': xs) > > The inductive style is satisfying, but I'm struggling to understand > exactly how GHC can pick one instance over the other. > > How is a constraint like Elem Int [Int] solved? Certainly the Elem x (x > ': xs) instance matches, with x = Int and xs = []. But, the second > instance is also equally valid with x = Int, y = Int, xs = []. Even > though *if *the second instance is chosen, the context cannot be > satisfied (no instance for Elem x []), it's my understanding that GHC > will not backtrack once it picks an instance. And because both instances > look valid to me, I don't understand why this code does not require > IncoherentInstances. > > Thanks. > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe > >

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Overlapping instances
by Mitchell Rosen 25 Feb '16

25 Feb '16

Consider this "list membership" typeclass and two overlapping instances: class Elem (x :: k) (xs :: [k]) instance {-# OVERLAPS #-} Elem x (x ': xs) instance {-# OVERLAPS #-} Elem x xs => Elem x (y ': xs) The inductive style is satisfying, but I'm struggling to understand exactly how GHC can pick one instance over the other. How is a constraint like Elem Int [Int] solved? Certainly the Elem x (x ': xs) instance matches, with x = Int and xs = []. But, the second instance is also equally valid with x = Int, y = Int, xs = []. Even though *if *the second instance is chosen, the context cannot be satisfied (no instance for Elem x []), it's my understanding that GHC will not backtrack once it picks an instance. And because both instances look valid to me, I don't understand why this code does not require IncoherentInstances. Thanks.

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Is there a nice place for extra stuff for Category?
by David Feuer 25 Feb '16

25 Feb '16

1. An explanation of a monoid as a category with exactly one object newtype Cat c a = Cat {getCat :: c a a} instance Category c => Monoid (c a) newtype Mon m a b = Mon {getMon :: m} instance Monoid m => Category (Mon m) --This would be more precisely written -- data Mon m a b where -- Mon :: m -> Mon a a -- but that hurts performance without seeming to offer much in return 2. newtype Op c a b = Op {getOp :: c b a} instance Category c => Category (Op c) instance Bifunctor c => Bifunctor (Op c) instance Bifunctor c => Functor (Op c a) instance Profunctor c => Contravariant (Op c a) I see that Control.Category.Dual in the categories package offers a version of this last defined as *data* for some reason.

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Re: [Haskell-cafe] Manual type-checking in graphs: Avoidable?
by Jeffrey Brown 24 Feb '16

24 Feb '16

It is possible! Off-list, Matt Parsons explained how to do it. With his permission I have copied that email below: On Sat, Feb 20, 2016 at 9:14 PM, Matt <parsonsmatt(a)gmail.com> wrote: > Phantom types can help disambiguate: > > data Hamster > data Person > > data Node a where > HNode :: String -> Node Hamster > PNode :: String -> Node Person > > type G = Gr (Some Node) String > > Your API functions will have the type: > > getPersonHamsters :: Node Person -> [Node Hamster] > getPersonHamsters (PNode personName) = ... > > Since the only way to construct a `Node Person` is with the `PNode` > constructor, you don't even have to check for the `HNode` constructor, and > it's a type error to provide it to the function. > > You'll need to wrap in `Some` to store in the graph and unwrap/pattern > match on the `Some` to get the data back out. This'll provide some > additional safety by constraining surface area of the less-strongly-typed > interface, but it won't take care of it entirely. > > Matt Parsons > > On Sat, Feb 20, 2016 at 9:27 PM, Jeffrey Brown <jeffbrown.the(a)gmail.com> > wrote: > >> Clever! That answers my first question, but still leaves me unmotivated. >> Would GADTs allow me to offload some kind of work onto the compiler that I >> currently have to do myself? >> >> On Sat, Feb 20, 2016 at 2:00 PM, Matt <parsonsmatt(a)gmail.com> wrote: >> >>> The pattern I've seen is: >>> >>> data Some f where >>> Some :: f a -> Some f >>> >>> type G = Gr (Some Box') String >>> >>> Ordinarily you lose the information about the `a`, but when you have a >>> GADT, that allows you to recover type information. So you can match on: >>> >>> f :: Some Box' -> String >>> f (Some (Bi i)) = show (i + 1) >>> f (Some (Bs s)) = s >>> >>> Matt Parsons >>> >>> On Sat, Feb 20, 2016 at 4:16 PM, Jeffrey Brown <jeffbrown.the(a)gmail.com> >>> wrote: >>> >>>> Interesting! I have two questions. >>>> >>>> (1) Given that Graph is of kind * -> * -> *, rather than (* -> *) -> * >>>> -> *, how can I use a GADT? The first graph using existentials defined >>>> earlier in this thread looked like: >>>> >>>> data Box = forall s. Show s => Box s >>>> type ExQuantGraph = Gr Box String >>>> >>>> If instead I use a GADT: >>>> >>>> data Box' a where >>>> Bi :: Int -> Box' Int >>>> Bs :: String -> Box' String >>>> >>>> then I can't define a graph on >>>> >>>> type G = Gr Box' String >>>> >>>> because Box is not a concrete type. I could specify (Box a) for some a, >>>> but then I lose the polymorphism that was the purpose of the GADT. >>>> >>>> (2) Would a GADT be better than what I'm already doing? Currently I >>>> define a Mindmap[1] as a graph where the nodes are a wrapper type called >>>> Expr ("expression"): >>>> >>>> type Mindmap = Gr Expr _ -- the edge type is irrelevant >>>> data Expr = Str String | Fl Float >>>> | Tplt [String] | Rel | Coll >>>> | RelSpecExpr RelVarSpec deriving(Show,Read,Eq,Ord) >>>> >>>> I do a lot of pattern matching on those constructors. If I used a GADT >>>> I would still be pattern matching on constructors. So do GADTs offer some >>>> advantage? >>>> >>>> [1] >>>> https://github.com/JeffreyBenjaminBrown/digraphs-with-text/blob/master/src/… >>>> >>>> On Sat, Feb 20, 2016 at 11:59 AM, Benjamin Edwards < >>>> edwards.benj(a)gmail.com> wrote: >>>> >>>>> if you are willing to have a closed universe, you can pattern match on >>>>> a gadt to do do the unpacking >>>>> >>>>> On Sat, 20 Feb 2016 at 19:19 Jeffrey Brown <jeffbrown.the(a)gmail.com> >>>>> wrote: >>>>> >>>>>> Yes, that is my point. Existentials cannot be unwrapped. >>>>>> >>>>>> On Sat, Feb 20, 2016 at 4:18 AM, Kosyrev Serge < >>>>>> _deepfire(a)feelingofgreen.ru> wrote: >>>>>> >>>>>>> Jeffrey Brown <jeffbrown.the(a)gmail.com> writes: >>>>>>> > After further study I believe existentials are not (at least alone) >>>>>>> > enough to solve the problem. >>>>>>> .. >>>>>>> > getInt :: ShowBox -> Int >>>>>>> > getInt (SB i) = i >>>>>>> > >>>>>>> > will not compile, because it cannot infer that i is an Int: >>>>>>> >>>>>>> You take a value of an existentially quantified type (which means it >>>>>>> can be anything at all, absent some extra context) and *proclaim* it >>>>>>> is an integer. >>>>>>> >>>>>>> On what grounds should the compiler accept your optimistic >>>>>>> restriction? >>>>>>> >>>>>>> -- >>>>>>> с уважениeм / respectfully, >>>>>>> Косырев Сергей >>>>>>> >>>>>> >>>>>> >>>>>> >>>>>> -- >>>>>> Jeffrey Benjamin Brown >>>>>> _______________________________________________ >>>>>> Haskell-Cafe mailing list >>>>>> Haskell-Cafe(a)haskell.org >>>>>> http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe >>>>>> >>>>> >>>> >>>> >>>> -- >>>> Jeffrey Benjamin Brown >>>> >>>> _______________________________________________ >>>> Haskell-Cafe mailing list >>>> Haskell-Cafe(a)haskell.org >>>> http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe >>>> >>>> >>> >> >> >> -- >> Jeffrey Benjamin Brown >> > > -- Jeffrey Benjamin Brown

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Problems with attoparsec (or maybe URI.ByteString)
by Brian Hurt 24 Feb '16

24 Feb '16

What I'm trying to do is write a function with the signature: data Urls = Txt Text.Text | Url Text.Text URI.URI deriving (Show) parseUrls :: Text.Text -> Either String [ Urls ] parseUrls text = ... Given a text block, it finds all the URLs, and breaks things into either URLs, or blocks of text which are not URLs. The full text is attached, for those who are interested. But the problem I'm hitting is using the Attoparsec parser URI.ByteString exports. When I do: *Base.DetectURL AP> AP.parseOnly (URI.uriParser URI.laxURIParserOptions) " http://foo/bar" Right (...) So, that works. But when I add a single space on the end of the string: *Base.DetectURL AP> AP.parseOnly (URI.uriParser URI.laxURIParserOptions) " http://foo/bar " Left "Failed reading: MalformedPath" It fails. Note that this isn't a problem with parseOnly- the real code looks like: parseAllUris :: AP.Parser (Bldr.Builder, [ Urls ]) parseAllUris = msum [ aUri, noUri, finished ] where finished = return (mempty, []) aUri = do (txt, url) <- AP.match $ URI.uriParser URI.laxURIParserOptions (bldr, us) <- msum [ noUri, finished ] return $ (mempty, (Url (E.decodeUtf8 txt) url : prependText bldr us)) noUri = do c <- AP.anyChar (bldr, us) <- parseAllUris return $ ((Bldr.charUtf8 c) `mappend` bldr, us) And this has the problem as well- parsing a URL with anything following it fails, and it doesn't detect any URLs. The parseOnly is just the easy way to demonstrate it. So, my question is, is there some way in attoparsec to tell it to just parse as much as makes sense, and leave the rest? Alternatively, is this a problem with the way URI.ByteString module constructed it's parser, and a different parser could work? Or, worst of all, is this a problem with the way that URIs are defined and no conforming parser will work? Thanks. Brian

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installing the Haskell Platform on Redhat without admin access
by Victor Miller 24 Feb '16

24 Feb '16

I'd like to install the latest version of the Haskell Platform for my own use on a redhat system. I don't have admin access so I can't run "sudo yum" as outlined on the haskell.org download page. How can I do this? Victor

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Hedis 0.7.0 release warning
by Kostiantyn Rybnikov 24 Feb '16

24 Feb '16

Hi! I've released hedis 0.7.0 with one change which might affect some of you, so I thought it makes sense to notify cafe. To fix the issue #23 [0] we decided that it's ok to sacrifice pipelining between runRedis calls, so from now on, there won't be pipelining in that scenario. I want to thank Kirill Zaborsky for the fix in pr #45 [1] Best regards, Kostia [0]: https://github.com/informatikr/hedis/issues/23 [1]: https://github.com/informatikr/hedis/pull/45

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Free and cofree
by David Laing 24 Feb '16

24 Feb '16

Hi all, A little while back I gave a talk at YOW Lambda Jam and wrote some posts on combing free monads and cofree comonads (inspired by some posts by Ed Kmett and Dan Piponi). I shared them on reddit, but recently realised that there might be some folks on this list who might be interested in the material. For those interested people, it's all here: http://dlaing.org/cofun/ The "coming soon" posts will turn up eventually, although not until I'm done preparing my proposal for this year's Lambda Jam :) Cheers, Dave

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