- Haskell-Cafe - Haskell.org

access programs own documentation
by Corentin Dupont 13 Jan '14

13 Jan '14

Hi guys, I'd like to access, from within my program, the program own haddock documentation. >From the cabal autogen path file, I can access the location of the program's data file, binaries etc. (getBinDir, getLibDir, getDataDir) but not the location of the doc. How to do that? For example on my machine: datadir = "/home/kau/.cabal/share/i386-linux-ghc-7.6.3/Nomyx-0.4.1" The documentation is generated in "/home/kau/.cabal/share/doc/i386-linux-ghc-7.6.3/Nomyx-0.4.1", but it depends on the configuration. Thanks, Corentin

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Haddock - How to write formulas ?
by Peter Caspers 13 Jan '14

13 Jan '14

Hi, I am still very new to Haskell, trying to start my very first project. For its documentation I want to use Haddock and suitable comments in the source code. I notice that (e.g. different from doxygen) there is no direct way of writing formulas, say in TeX style. Looking into some projects on Hackage, formulas there seem to be written in "pseudo-code" more or less like TeX but not following any strict standard. As far as I can see. What would be your recommendations concerning this ? Is there some guideline on how to include formulas ? I understand that there is "literal programming" where you can e.g. write a TeX article with embedded code blocks that can be extracted for the compiler. However, I do not want to follow this path, also the result is a bit different from what is produced in the "traditional" approach, isn't it. Thanks a lot Peter

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Re: [Haskell-cafe] Licenses and dependencies
by Mike Meyer 13 Jan '14

13 Jan '14

On Sat, Jan 11, 2014 at 2:17 PM, Mateusz Kowalczyk <fuuzetsu(a)fuuzetsu.co.uk> wrote: > Hey, > Perhaps you should have CC'd the list as well for this reply. Oops. I've moved it back, since you suggested it. > On 11/01/14 20:04, Mike Meyer wrote: > >> On 11/01/14 18:44, Ben Foppa wrote: > >> Once you distribute the binary, you have probably pulled in and used the > >> licensed libraries (unless replaced) so you're subject to the strongest > >> license used. > > > > Not "strongest", but all. Well, the GPL licenses sort of "stack", > > since they explicitly allow you to use newer (and usually stronger) > > versions of themselves, but other license don't. > > > > The GNU project provides a description of many of the available > > licenses at: http://www.gnu.org/licenses/license-list.html, including > > compatibility notes. > Right, all, but what this usually entices is that the strongest one > prevails. For example, code with BSD3 and GPLv3 sources will have to > be released under GPLv3 as a whole (although each part is still what > it was licensed under individually). I worked under the assumption > that all dependencies are have compatible licenses (and their > dependencies too &c). No, it's not a contest. You *have* to abide by them all. If you're dealing with the GPL, it may look like the "strongest" prevails, but that's a property of the GPL. It specifically requires any derived work be distributed under the GPL, with no added restrictions. So a compatible license will have no extra restrictions, and appear "weaker". A "stronger" license with extra restrictions (like the 4-clause BSD license) will be incompatible with the GPL, so you can't distributed a derived work at all. If you take the GPL out of the mix, and wanted to distributed a work derived from something covered by (for instance) BSD4 and the CPL - well, that would be fine, because I don't think either prevents the restrictions in the other. But the derived work would be covered by both, not just the stronger of the two (whichever that is). Your usual case may well be GPL'ed code. I happen to work with BSD-licensed code bases more often than not. > >> I think you'll be fine just choosing the PublicDomain license option. [...] > >> Things you can't do with such an option: > >> > >> * bundle other people's code which isn't under public domain > > > > This is vague. If your bundle is a single work (like a static binary), > > then it has to obey all the applicable licenses, and probably can't be > > in the public domain. If it's a collection of works, then it can > > include anything you want, so long as each element in the collection > > obeys it's license. > > I meant bundle as in the repository/source tarball/however you publish > your source, _not_ the binary (I explicitly mention binary somewhere else). In that case, you're perfectly free to include sources with other licenses in the tarball. You just need to make sure you follow their license for their entry in the collection. It's well-established that a simple collection of works (a tarball, etc.) is not a derived work. > >> * somehow enforce that only certain libraries will run with your > >> program: the user should be able to replace the BSD3 licensed > >> dependencies with their own if they want to > > Is there any license that enforces that? That seems more like a > > technical issue than a legal one. Though if you're dynamically linking > > the libraries, things get incredibly confusing. > LGPL, does it not? It even outlines what constitutes as a prevention > of other libraries. I believe the LGPL does just the opposite - it *permits* linking with non-free code without creating a derived work. I think I may not be understanding what you're saying, though. I think you're implying that it's possible to force users to use specific libraries. That is indeed the case - you could write a modified version of the GPL that says "You may only redistribute works derived from my project if they use my libraries". You are correct in that you can't do that if you place the work in the public domain. On the other hand, I don't know of any commonly used open source library that does that. <mike

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different behaviours with or without putStrLn
by Corentin Dupont 13 Jan '14

13 Jan '14

Hi guys, I'm experimenting different behaviours with or without a "putStrLn"! :( Basically, with the following code, I want the evaluation to really happen on the "evaluate". I found out that it doesn't: it is evaluated elsewhere (I don't know where). If I put a putStrLn (commented below), the evaluation really happens there. *execCommand :: (TVar MyState) -> StateT MyState IO () -> IO ()execCommand ts sm = do s <- atomically $ readTVar ts s' <- execStateT sm s s'' <- evaluate s' --evaluation should happen here, but it doesn't --putStrLn $ displayMulti $ _multi s'' atomically $ writeTVar ts s''* To give you more context, I have a state that, when evaluated, might not terminate. So I added a watchdog (like in mueval), that will kill the thread in case the evaluation doesn't terminate. That's why I need to be sure of where the evaluation takes place. Thanks! Corentin

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Parse Error ( Parsec )
by mukesh tiwari 12 Jan '14

12 Jan '14

Hello Cafe, I am trying to write a parser for propositional logic[1]. It's working fine for every input except equivalence ( <=> ). *Main> calculator "a=>b" Imp (Lit 'a') (Lit 'b') *Main> calculator "a<=b" Red (Lit 'a') (Lit 'b') *Main> calculator "a<=>b" *** Exception: failed to parse I think, the reason is parser taking equivalence ( <=> ) as reduction ( <= ) and next character is '>' so it is parse error . If I remove both implication and reduction then equivalence is working fine. *Main> calculator "a<=>b" Eqi (Lit 'a') (Lit 'b') Could some please tell me how to solve this problem. I also tried fixity declaration but got this error LogicPraser.hs:12:10: The fixity signature for `<=>' lacks an accompanying binding -Mukesh Tiwari [1] http://logic.stanford.edu/classes/cs157/2010/notes/chap02.html {-# LANGUAGE NoMonomorphismRestriction #-} import Text.Parsec.Token import Text.Parsec.Prim import Text.Parsec.Char import Text.Parsec.Expr import Text.Parsec.Combinator import Text.Parsec.Language import Control.Applicative hiding ( ( <|> ) , many ) import Data.Maybe ( fromJust ) --infixl 9 <=> data LExpr = Lit Char | Not LExpr | And LExpr LExpr | Or LExpr LExpr | Imp LExpr LExpr -- (=>) | Red LExpr LExpr -- ( <= ) | Eqi LExpr LExpr -- ( <=> ) deriving Show exprCal = buildExpressionParser table atom table = [ [ Prefix ( Not <$ string "~" ) ] , [ Infix ( And <$ string "&" ) AssocLeft ] , [ Infix ( Or <$ string "|" ) AssocLeft ] , [ Infix ( Imp <$ string "=>" ) AssocLeft , Infix ( Red <$ string "<=" ) AssocLeft , Infix ( Eqi <$ string "<=>" ) AssocLeft ] ] atom = char '(' *> exprCal <* char ')' <|> ( Lit <$> letter ) calculator :: String -> LExpr calculator expr = case parse exprCal "" expr of Left msg -> error "failed to parse" Right ( val ) -> val ~

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TypeLits and ScopedTypeVariables
by Ben Foppa 12 Jan '14

12 Jan '14

I've been playing with TypeLits recently, trying to create a vector with type-fixed size. I've hit a stumbling block trying to compile this (simplified) code: {-# LANGUAGE DataKinds #-} {-# LANGUAGE KindSignatures #-} import Control.Exception (assert) import GHC.TypeLits data Foo (n :: Nat) a = Foo a size :: SingI n => Foo n a -> Int size = fromInteger . fromSing . sizeSing where sizeSing :: Foo n a -> Sing n sizeSing _ = sing The error is: Could not deduce (SingI Nat n1) arising from a use of `sing' from the context (SingI Nat n) bound by the type signature for size :: SingI Nat n => Foo n a -> Int at /home/ben/test.hs:10:9-33 Possible fix: add an instance declaration for (SingI Nat n1) In the expression: sing In an equation for `sizeSing': sizeSing _ = sing In an equation for `size': size = fromInteger . fromSing . sizeSing where sizeSing :: Foo n a -> Sing n sizeSing _ = sing Can anybody shed light on this? It seems like the kind of thing that ScopedTypeVariables should solve, but it doesn't make a difference.

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Licenses and dependencies
by Ben Foppa 12 Jan '14

12 Jan '14

Many permissive licenses (I'm specifically looking at MIT and BSD3) include a clause that the license file must be included in distributions of the source or the binary. It seems to me that if I have a project on Github, that qualifies as redistribution of the source. My questions are thus: Do I need to include the license files from all my dependencies in my Github repository? Does having them listed as dependencies in my .cabal file "implicitly" include them? Does making a project "PublicDomain" only refer to the code contained in that project, or would it (unintentionally and unlawfully) give distributors of that source code the freedom to not include the BSD3/MIT license files from dependencies? Essentially my goal is to waive all intellectual property rights to most of my Haskell projects, to the extent that, were I to unintentionally sign away my intellectual property, my open-source contributions would be safe - what's the easiest way to do this? Thanks, Ben Foppa

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ANN: xxHash, fast, high quality, non-cryptographic checksums.
by christian＠ponies.io 10 Jan '14

10 Jan '14

I've just finished implementing a haskell version of the xxHash algorithm, which is documented briefly here (mostly in code): https://code.google.com/p/xxhash/ The haskell version is *almost* as fast as C and allows the generation of a better quality checksum than adler32, in less time. Benchmarks are here: http://ponies.io/posts/2013-01-10-xxhash.html Code is here: https://github.com/christian-marie/xxhash Critique welcome!

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Slow mvar when compiled with threaded
by Branimir Maksimovic 10 Jan '14

10 Jan '14

I have test network client, something like apache bench tool. It uses mvars to synchronize and everything is ok when compiled without -threaded. real 0m2.995s user 0m0.601s sys 0m2.391s With -threaded compile option result is following: real 0m18.196s user 0m2.054s sys 0m3.313s Seems that program is sleeping most of the time for some reason. I can't explain behavior as it seems that program is ok. It starts `concurrency` threads which wait on mvar to process next task. Program follows: {-# Language OverloadedStrings #-} import System.CPUTime import System.IO --import System.IO.Error import Network.Socket hiding(recv) import Network.Socket.ByteString import System.Environment import Control.Concurrent import Control.Exception main = do n <- getArgs let (host,port,conc,reqs) = parse n putStrLn $ "Connecting to " ++ host ++ " " ++ port s <- getAddrInfo Nothing (Just host) (Just port) let servAddr = head s begin <- getCPUTime process servAddr conc reqs end <- getCPUTime let diff = (fromIntegral (end - begin))/(10^12) :: Double putStrLn $ show (round (fromIntegral reqs / diff)) ++ " r/s" parse [h,p,conc,reqs] = (h,p,read conc::Int,read reqs::Int) parse _ = error "usage client host port concurrency requests" process servAddr conc reqs = do let niter = if reqs >= conc then conc else reqs putStrLn $ "loop " ++ show niter mvars <- initThreads niter [] putStrLn $ "Initialized " ++ show niter let loop n (m:mvs) f | n>0 = do flag <- isEmptyMVar m if f > length mvars then putStrLn "busy" else return () if flag || f > length mvars then do putMVar m () loop (n-1) mvs 0 else loop n mvs (f+1) | otherwise = return () loop n [] f = if n>0 then loop n mvars f else return () putStrLn $ "length " ++ show (length mvars) loop (reqs-niter) mvars 0 where initThreads niter vars | niter > 0 = do mvar <- newMVar () forkIO $ process mvar initThreads (niter-1) (mvar:vars) | otherwise = return vars process mvar = do sock <- socket (addrFamily servAddr) Stream defaultProtocol connect sock (addrAddress servAddr) sendAll sock "Hello World!\n" buf <- recv sock 1024 close sock takeMVar mvar process mvar

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Re: [Haskell-cafe] Why Maybe exists if there is Either?
by Niklas Larsson 09 Jan '14

09 Jan '14

Maybe doesn't have to be used for error reporting. Optional values is a very big use case for it. ----- Ursprungligt meddelande ----- Från: "Vlatko Basic" <vlatko.basic(a)gmail.com> Skickat: ‎2014-‎01-‎09 16:37 Till: "haskell-cafe(a)haskell.org" <haskell-cafe(a)haskell.org> Ämne: Re: [Haskell-cafe] Why Maybe exists if there is Either? Hi Daniel, > If you just want to signalize a fail case without any additional > information, then a Maybe fits better than an Either, because > why should you need this bogus empty string? Now it looks to me that it might be better and more consistent to write an empty bogus string (as with: nothing = Left "") than to have two distinct ways of error reporting, and both are used widely and often should be intermixed. And at the end, call site could decide does it want to use the string or not. vlatko -------- Original Message -------- Subject: Re: [Haskell-cafe] Why Maybe exists if there is Either? From: Daniel Trstenjak <daniel.trstenjak(a)gmail.com> To: haskell-cafe(a)haskell.org Date: 09.01.2014 16:13 > > Hi Vlatko, > > On Thu, Jan 09, 2014 at 03:50:16PM +0100, Vlatko Basic wrote: >> I'm curious to find out what was the reasoning to make Maybe? >> What is the added value with introducing it? >> In which situations the above substitution does not hold? > > If you just want to signalize a fail case without any additional > information, then a Maybe fits better than an Either, because > why should you need this bogus empty string? > > How should you know that the string doesn't contain something relevant? > > > Greetings, > Daniel > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe(a)haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

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How to determine the right path to haddock html documentation?
by Carlo Hamalainen 09 Jan '14

09 Jan '14

Hi, I'm writing a patch for ghcmod-vim so that a user can look up the haddock html documentation for a symbol. My code makes three system calls, e.g. to look up "Just" it does: $ ghc-mod info foo.hs Foo Just data Maybe a = ... | Just a -- Defined in `Data.Maybe' $ ghc-pkg find-module Data.Maybe --simple-output haskell2010-1.1.1.0 base-4.6.0.1 $ ghc-pkg field base-4.6.0.1 haddock-html haddock-html: /home/carlo/opt/ghc-7.6.3_build/share/doc/ghc/html/libraries/base-4.6.0.1 Then "Data.Maybe" is changed to Data-Maybe.html" and appended to the haddock-html value. But this procedure doesn't work in general, for example when looking up String: $ ghc-mod info foo.hs Foo String type String = [Char] -- Defined in `GHC.Base' $ ghc-pkg find-module GHC.Base /home/carlo/opt/ghc-7.6.3_build/lib/ghc-7.6.3/package.conf.d base-4.6.0.1 /home/carlo/.ghc/x86_64-linux-7.6.3/package.conf.d $ ghc-pkg field base-4.6.0.1 haddock-html haddock-html: /home/carlo/opt/ghc-7.6.3_build/share/doc/ghc/html/libraries/base-4.6.0.1 $ ls /home/carlo/opt/ghc-7.6.3_build/share/doc/ghc/html/libraries/base-4.6.0.1/GHC-Base.html ls: cannot access /home/carlo/opt/ghc-7.6.3_build/share/doc/ghc/html/libraries/base-4.6.0.1/GHC-Base.html: No such file or directory because the html is in the src subdirectory: $ ls /home/carlo/opt/ghc-7.6.3_build/share/doc/ghc/html/libraries/base-4.6.0.1/src/GHC-Base.html /home/carlo/opt/ghc-7.6.3_build/share/doc/ghc/html/libraries/base-4.6.0.1/src/GHC-Base.html But this isn't even the right place to go, one should look at the Prelude file http://hackage.haskell.org/package/base-4.6.0.1/docs/Prelude.html#t:String which is what Hoogle suggests. How does Hoogle get it right? It looks like haskell-mode for emacs runs into a similar problem: https://github.com/haskell/haskell-mode/blob/master/inf-haskell.el#L734-L740 ------------------------------- (defvar inferior-haskell-ghc-internal-ident-alist ;; FIXME: Fill this table, ideally semi-automatically. '(("GHC.Base.return" . "Control.Monad.return") ("GHC.List" . "Data.List"))) (defun inferior-haskell-map-internal-ghc-ident (ident) "Try to translate some internal GHC identifier to its alter ego in haskell docs." ------------------------------- Thanks, -- Carlo Hamalainen http://carlo-hamalainen.net

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Altering Class hierarchy
by Frantisek Farka 09 Jan '14

09 Jan '14

Hi all, I was toying with Haskell classes and came across Functor-Applicative-Monad proposal[1]. It shows out that adding a super class (Applicative) to a class (Monad) is not an easy task when the class hierarchy is already in use. In this case it result in stepwise approach (step1 e. g. [2]) in order not to break already existing code which uses Monad. I am interested in other examples of such changes. Which alterings of your class hierarchy do you usually need when rewriting your code? I don't mean only adding superclass here, other modifications could be also handy, e. g. dropping out a class or splitting a class into two. Can you point me to some actual examples on real source code? How do you deal with this kind of problem on your projects? Thanks for a reply :) FF [1] http://www.haskell.org/haskellwiki/Functor-Applicative-Monad_Proposal [2] https://ghc.haskell.org/trac/ghc/ticket/8004

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ANN: MiniAgda-0.2014.1.9 Toy language with dependent and sized types
by Andreas Abel 09 Jan '14

09 Jan '14

For what it's worth, I finally uploaded MiniAgda to hackage. If you want to play around with type-based termination and coinduction in a dependently-typed setting, try: cabal install MiniAgda Some examples and pointers to literature are (still) on my old homepage: http://www2.tcs.ifi.lmu.de/~abel/miniagda/ Source code and issue tracker are on the darcs hub: http://hub.darcs.net/abel/miniagda/ Have fun playing (or despair of the horrible error messages), and have a happy new year, Andreas -- Andreas Abel <>< Du bist der geliebte Mensch.

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Drop or update the upper bounds of numerals and numerals-base
by Nikita Karetnikov 09 Jan '14

09 Jan '14

I’ve built both packages locally, and they seem to work fine with GHC 7.6.3. However, I can’t install them from Hackage due to the required dependencies. Could anyone drop or update the upper bounds? Alternatively, I could try to do so myself if someone provides the required permissions.

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Class Instance with ExistentialQuantification
by Andrew Gibiansky 08 Jan '14

08 Jan '14

Why is the following not allowed? {-# LANGUAGE ExistentialQuantification, ExplicitForAll, RankNTypes, FlexibleInstances #-} class Class a where test :: a -> Bool instance Class (forall m. m -> m) where test _ = True main = do putStrLn $ test id Is there a reason that this is forbidden? Just curious. -- Andrew

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ANN: cheapskate 0.1, markdown parser
by John MacFarlane 08 Jan '14

08 Jan '14

I've released a new markdown library on Hackage: http://hackage.haskell.org/package/cheapskate This library is designed to be used in web applications. It is small, accurate, and fast, in pure Haskell with few dependencies. All output is sanitized through a whitelist by default. It is designed to have performance that varies linearly with the input size, even with garbage input. To illustrate: % head -c 100000 /dev/urandom | iconv -f latin1 -t utf-8 | time cheapskate >/dev/null cheapskate > /dev/null 0.15s user 0.01s system 82% cpu 0.199 total % head -c 1000000 /dev/urandom | iconv -f latin1 -t utf-8 | time cheapskate >/dev/null cheapskate > /dev/null 1.53s user 0.03s system 88% cpu 1.770 total % head -c 10000000 /dev/urandom | iconv -f latin1 -t utf-8 | time cheapskate >/dev/null cheapskate > /dev/null 15.50s user 0.20s system 89% cpu 17.632 total This is a test that many markdown parsers fail (including my own pandoc and the markdown package on Hackage). Performance is about seven times faster than pandoc (with five times less memory used), and about 25% faster than the markdown package on Hackage. Generic functions are provided that allow transformations of the AST prior to rendering (e.g., promotion of headers, insertion of syntax highlighting, or the conversion of specially marked code blocks into diagrams). Several markdown extensions are supported, and sensible decisions have been made about several aspects of markdown syntax that are left vague by John Gruber's specification. For details, see the README at https://github.com/jgm/cheapskate.

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Right approach to profiling and optimizing a concurrent data structure?
by Brandon Simmons 08 Jan '14

08 Jan '14

Happy New Year, all, I started what I thought would be a pretty straightforward project to implement a concurrent queue (with semantics like Chan) which I hoped would be faster, but the process of trying to measure and test performance has been super frustrating. I started with a really big criterion benchmark suite that ran through a bunch of Chan-like implementations as well as comparing different var primitives; I was compiling that with `-O2 -threaded` and running with +RTS -N (as that seemed realistic, and results were very consistent). Short version: at some point I realized I had (in my cabal config) enabled executable-profiling, which when disabled completely changed all timing and actually *hurt* performance. Then after a lot more head-banging I realized that +RTS -N seems to run on only one core when compiled with -prof (I didn't see that documented anywhere) although I could *force* the -prof version to use more with -N2, and so apparently for my tests[1], running on a single core just *happened* to be faster (I can see why it might; I probably can't expect a speedup when I'm just measuring throughput). I'd be interested in any comments on above, but mostly I'm trying to understand what my approach should be at this point; should I be benchmarking on 1 core and trying to maximize throughput? Should I also profile on just 1 core? How should I benchmark the effects of lots of contention and interpret the results? How can I avoid benchmarking arbitrary decisions of the thread scheduler, while still having my benchmarks be realistic? Are there any RTS flags or compile-time settings that I should *definitely* have on? Thanks for any clarity on this, Brandon http://brandon.si [1] Here's the test I used while most of the forehead-bloodying occurred, here using `Control.Concurrent.Chan`; for no combination of readers/writers/messages could I manage to get this going as fast on 2 cores as on the single-core bound -prof version runC :: Int -> Int -> Int -> IO () runC writers readers n = do let nNice = n - rem n (lcm writers readers) perReader = nNice `quot` readers perWriter = (nNice `quot` writers) vs <- replicateM readers newEmptyMVar c <- C.newChan let doRead = replicateM_ perReader $ theRead theRead = C.readChan c doWrite = replicateM_ perWriter $ theWrite theWrite = C.writeChan c (1 :: Int) mapM_ (\v-> forkIO (doRead >> putMVar v ())) vs replicateM writers $ forkIO $ doWrite mapM_ takeMVar vs -- await readers

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ICFP 2014: Call for papers
by David Van Horn 08 Jan '14

08 Jan '14

===================================================================== 19th ACM SIGPLAN International Conference on Functional Programming ICFP 2014 Gothenburg, Sweden, 1-3 September 2014 http://www.icfpconference.org/icfp2014 ===================================================================== Important Dates ~~~~~~~~~~~~~~~ Submissions due: Saturday, 1 March 2014, 23:59 UTC-11 (anywhere in the world) Author response: Wednesday, 23 April, 2014 Friday, 25 April, 2014 Notification: Monday, 5 May, 2014 Final copy due: Wednesday, 11 June, 2014 Scope ~~~~~ ICFP 2014 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 and distribution; modules; components and composition; metaprogramming; interoperability; type systems; relations to imperative, object-oriented, or logic programming * Implementation: abstract machines; virtual machines; interpretation; compilation; compile-time and run-time optimization; 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 and Domain-Specific Languages: 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 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 Saturday, 1 March 2014, 23:59 UTC-11 (anywhere in the world), submit a full paper of at most 12 pages (6 pages for an Experience Report), including bibliography and figures. The deadlines will be strictly enforced and papers exceeding the page limits will be summarily rejected. * Authors have the option to attach supplementary material to a submission, on the understanding that reviewers may choose not to look at it. * 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 on the conference web site. Proceedings will be published by ACM Press. Authors of accepted submissions are expected to transfer the copyright to the ACM. 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 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 ACM 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: http://www.acm.org/sigs/sigplan/authorInformation.htm Submission: Submissions will be accepted on the web at https://www.easychair.org/conferences/?conf=icfp2014 . 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 0:00 UTC-11 on Wednesday, 23 April 2014, 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 fee downloads from the ACM DL. 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1 0

happstack-fastcgi maintenance
by Thomas Bereknyei 08 Jan '14

08 Jan '14

I've patched happstack-fastcgi to work with the latest versions of ghc and happstack. The maintainers have expressed that they consider it unmaintained. I would like to gauge the interest in the library and perhaps maintain it. Let me know if this is of interest to anyone. Have you used it in the past? What features would you need added? Any changes? -Tom

2 1

relocation R_X86_64_PC32 against undefined symbol?
by Magnus Therning 07 Jan '14

07 Jan '14

I get the following error when trying to build hsimports: [7 of 7] Compiling HsImport ( src/HsImport.hs, dist/build/HsImport.dyn_o ) /usr/bin/ld: dist/build/HsImport/Args.dyn_o: relocation R_X86_64_PC32 against undefined symbol `hsimportzm0zi2zi6zi3_Pathszuhsimport_version1_closure' can not be used when making a shared object; recompile with -fPIC /usr/bin/ld: final link failed: Bad value collect2: error: ld returned 1 exit status What could be causing this? /M -- Magnus Therning OpenPGP: 0xAB4DFBA4 email: magnus(a)therning.org jabber: magnus(a)therning.org twitter: magthe http://therning.org/magnus Do not meddle in the affairs of Wizards, for they are subtle and quick to anger. -- J.R.R Tolkien

2 7

"Segmentation fault/access violation" when working with FFI
by Leza Morais Lutonda 07 Jan '14

07 Jan '14

Hi, I has trying to use the portaudio [1] package to play some sound, and running the examples [2] and it results in the "Segmentation fault/access violation" error. Any idea? Thanks. [1] http://hackage.haskell.org/package/portaudio [2] https://github.com/sw17ch/portaudio/blob/master/examples/Example1.h

2 2

Associated types, kind constraints & typelits
by Nicolas Trangez 07 Jan '14

07 Jan '14

All, While toying with typelits I wanted to get the following to work, but failed to. Is it intended not to work at all, should I change something to get it to work, or is this something which needs some GHC support? Note I obviously tried to add the constraint mentioned in the compiler error, but failed to: I seem to add too many type arguments to SingI, which somewhat puzzles me. Thanks, Nicolas {-# LANGUAGE TypeFamilies, DataKinds #-} module Main where import GHC.TypeLits class C a where type T a :: Nat data D = D instance C D where type T D = 10 {- This is not allowed, as intended: data E = E instance C E where type T E = Int -} -- This works: tOfD :: D -> Integer tOfD D = fromSing $ (sing :: Sing (T D)) {- This doesn't work: - Could not deduce (SingI Nat (T a1)) arising from a use of `sing' - from the context (C a) tOf :: C a => a -> Integer tOf _ = fromSing $ (sing :: Sing (T a)) -} main :: IO () main = return ()

3 4

Please fix documentation for you Hackage packages!
by Mateusz Kowalczyk 07 Jan '14

07 Jan '14

Greetings café, As some of you might have noticed recently, there seems to be quite a few packages with broken documentation on Hackage recently. If you are an owner of such package, please consider fixing it. There's a thread on cabal-devel about this if you're interested in details. Here's a list of packages uploaded since beginning of 2013 for which the documentation was broken as of yesterday: http://fuuzetsu.co.uk/misc/sorted.txt If your package is on that list, your documentation is broken. Only the most recent versions of packages were being considered. I outline how to fix your documentation (in most cases this means uploading it by hand) in a blog post I just published. Please refer to: http://fuuzetsu.co.uk/blog/posts/2014-01-06-Fix-your-Hackage-documentation.… The post contains a link to as script which naively attempts to automate the burden of uploading the docs manually. If your package can't be built simply with ‘cabal configure && cabal build && cabal haddock --hyperlink-source’, you'll have to adapt it to your situation. Thanks. I hope to see fewer packages with broken documentation in the future. -- Mateusz K.

11 21

Re: [Haskell-cafe] preprocessor, how to?
by Erik Hesselink 07 Jan '14

07 Jan '14

I don't quite understand why you can't just use pathSeparator (or </>) here. The code in the quotes is still Haskell, and it's going to compile on the same platform it's going to run on, right? Erik On Mon, Jan 6, 2014 at 9:13 AM, Kees Bleijenberg <k.bleijenberg(a)lijbrandt.nl> wrote: > In a program I use a lot of hamlet files. All these hamlet files are in a > submap of the source map. In code: > allHtml = Mconcat [ $(hamletFile "hamlet\\belastingen.hamlet") renderUrl > ,$(hamletFile "hamlet\\winddruk.hamlet") renderUrl > .. > ,$(hamletFile "hamlet\\berekeningDruk.hamlet") > renderUrl > ,$(hamletFile "hamlet\\drukNaarBuigTrek.hamlet") > renderUrl] > This is a Windows program. > > For another project I want to use this piece of code on Linux. The code > should now be: > .. $(hamletFile "hamlet/winddruk.hamlet") renderUrl > > I can't use pathSeparator here, because this is all happening at compile > time. So I think I need a preprocessor. > I would like to have a kind of hamletDir macro and change the code to > something like: > > #ifdef Win32 > hamletDir = "hamlet\\" > #else > hamletDir = "hamlet/" > #endif > ... > $(hamletFile "{hamletDir}winddruk.hamlet") renderUrl > Is it possible in Haskell and how do I it? > > All examples of preprocessing in Haskell I found are something like this: > #ifdef xxx > aaaa > #else > Bbbbb > #endif > I could use that. But a user defined macro is less code. > > Kees > > > > > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe

1 0

Infelicity in StdGen?
by Rob Arthan 06 Jan '14

06 Jan '14

Either I am misunderstanding something or there is an infelicity in the implementation of StdGen. The documentation on mkStdGen says that distinct arguments should be likely to produce distinct generators. This made me think that I would get a reasonable pseudo-random function to simulate n rolls of a die by using n to seed the random number generator: import System.Random roll :: Int -> String roll n = take n . randomRs ('1', '6') . mkStdGen $ n However, this produces a string beginning with a '6' for 0 <= n <= 53667. In fact the dependency of the first value on the seed seems to be far from random: map (\l -> (head l, length l)) . group . map (fst . randomR (1, 6) . mkStdGen) $ [0..25*53668+6] returns: [(6,53668),(5,53668),(4,53668),(3,53669),(2,53668),(1,53668),(6,53669),(5,53668),(4,53668),(3,53669),(2,53668),(1,53668),(6,53668),(5,53669),(4,53668),(3,53668),(2,53669),(1,53668),(6,53668),(5,53669),(4,53668),(3,53668),(2,53669),(1,53668),(6,53668)] The behaviour seems to be related to the length of the range. You get similar behaviour for ranges of length 2, 3, 6 and 9 for example, but not for 4, 5, 7 or 8. If it is relevant, I am using ghc version 7.6.3. Regards, Rob.

3 4

Re: [Haskell-cafe] Please fix documentation for you Hackage packages!
by timmy tofu 06 Jan '14

06 Jan '14

With authors (no emails), quick and dirty: http://lpaste.net/raw/98035 Date: Sun, 5 Jan 2014 20:47:27 -0500 > From: Clark Gaebel <cgaebel(a)uwaterloo.ca> > To: Mateusz Kowalczyk <fuuzetsu(a)fuuzetsu.co.uk> > Cc: haskell-cafe <haskell-cafe(a)haskell.org> > Subject: Re: [Haskell-cafe] Please fix documentation for you Hackage > packages! > Message-ID: > <CA+aES6NVWY_EJKzpWSTap1b3BTEWVzh2=+ > qTchiCMeLgtjeFbw(a)mail.gmail.com> > Content-Type: text/plain; charset="utf-8" > > Is there any way you could tag that list with the author so that we can > Ctrl+f ourselves? > > > On Sun, Jan 5, 2014 at 8:37 PM, Mateusz Kowalczyk > <fuuzetsu(a)fuuzetsu.co.uk>wrote: > > > Greetings caf?, > > > > As some of you might have noticed recently, there seems to be quite a > > few packages with broken documentation on Hackage recently. If you are > > an owner of such package, please consider fixing it. There's a thread on > > cabal-devel about this if you're interested in details. > > > > Here's a list of packages uploaded since beginning of 2013 for which the > > documentation was broken as of yesterday: > > http://fuuzetsu.co.uk/misc/sorted.txt > > > > If your package is on that list, your documentation is broken. Only the > > most recent versions of packages were being considered. > > > > I outline how to fix your documentation (in most cases this means > > uploading it by hand) in a blog post I just published. Please refer to: > > > > > http://fuuzetsu.co.uk/blog/posts/2014-01-06-Fix-your-Hackage-documentation.… > > > > The post contains a link to as script which naively attempts to automate > > the burden of uploading the docs manually. If your package can't be > > built simply with ?cabal configure && cabal build && cabal haddock > > --hyperlink-source?, you'll have to adapt it to your situation. > > > > Thanks. I hope to see fewer packages with broken documentation in the > > future. > > -- > > Mateusz K. > > _______________________________________________ > > Haskell-Cafe mailing list > > Haskell-Cafe(a)haskell.org > > http://www.haskell.org/mailman/listinfo/haskell-cafe > > > > > > -- > Clark. > > Key ID : 0x78099922 > Fingerprint: B292 493C 51AE F3AB D016 DD04 E5E3 C36F 5534 F907 >

1 0

Cloud Haskell, spawn, link and spawnLink
by Alexander Alexeev 06 Jan '14

06 Jan '14

Hi. Let's consider the following code: pid <- spawnLocal ... link pid Is it true, that a parent process will always be terminated when child process terminates, even if child process will be terminated between calls of spawnLocal and link? I wrote a test program and it seems to be so. But I wanted to be sure, that this is not some coincidence, but a real Cloud Haskell semantics. Is the same true for monitors? -- Best regards, Alexander Alexeev http://eax.me/

1 1

Re: [Haskell-cafe] preprocessor, how to?
by Chris Wong 06 Jan '14

06 Jan '14

You can use forward slashes in Windows as well. On Jan 6, 2014 9:13 PM, "Kees Bleijenberg" <k.bleijenberg(a)lijbrandt.nl> wrote: > In a program I use a lot of hamlet files. All these hamlet files are in a > submap of the source map. In code: > allHtml = Mconcat [ $(hamletFile "hamlet\\belastingen.hamlet") renderUrl > ,$(hamletFile "hamlet\\winddruk.hamlet") renderUrl > .. > ,$(hamletFile "hamlet\\berekeningDruk.hamlet") > renderUrl > ,$(hamletFile "hamlet\\drukNaarBuigTrek.hamlet") > renderUrl] > This is a Windows program. > > For another project I want to use this piece of code on Linux. The code > should now be: > .. $(hamletFile "hamlet/winddruk.hamlet") renderUrl > > I can't use pathSeparator here, because this is all happening at compile > time. So I think I need a preprocessor. > I would like to have a kind of hamletDir macro and change the code to > something like: > > #ifdef Win32 > hamletDir = "hamlet\\" > #else > hamletDir = "hamlet/" > #endif > ... > $(hamletFile "{hamletDir}winddruk.hamlet") renderUrl > Is it possible in Haskell and how do I it? > > All examples of preprocessing in Haskell I found are something like this: > #ifdef xxx > aaaa > #else > Bbbbb > #endif > I could use that. But a user defined macro is less code. > > Kees > > > > > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe >

2 1

Turning off warning in ghc-mod (in vim)
by Magnus Therning 06 Jan '14

06 Jan '14

On the command line I can do the following: % ghc-mod check -g -isrc -g dist/build/autogen src/Main.hs | grep Top-level Binary file (standard input) matches % ghc-mod check -g -isrc -g dist/build/autogen -g -fno-warn-missing-signatures src/Main.hs | grep Top-level % That is, there is no output on the second call, just as expected. However, I can't work out out to turn off the warning in Vim. AFAIU the following should work: g:ghcmod_ghc_options = ['-isrc', '-idist/build/autogen', '-fno-warn-missing-signatures'] but it doesn't. I'm obviously missing something, but what? /M -- Magnus Therning OpenPGP: 0xAB4DFBA4 email: magnus(a)therning.org jabber: magnus(a)therning.org twitter: magthe http://therning.org/magnus Beauty is more important in computing than anywhere else in technology because software is so complicated. Beauty is the ultimate defence against complexity. -- David Gelernter

1 1

preprocessor, how to?
by Kees Bleijenberg 06 Jan '14

06 Jan '14

In a program I use a lot of hamlet files. All these hamlet files are in a submap of the source map. In code: allHtml = Mconcat [ $(hamletFile "hamlet\\belastingen.hamlet") renderUrl ,$(hamletFile "hamlet\\winddruk.hamlet") renderUrl .. ,$(hamletFile "hamlet\\berekeningDruk.hamlet") renderUrl ,$(hamletFile "hamlet\\drukNaarBuigTrek.hamlet") renderUrl] This is a Windows program. For another project I want to use this piece of code on Linux. The code should now be: .. $(hamletFile "hamlet/winddruk.hamlet") renderUrl I can't use pathSeparator here, because this is all happening at compile time. So I think I need a preprocessor. I would like to have a kind of hamletDir macro and change the code to something like: #ifdef Win32 hamletDir = "hamlet\\" #else hamletDir = "hamlet/" #endif ... $(hamletFile "{hamletDir}winddruk.hamlet") renderUrl Is it possible in Haskell and how do I it? All examples of preprocessing in Haskell I found are something like this: #ifdef xxx aaaa #else Bbbbb #endif I could use that. But a user defined macro is less code. Kees

1 0

hackage build failure (?) - but no log
by Rogan Creswick 06 Jan '14

06 Jan '14

Mateusz Kowalczyk's recent message about fixing hackage documentation made me realize that one of my packages (chatter) has no build log, and indeed, the documentation was not showing up on hackage. Is there any way to figure out what is causing the failure? Chatter has no non-Haskell dependencies, and it builds just fine in a sandbox on the major three architectures (including building documentation on at least two of them - I haven't tested haddock on Windows). Manually posting the documentation is more than a little inconvenient at the moment, and given the nature of the library, I'm pretty surprised this is an issue. (I did post the docs for chatter-0.0.0.3 manually, if you're wondering why they appear despite my complaints...) Does anyone have suggestions? Thanks! Rogan

1 0

Intercept stdin in Haskell
by Andrew Gibiansky 06 Jan '14

06 Jan '14

Hello all, I need to do something strange and terrible in Haskell: intercept `stdin`. In other words, I need to detect (in another thread, probably?) when my running program is trying to read from `stdin`, and then feed it some data. I know I can use `hDupTo` and other similar things to replace the stdin handle with my own handle, and I know I could probably use `createPipe` or similar from the `unix` package in order to write things to these handles, but I have no idea how I might go about detecting that a handle is being read from. Any ideas? I've racked my brain and cannot come up with a way to do this. I am using the GHC API elsewhere if that leads to any sort of crazy hackery that might save the day. -- Andrew

6 14

[ANN] vim-hsimport - automatically create Haskell import statements
by Daniel Trstenjak 05 Jan '14

05 Jan '14

Hi all, vim-hsimport is a Vim plugin that automatically creates import statements for Haskell source files for the symbol/identifier under the cursor. vim-hsimport can create import statements for the whole module, for only the desired symbol/indentifier and is also able to create qualified module import statements. By using hdevtools in conjunction with a cabal sandbox, dynamically only the modules of packages are considered, which your project depends on. vim-hsimport does also consider the modules of your current project. If the symbol/identifier is contained in multiple modules, then a selection dialog is shown. In conjunction to the hsimport command the Vim plugin also uses the command hdevtools and the Vim plugin vim-hdevtools. Currently you need to use forks of hdevtools and vim-hdevtools to get a working version of vim-hsimport, please see the installation section for details. Currently the biggest issue is if and how modules of your current project are considered for the import, please see the issues section. https://github.com/dan-t/vim-hsimport Greetings, Daniel

4 6

(automatically) adding import statements in vim
by cdep.illabout＠gmail.com 05 Jan '14

05 Jan '14

Are there any tools or vim-plugins that will automatically add import statements? For instance, I have the following code: ... import Data.Maybe (fromJust) import Text.ParserCombinators.Parsec (ParseError) import Text.Printf (printf) ... parseMyString :: String -> Either ParseError String parseMyString stringToParse = parse myParser "(unknown)" stringToParse Here's what I want to have happen. I put my cursor over "parse" and press some keystroke. It automatically adds the parse function to the list of import statements from the Text.ParserCombinators.Parsec module (hopefully sorted alphabetically). So the import statements end up looking like this: ... import Data.Maybe (fromJust) import Text.ParserCombinators.Parsec (ParseError, parse) import Text.Printf (printf) ... The haskellmode vim plugin (https://github.com/lukerandall/haskellmode-vim) has functionality similar to this, but it can only add new import lines. It can't add functions/types to existing import lines. The best it can do is something like this: ... import Data.Maybe (fromJust) import Text.ParserCombinators.Parsec (ParseError) import Text.Printf (printf) import Text.ParserCombinators.Parsec (parse) ... This is alright, but I have to go back and edit it by hand to make it look like the above. Are there any tools or vim-plugins that can do this automatically?

5 6

An Invitation to Gordian Knot, a mathematics and computing problem solving contest
by Threads 04 Jan '14

04 Jan '14

Hello everyone! Felicity, the annual festival of IIIT Hyderabad, brings to you Felicity Threads 2014, the tenth annual edition of the celebration of spirit of computing and engineering. We bring to you a wide range of online contests in various fields of programming and mathematics. Our series of online events includes contests on algorithmic programming (Codecraft), parallel programming (Kernel Cruise), combinatorial search and game playing bot design (Strange Loop), and a CTF-style security contest (Break In). Our first event, Gordian Knot, a Project Euler-inspired contest full of mathematical problems, starts TODAY at 1600 Hrs (IST), and will run for atleast a day. Being a mathematics and computing contest, Gordian Knot challenges you to come up with solutions to problems and untangle complex yet elegant knots, using any methods and computational tools at your disposal. http://www.felicity.iiit.ac.in/threads/gordian-knot/ [1] The only prerequisite to participate is the urge to learn. Let us learn and rejoice the spirit of computing and engineering together. There are exciting prizes for the event too! So let the mathematician inside you come out and to know more about Threads, visit us at : http://felicity.iiit.ac.in/threads/ [2] Sincerely, Threads 2014 Team Links: ------ [1] http://www.felicity.iiit.ac.in/threads/gordian-knot/ [2] http://felicity.iiit.ac.in/threads/

1 0

Designing somewhat-type-safe RPC
by Nicolas Trangez 03 Jan '14

03 Jan '14

Hi, While working on the design of an RPC library (for an existing protocol), I got somewhat stuck. The system is fairly simple: for some call, a client first sends an identifier of the call, followed by a serialized form of the argument. Then the server returns some serialized result. A server exposes several procedures, all taking a certain argument type and returning a certain result type. Below is some code which sketches my current approach. The 'client' side seems straight-forward and working (hence 'runCall'), but I didn't manage to implement the server side as I imagine it to be (i.e. the parts commented out). Any pointers would be appreciated. Thanks, Nicolas {-# LANGUAGE GADTs, RankNTypes, OverloadedStrings, KindSignatures, ScopedTypeVariables #-} module RPC where import Data.Word (Word32) import Data.Binary (Binary, decode, encode) class RPC (a :: * -> * -> *) where rpcProcedureId :: a req res -> Word32 {- rpcProcedure :: Word32 -> Maybe (a req res) -} data Service req res where Ping :: Service () () Add :: Service (Word32, Word32) Word32 instance RPC Service where rpcProcedureId p = case p of Ping -> 1 Add -> 2 {- rpcProcedure i = case i of 1 -> Just Ping 2 -> Just Add _ -> Nothing -} runCall :: forall call req res. (RPC call, Binary req, Binary res) => call req res -> req -> IO res runCall call req = do let bs = encode req idx = rpcProcedureId call -- Send idx & bs to network, read stuff from network and interpret s <- return $ encode (3 :: Word32) return $ decode s runServer :: (RPC call, Binary req, Binary res) => (call req res -> req -> IO res) -> IO () {- runServer handler = do i <- return 2 -- Read from network case rpcProcedure i of Nothing -> error "No such procedure" Just (call :: call req res) -> do -- Read request from network s <- return $ encode (1 :: Word32, 2 :: Word32) let (req :: req) = decode s (res :: res) <- handler call req -- Send reply to network let res' = encode res return () -} runServer handler = undefined main :: IO () main = do runCall Ping () >>= print runCall Add (1, 2) >>= print {- runServer handler where handler :: Service req res -> req -> IO res handler c (r :: req) = case c of Ping -> return () Add -> case r of (a, b) -> return (a + b) -}

2 2

Re: [Haskell-cafe] SYB supports genuine gmap
by oleg＠okmij.org 03 Jan '14

03 Jan '14

For the record, I have described the implementation of generic map (which is fmap implemented generically, once and for all). http://okmij.org/ftp/Haskell/generics.html#gmap The article stresses a drawback of a fake gmap in terms of gfoldl: a fake gmap is non-parametric. It fails the law gmap f . gmap g == gmap (f.g). OTH, the genuine gmap satisfies the law, as behooves to the generic version of fmap. Although gmap is implemented in terms of the parametricity-breaking type-introspecting SYB, the gmap is parametric nevertheless.

1 0

Re: [Haskell-cafe] Designing somewhat-type-safe RPC
by oleg＠okmij.org 03 Jan '14

03 Jan '14

The remote procedure call is obviously a partial function: first of all, it may fail because of various network problems. It may also fail if a client and a server disagree on the types of the arguments and the results of the function call. For example, the client may think that "Add" service adds integers while the server takes "Add" to sum floats. There is nothing in the type system that can enforce the agreement between distributed entities. So, we are liable to get serialization/deserialization errors. It is inevitable that the communication part is a big "Dynamic", and getting data from that Dynamic may fail because of `type' errors (the data were serialized at a different type than expected, or the data were corrupted in transit). With these assumptions, the implementation is straightforward (enclosed). Both the server and the client operations are typed (but the middle part, the communication, is necessarily `untyped'). Incidentally, some three years ago I wrote a quite more advanced RPC library, in OCaml. It didn't use any GADTs and other bleeding stuff (first, OCaml did not have GADTs at the time; second, I'm minimalist). It did much more, including semi-automatic request batching and some fairly complex server programs including conditionals. It already does more than X protocol and Java RPC. If I added server-side loops, it would do even more. Alas, I didn't have time to come back to that project since. http://okmij.org/ftp/meta-future/meta-future.html {-# LANGUAGE ExistentialQuantification #-} module RPC where import System.IO import qualified Data.Map as M -- identifiers of functions to call type ServiceID = String -- ------------------------------------------------------------------------ -- Server part -- For simplicity, we use Read for deserialization and Show for -- serialization. Binary would've been a better choice for both -- All functions are supposed to be uncurried. -- ServerFn essentially packs a function together with the serializer -- of the result and the deserializer for arguments. data ServerFn = forall a b. (Read a, Show b) => ServerFn (a->b) type Services = M.Map ServiceID ServerFn -- For simplicity, we handle just one request, which we read from -- the handle. We write the result to stdio. It is easy to generalize: -- write the result to an output handle and loop. runServer :: Services -> Handle -> IO () runServer services h = do service_id <- hGetLine h putStrLn $ service_id args <- hGetLine h maybe (fail $ "no such service: " ++ service_id) (handle args) $ M.lookup service_id services where handle sargs (ServerFn f) = do let args = read sargs print $ f args -- Sample services services :: Services services = M.fromList [ ("Ping", ServerFn (\ () -> ())), ("Add", ServerFn (\ (x,y) -> x + y :: Int)) ] -- ------------------------------------------------------------------------ -- Client part -- Stubs of server fn -- ClientFn a b represents a function a->b to be executed by a server data ClientFn a b = ClientFn ServiceID ping :: ClientFn () () ping = ClientFn "Ping" add :: ClientFn (Int,Int) Int add = ClientFn "Add" -- the set of functions is open; more can be added at any time -- Do the remote function application rpc :: (Show a, Read b) => Handle -> ClientFn a b -> a -> IO b rpc h (ClientFn fid) x = do hPutStrLn h fid hPutStrLn h (show x) -- read the result: currently stabbed result_str <- return "stubbed" return $ read result_str -- ------------------------------------------------------------------------ -- Test comm_file = "/tmp/connection" main = do h <- openFile comm_file WriteMode -- send the request down to h. In this example, the return communication -- is not implemented res <- rpc h add (2::Int,3::Int) -- don't look at the result: it this example, it is undefined hClose h h <- openFile comm_file ReadMode runServer services h

1 0

Standard way to translate Haskell to other languages?
by Joey 02 Jan '14

02 Jan '14

I'm looking at writing a simple Haskell to Elm translator. Thankfully, the Elm compiler is in Haskell, so I'm just trying transforming syntax trees. I'm wondering, is there a standard way to do this sort of thing? Template Haskell looked promising, but it seems there isn't much of a way to do introspection on function definition. Has there been any work/progress on this? The GHC API seemed useful for compiling, but it seems tricky to use, and I haven't been able to find many tutorials for this sort of thing. I could just be looking in the wrong place. What are the ways I could go about this? Are there any similar projects I could use as reference? What packages are useful for generating and manipulating Haskell ASTs? Does anybody have experience in this area they could share? I've posted a similar question to the Haskell Reddit at http://www.reddit.com/r/haskell/comments/1u55rl/standard_way_to_translate_h…. Thanks! Joey Eremondi

4 6

Fwd: HFuse Patch
by Diony Rosa 02 Jan '14

02 Jan '14

I was told to email this list: Hello, I wish to become maintainer of the HFuse (FUSE bindings for Haskell) package on hackage. The package's github URL and email are both dead, and the package doesn't compile under GHC7. I've gotten the package to work under GHC7 with a patch, and have gotten permission from the original maintainer to take over. My hackage username is dhrosa, and here's the package page: http://hackage.haskell.org/package/HFuse , and the referenced patch is here: http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=703417 Are there any objections? ---------- Forwarded message ---------- From: Paul van der Walt <Paul.vanderWalt(a)inria.fr> Date: Wed, Jan 1, 2014 at 7:06 PM Subject: Re: HFuse Patch To: Diony Rosa <dhrosa(a)mit.edu> On Wed, Jan 01, 2014 at 19:00:18 -0500, quoth Diony Rosa: In that case, may I take it over? I know how to fix it and am interested in > this package. > That would suit me down to the ground! Feel free to remove all references to me, my name or my website! Thank you very much! Paul

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Consistency issue with type level numeric literals
by oleg＠okmij.org 02 Jan '14

02 Jan '14

GHC 7.6.3 has quite convenient type-level numeric literals. We can use numbers like 1 and 2 in types. However, using the type level numeral has been quite a bit of a challenge, illustrated, for example, by the following SO question. http://stackoverflow.com/questions/20809998/type-level-nats-with-literals-a… It seems the challenge has the reason: the type level numerals and their operations provided in GHC.TypeLits have a consistency issue. The following code demonstrates the issue: it constructs two distinct values of the type Sing 2. Singletons aren't singletons. {-# LANGUAGE KindSignatures #-} {-# LANGUAGE DataKinds #-} {-# LANGUAGE ScopedTypeVariables #-} {-# LANGUAGE TypeOperators #-} {-# LANGUAGE TypeFamilies #-} {-# LANGUAGE GADTs #-} {-# LANGUAGE PolyKinds #-} module NotSing where import GHC.TypeLits -- GHC strangely enough lets us define instances for (+) type instance 1 + 1 = 0 type instance 0 + 1 = 2 -- A singular representative of 1::Nat s1 :: Sing 1 s1 = withSing id -- A singular representative of 2::Nat s2 :: Sing 2 s2 = withSing id is2 :: IsZero 0 is2 = IsSucc s1 tran :: IsZero 0 -> Sing 2 tran (IsSucc x) = case isZero x of IsSucc y -> case isZero y of IsZero -> x -- Another singular representative of 2::Nat -- A singular representative of 2::Nat s2' :: Sing 2 s2' = tran is2 {- *NotSing> s2 2 *NotSing> s2' 1 -}

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Get expected type of expression using GHC API?
by Patrick Hurst 01 Jan '14

01 Jan '14

Using the GHC API, you can get the inferred type of an `LHsExpr Id` via a `TypecheckedModule`, as hdevtools does[1]. Is there any way to get the expected type? This would be useful for cases where the expected type is much more polymorphic than the inferred type (or possibly for debugging type errors with -fdefer-type-errors, depending on what GHC does in that case). [1] https://github.com/bennofs/hdevtools/blob/master/src/Info.hs#L133

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[ANN] Laborantin: experimentation framework
by lucas di cioccio 31 Dec '13

31 Dec '13

Dear all, I am happy to announce Laborantin. Laborantin is a Haskell library and DSL for running and analyzing controlled experiments. Repository: https://github.com/lucasdicioccio/laborantin-hs Hackage page: http://hackage.haskell.org/package/laborantin-hs Laborantin's opinion is that running proper experiments is a non-trivial and often overlooked problem. Therefore, we should provide good tools to assist experimenters. The hope is that, with Laborantin, experimenters will spend more time on their core problem while racing through the menial tasks of editing scripts because one data point is missing in a plot. At the same time, Laborantin is also an effort within the broad open-science movement. Indeed, Laborantin's DSL separates boilerplate from the actual experiment implementation. Thus, Laborantin could reduce the friction for code and data-reuse. One family of experiments that fit well Laborantin are benchmarks with tedious setup and teardown procedures (for instance starting, configuring, and stopping remote machines). Analyses that require measurements from a variety of data points in a multi-dimensional parameter space also fall in the scope of Laborantin. When using Laborantin, the experimenter: * Can express experimental scenarios using a readable and familiar DSL. This feature, albeit subjective, was confirmed by non-Haskeller colleagues. * Saves time on boilerplate such as writing command-line parsers or encoding dependencies between experiments and analysis results in a Makefile. * Benefits from auto-documentation and result introspection features when one comes back to a project, possibly months or weeks later. * Harnesses the power of Haskell type-system to catch common errors at compile time If you had to read one story to understand the pain points that Laborantin tries to address, it should be Section 5 of "Strategies for Sound Internet Measurement" (V. Paxson, IMC 2004). I'd be glad to take question and comments (or, even better, code reviews and pull requests). Kind regards, --Lucas DiCioccio (@lucasdicioccio on GitHub/Twitter)

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parametrized data types and Template Haskell
by Maarten Faddegon 31 Dec '13

31 Dec '13

Dear Cafe, Hope you all had a nice Christmas. I have been playing with generating method instances using Template Haskell but am a bit stuck now trying to generate an instance for a parametrized data type. I would like to generate the following: > instance (MyClass a) => MyClass (Tree a) where > mymethod _ = "todo" I defined a genMyClassInstance that is working fine for unparametrized data types, but clearly there is nothing here that inserts the '(MyClass a) =>' part here. My first question is: how should I instruct Template Haskell to insert the beforementioned code when appropriate? > genMyClassInstance :: Name -> Q [Dec] > genMyClassInstance name > = [d|instance MyClass $(conT name) where > mymethod _ = "todo" > |] My second question is how to pass the Name of a parametrized data type? I tried the following, but GHC does not seem to like that: "Not in scope: type constructor or class `Tree a' Perhaps you meant `Tree'" > $(genMyInstance (mkName "Tree a")) Thank you! Maarten Faddegon

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December 31 deadline for Haskell.org donations
by Adam Foltzer 31 Dec '13

31 Dec '13

Dear Haskellers, HaskellWiki, Hackage, Hoogle, and all the other haskell.org sites we depend on are hosted with funds donated by you, the Haskell community. As many of us are in tax jurisdictions where December 31st is a deadline for charitable giving, I’d like to remind you that donations to support these activities are tax-deductible. You can donate online right now at https://co.clickandpledge.com/advanced/default.aspx?wid=69561. Thank you to Jason Dagit and the rest of last year’s Haskell.org Committee for making these donations possible by establishing a partnership with Software in the Public Interest, and thank you to all those who have already donated both money and time to advance our infrastructure and community. It has been a momentous year for Haskell.org: Hackage 2 was launched on a community server, and the HaskellWiki is now available via HTTPS. Let’s keep making progress in 2014! Thank you and best wishes for a happy new year, Adam Foltzer

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ANN: treeviz-0.0.4 - a tool for visualizing Divide&Conquer computational breakdown.
by David Banas 31 Dec '13

31 Dec '13

I just posted v0.0.4 of treeviz, a Haskell package for visualizing how various divide & conquer algorithms break down computations. The specific example provided is the classic fast Fourier transform (FFT). http://hackage.haskell.org/package/treeviz-0.0.4 https://github.com/capn-freako/treeviz Happy holidays, -db

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Top-down inserts in Persistent
by Michael Orlitzky 31 Dec '13

31 Dec '13

I'm loading data from XML into a Haskell data type, and I'd like to use Persistent to save it to a database. The examples from the Yesod book have you manually define a FooId field and create the relationships yourself from the bottom up. For example, "a person has many cars": blah blah [persistLowerCase| Person name String Car ownerId PersonId Eq name String |] This works well if you're responsible for creating every person/car manually. But what if the data are given to you? If I were to parse people from an XML file, the cars wouldn't have people_ids in them. Instead I'd get, blah blah [persistLowerCase| Person name String cars [Car] Car name String |] As long as the cars list contains another Persistent type, it seems like I should be able to insert a person and have it insert the cars, with proper foreign keys, automatically. Doing it manually isn't straight-forward because I can't add the "ownerId" field to my Car type and still expect to parse it from the XML (which has no such field). Any ideas? I'm not married to Persistent yet; I just want to read in some XML and save it to a database without having to specify the names and types in three places (preferred place: in Haskell). I don't care too much about the schema I get as long as it's relational.

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Re: [Haskell-cafe] Haskell-Cafe Digest, Vol 124, Issue 39
by Hans Höglund 30 Dec '13

30 Dec '13

> On Sat, Dec 28, 2013 at 5:13 PM, Tom Ellis > <tom-lists-haskell-cafe-2013 at jaguarpaw.co.uk> wrote: > > data Wrap m n a = Wrap (m (n a)) > > This is Compose [0] from traversable. It doesn't have a Monad > instance, though... > > Erik > > [0] http://hackage.haskell.org/package/transformers-0.3.0.0/docs/Data-Functor-C… According to [1], Wrap/Compose does indeed have a Monad instance terms of traverse (here called 'swap'). As far as I understand the paper includes a proof that all the Monad laws holds for such as composition, however there is no mention of transformers as the paper predates them. Personally I am not sure that the transformer composition is useful. Would it be as simple as adding this instance to Data.Functor.Compose: > instance (Functor m, Traversable n, Monad m, Monad n) => Monad (Compose m n) or can anyone spot a problem with this approach? Hans [1]: http://web.cecs.pdx.edu/~mpj/pubs/RR-1004.pdf, page 9 [2]: http://strictlypositive.org/IdiomLite.pdf

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Re: [Haskell-cafe] Question on nested State monads.
by Benjamin Edwards 29 Dec '13

29 Dec '13

(Forwarding to the list, sorry Albert) On Sunday, 29 December 2013 08:32:06, Benjamin Edwards < edwards.benj(a)gmail.com> wrote: I would also recommend looking at use / zoom from the lens package. I find them indispensable for when you want to use a compound state. If you just have two parts you can write all your state calculations on the separate parts then user StateT m (a,b) r and zoom _1 / zoom _2 to access them in the same do block.

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Question on nested State monads.
by David Banas 29 Dec '13

29 Dec '13

Hi all, If I want to use a second, nested State monad inside another State monad, must I thread the outer monad’s state through the processing of the inner one, even if the inner one falls out of scope before I return from the processing body of the outer one? Thanks, -db

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