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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 11 Mar '16

11 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 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. >> >> >> _______________________________________________ >> 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 10 Mar '16

10 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 08 Mar '16

08 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 08 Mar '16

08 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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