February 2013 - Haskell-Cafe

Yet another Conduit question
by Simon Marechal 04 Feb '13

04 Feb '13

I am working with bulk sources and sinks, that is with a type like: Source m [a] Sink [a] m () The problem is that I would like to work on individual values in my conduit. I can have this: concat :: (Monad m) => Conduit [a] m a concat = awaitForever (mapM_ yield) But how can I do it the other way around ? I suppose it involves pattern matching on the different states my conduit might me in. But is that even possible to do it in a "non blocking" way, that is catenate data while there is something to read (up to a certain threshold), and send it as soon as there is nothing left to read ? Or doesn't that make any sense in the context of Conduits (in the sense that this conduit will be recheck for input before the upstream conduits will have a chance to operate) ? Another approach would be to have a map equivalent: conduitMap :: Conduit i m o -> Conduit [i] m [o] But I am not sure how to do this either ...

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Re: [Haskell-cafe] Monadic parser vs. combinator parser
by Jan Stolarek 04 Feb '13

04 Feb '13

> Of course that is fine. But keep in mind that all parser combinator based > libraries are based on a top-down parsing strategy which in most places, > BUT NOT ALL, fits Haskell's syntax. Try to use the chaining combinators > where-ever possible. In case of doubt you can always spy on how things are > done in the UHC. Luckily I'm not building a parser for Haskell, but for a very small set of instructions given to a program from the command line. In case of emergency I will be looking at the source code of uulib and uu-parsinglib (which, by the way, could use some line wrapping to fit in 80 columns). Janek

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Call for Nominations: Haskell Prime language committee
by Malcolm Wallace 04 Feb '13

04 Feb '13

Dear Haskell lovers, The Haskell Prime process for standardisation of new versions of the Haskell language is at something of an impasse. Since the Haskell 2010 Report was issued (at the end of 2009), there has been very little momentum to formalise existing extensions and generalisations, nor appetite to decide on whether any such extensions should be adopted as part of the core language standard. We therefore seek nominations for new members of the Haskell Prime language committee; people who have the enthusiasm to take this forward, as well as some relevant experience. If you think you would like to contribute, please nominate yourself by sending a email to haskell-2011-committee(a)haskell.org describing who you are, and what you think the committee might hope to achieve. The address above is populated by the existing committee members, but is now (temporarily) open to all during the nomination period. Nominations close in 3 weeks time, i.e. to be received by end of Sunday 24th February 2013. The process for deciding a new language committee is described at http://hackage.haskell.org/trac/haskell-prime/wiki/Committee Regards, Malcolm

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Parsec Vs. src exts
by Sean Cormican 03 Feb '13

03 Feb '13

Hi, I've been spending a bit of time looking into using Parsec as a parser for a subset of the syntax of the Haskell language, a high level description of what I'm looking to create is a parser which confirms whether a given Haskell code file is valid or in the case that it isn't returns some helpful error messages to the user. (Please bear with me, I know this is a strange implementation) My question is that while I have a modest understanding of Parsec from tutorials, the haskell-src package has also been recommended of which I can find very limited documentation in comparison. However, the issue arises from both my limited understanding of haskell and lack of information on the usage of haskell-src. Can anybody provide me with some guidance on whether to try and create the parser using Parsec or if haskell-src is a better option. In the case that haskell-src is a better option, are there any tutorials or documents that I would benefit from reading? Thanks in advance, Seán

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Most used functions in hackage
by Casey Basichis 03 Feb '13

03 Feb '13

Hi, I'm guessing this is a long shot, but Is there any link that counts the use of all functions in all packages in Hackage and lists them by frequency or by other stats? I'm still new to haskell but I've been working my way through tons and tons of tutorials and books. It would be very helpful to target in on the current reality of the most critical functions. Anything like this out there? Thanks, Casey

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linking errors while compile hugs98 in macos
by Junior White 03 Feb '13

03 Feb '13

Hi Cafe, I downloaded the latest hugs98 source package, unzip and build, I get the following link errors. It seems many symbols are not defined, am I missing same depending libraries? This is my machine info: ➜ hugs98-plus-Sep2006 git:(master) ✗ uname -a Darwin lan-seimatoMacBook-Air.local 11.4.2 Darwin Kernel Version 11.4.2: Thu Aug 23 16:25:48 PDT 2012; root:xnu-1699.32.7~1/RELEASE_X86_64 x86_64 ================================ Preprocessing Network/Hackage/Version Undefined symbols for architecture x86_64: "_environ", referenced from: ___hscore_environ in ccuTP291.o (maybe you meant: ___hscore_environ) ld: symbol(s) not found for architecture x86_64 collect2: ld returned 1 exit status runhugs: Error occurred ERROR "libraries/bootlib/Foreign/C/Error.hs" - Error while running compilation command 'gcc -DNDEBUG=1 -g -no-cpp-precomp -flat_namespace -shared -fPIC -D__HUGS__ "-Ihugsdir/include" -o "libraries/bootlib/Foreign/C/Error.so" "libraries/bootlib/Foreign/C/Error.c" -Ipackages/base/include -Ighc/includes packages/base/cbits/PrelIOUtils.c' Undefined symbols for architecture x86_64: "_environ", referenced from: ___hscore_environ in ccYZo1zT.o (maybe you meant: ___hscore_environ) ld: symbol(s) not found for architecture x86_64 collect2: ld returned 1 exit status runhugs: Error occurred ERROR "libraries/bootlib/Foreign/Marshal/Alloc.hs" - Error while running compilation command 'gcc -DNDEBUG=1 -g -no-cpp-precomp -flat_namespace -shared -fPIC -D__HUGS__ "-Ihugsdir/include" -o "libraries/bootlib/Foreign/Marshal/Alloc.so" "libraries/bootlib/Foreign/Marshal/Alloc.c" -Ipackages/base/include -Ighc/includes packages/base/cbits/PrelIOUtils.c packages/base/cbits/dirUtils.c packages/base/cbits/consUtils.c'

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why GHC cannot infer type in this case?
by oleg＠okmij.org 03 Feb '13

03 Feb '13

Dmitry Kulagin wrote: > I try to implement little typed DSL with functions, but there is a problem: > compiler is unable to infer type for my "functions". One way to avoid the problem is to start with the tagless final representation. It imposes fewer requirements on the type system, and is a quite mechanical way of embedding DSL. The enclosed code re-implements your example in the tagless final style. If later you must have a GADT representation, one can easily write an interpreter that interprets your terms as GADTs (this is mechanical). For more examples, see the (relatively recent) lecture notes http://okmij.org/ftp/tagless-final/course/lecture.pdf {-# LANGUAGE TypeOperators, KindSignatures, DataKinds #-} {-# LANGUAGE NoMonomorphismRestriction, TypeFamilies #-} module TestExp where -- types of DSL terms data Ty = Int16 | TFun [Ty] Ty | TSeq [Ty] -- DSL terms class Exp (repr :: Ty -> *) where int16:: Int -> repr Int16 add :: repr Int16 -> repr Int16 -> repr Int16 decl :: (repr (TSeq ts) -> repr t) -> repr (TFun ts t) call :: repr (TFun ts t) -> repr (TSeq ts) -> repr t -- building and deconstructing sequences unit :: repr (TSeq '[]) cons :: repr t -> repr (TSeq ts) -> repr (TSeq (t ': ts)) deco :: (repr t -> repr (TSeq ts) -> repr w) -> repr (TSeq (t ': ts)) -> repr w -- A few convenience functions deun :: repr (TSeq '[]) -> repr w -> repr w deun _ x = x singleton :: Exp repr => (repr t -> repr w) -> repr (TSeq '[t]) -> repr w singleton body = deco (\x _ -> body x) -- sample terms fun = decl $ singleton (\x -> add (int16 2) x) -- Inferred type -- fun :: Exp repr => repr (TFun ((:) Ty 'Int16 ([] Ty)) 'Int16) test = call fun (cons (int16 3) unit) -- Inferred type -- test :: Exp repr => repr 'Int16 fun2 = decl $ deco (\x -> singleton (\y -> add (int16 2) (add x y))) {- inferred fun2 :: Exp repr => repr (TFun ((:) Ty 'Int16 ((:) Ty 'Int16 ([] Ty))) 'Int16) -} test2 = call fun2 (int16 3 `cons` (int16 4 `cons` unit)) -- Simple evaluator -- Interpret the object type Ty as Haskell type * type family TInterp (t :: Ty) :: * type instance TInterp Int16 = Int type instance TInterp (TFun ts t) = TISeq ts -> TInterp t type instance TInterp (TSeq ts) = TISeq ts type family TISeq (t :: [Ty]) :: * type instance TISeq '[] = () type instance TISeq (t1 ': ts) = (TInterp t1, TISeq ts) newtype R t = R{unR:: TInterp t} instance Exp R where int16 = R add (R x) (R y) = R $ x + y decl f = R $ \args -> unR . f . R $ args call (R f) (R args) = R $ f args unit = R () cons (R x) (R y) = R (x,y) deco f (R (x,y)) = f (R x) (R y) testv = unR test -- 5 tes2tv = unR test2 -- 9

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Monadic parser vs. combinator parser
by Jan Stolarek 03 Feb '13

03 Feb '13

I will be writing a parser in Haskell and I wonder how to approach the problem. My first thought was to use monadic parser, e.g. like the one described by Hutton and Meijer in "Monadic Parsing in Haskell" functional pearl. But then I stumbled upon this: https://github.com/alephnullplex/cradle/tree/master/code/src/Lbach/Parser Monadic parser seems extremely verbose and not very straightforward compared to this one. I started to wonder whether I should use monadic parser for the sake of it being monadic or should I just go with the combinator approach? Any thoughts will be appreciated before I shoot myself in the foot :) Janek

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A question about "An introduction to Category theory in four easy movements"
by Jan Stolarek 03 Feb '13

03 Feb '13

Hi all, I was thinking about reading "An introduction to Category theory in four easy movements" available online at http://www.cs.man.ac.uk/~hsimmons/BOOKS/books.html I noticed however that these notes were published as a book. Looking at the table of contents at Amazon, it seems that organization of material is different. About 40% of online version are solutions to the exercises and this is seems not to be the case for the book. Were these solutions removed from the printed version or are they just contained within chapters? Are there any significant differences in scopes of these two versions? I would be thankful if anyone owning the book could give a short comparison with online notes. Janek

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Substantial (1:10??) system dependencies of runtime performance??
by Nick Rudnick 02 Feb '13

02 Feb '13

Dear all, for quite a while now, I have experienced this issue with some curiosity; yesterday I had it again, when a program that took well over one hour before only needed about ten minutes, after a system reboot (recent Ubuntu) and with no browser started -- finally deciding to post this. I still can't reproduce these effects, but there is indication it is connected with browser use (mostly Google Chrome, with usually 10's of windows and ~100 folders open) and especially use of video players; closing or killing doesn't seem to set free resources, a reboot or at least suspend to disk seems to be necessary (suspend to RAM doesn't seem enough). Roughly, I would say the differences in runtime can reach a factor as much as 1:10 at many times -- and so I am curious whether this subject has already been observed or even better discussed elsewhere. I have spoken to somebody, and our only plausible conclusion was that software like web browsers is able to somewhat aggressively claim system resources higher in the privilege hierarchy (cache?? register??), so that they are not available to other programs any more. I hope this is interesting to others, too, I guess it is an issue for anybody programming computation intensive code to be run on standard systems with other applications running there, too, and having to predict the estimated runtime to the client. Maybe I have overseen some libs which are already able to scan the system state in this regard, or even tell Haskell to behave 'less nice' when other applications are known to be of lower priority?? Thanks a lot in advance, Nick

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