- Glasgow-haskell-users

Question regarding the GHC users manual
by Tyson Whitehead 25 Jan '10

25 Jan '10

Hi all, Just wondering if there could be a typo in section 7.7.2.1 of the GHC manual (type family declarations) http://www.haskell.org/ghc/docs/latest/html/users_guide/type-families.html Specifically, it says type family F a b :: * -> * -- F's arity is 2, -- although its overall kind is * -> * -> * -> * along with F Char [Int] -- OK! Kind: * -> * F Char [Int] Bool -- OK! Kind: * F IO Bool -- WRONG: kind mismatch in the first argument F Bool -- WRONG: unsaturated application and I'm wondering about the overall kind. Shouldn't that be * -> * -> *, or am I not understanding something? Thanks! -Tyson

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Trying to build from the darcs repository
by Florian Weimer 25 Jan '10

25 Jan '10

How much memory does GHC need to build on amd64 GNU/Linux? I'm trying to build from the darcs repository, and this command "inplace/bin/ghc-stage1" -H32m -O -package-name terminfo-0.3.1.1 -hide-all-packages -i ilibraries/terminfo/. --ilibraries/terminfo/dist-install/build -ilibraries/terminfo/dist-install/build/autogen -Ilibraries/terminfo/dist-install/build -Ilibraries/terminfo/dist-install/build/autogen --Ilibraries/terminfo/. optP-include -optPlibraries/terminfo/dist-install/build/autogen/cabal_macros.h -package base-4.2.0.0 -package extensible-exceptions-0.1.1.1 -split-objs -Wall -XForeignFunctionInterface -XDeriveDataTypeable -XEmptyDataDecls -XScopedTypeVariables -XFlexibleInstances -O2 -XGenerics -fno-warn-deprecated-flags -odir -libraries/terminfo/dist-install/build -hidir -libraries/terminfo/dist-install/build -stubdir -libraries/terminfo/dist-install/build -hisuf hi -osuf o -hcsuf hc --c libraries/terminfo/./System/Console/Terminfo/Effects.hs -o -libraries/terminfo/dist-install/build/System/Console/Terminfo/Effects.o needs more than 3.5 GB of RAM. Or is this some sort of miscompilation problem? (I'm trying to bootstrap with GHC 6.12.1, but saw this with GHC 6.8 as well.) The start of the context is this: [Remove an out-of-date comment Ian Lynagh <igloo(a)earth.li>**20100122130853] [fix build on Windows Simon Marlow <marlowsd(a)gmail.com>**20100122121016 Ignore-this: cd690a8eff71ca4f87fe67a41a20f493 ] [fix warning on Windows Simon Marlow <marlowsd(a)gmail.com>**20100122120328 Ignore-this: 507a91e356a667dcecf8bec621fd2b95 ]

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Scriptable error messages
by Daniel Gustafsson 22 Jan '10

22 Jan '10

Hello Me and a fellow student are taking a course named Frontiers of Programming Languages, and in this course we should do a project with something. A while ago we were at a presentation of Feldspar, a Embedded Domain Specific Language, and one of the negative points of using haskell was that the error messages could be cryptic due to the various ways it was implemented. So we are now looking into the possibility of adding your own error messages to GHC as our project. We have about 8 weeks to implement, either all or some proof of concept. As such we are looking for comments, ideas and suggestions. Is this doable in GHC?, or what is needed. Thanks Daniel Gustafsson Simon Edwardsson Chalmers University of Technology (Gothenburg, Sweden)

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ghc API in platforms with no GHCi
by Marco Túlio Gontijo e Silva 21 Jan '10

21 Jan '10

Hi. I noticed that in Debian/KFreeBSD the module ByteCodeLink is not available, which cause the build error of haskell-hint in Debian's buildds. http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=564136 This is also the case of alpha, armel, hppa, mips, powerpc and s390. I haven't checked if these are the architectures without GHCi but I suspect they are. Is the full ghc API available only in arches in which GHCi is available? Greetings. -- marcot http://marcot.iaaeee.org/

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Adding support for CLDouble
by Denys Rtveliashvili 21 Jan '10

21 Jan '10

Hi, Currently ghc 6.12 does not have support for CLDouble type (and there is an open issue on trac about it http://hackage.haskell.org/trac/ghc/ticket/3353 ). I have been trying to figure out what is necessary to add it. After playing with the code a bit I've got a few questions... 1. Should there be a LDouble type, just like for CFloat and CDouble there there are Haskell-native types Float and Double? 2. The ghc compiler references Float and Double in its code. When a similar code is added for LDouble, the compiler does not get built. In particular, 'compiler/coreSyn/CoreSyn.lhs' does not see 'LDouble' despite I declared and exported it in 'libraries/ghc-prim/GHC/Types.hs'. This probably makes sense in case compiler is built first. But then in turns out to be a chicken and egg situation as to support a new type in GHC one needs to have another GHC that already supports it. Or, perhaps, I am missing something? 3. http://hackage.haskell.org/trac/ghc/wiki/Commentary/Rts/HaskellExecution/Re… mentions that there are registers for floats and doubles. What should be done about long doubles then? With kind regards and hope that gurus may shed some light on the above, Denys Rtveliashvili

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UTF-8 + Hsc2hs
by Taru Karttunen 20 Jan '10

20 Jan '10

GHC 6.12.1 hsc2hs seems to have an issue with compiling sources that contain certain UTF-8 charactes in the comments. It contains code like: showCChar c = ['\\', intToDigit (ord c `quot` 64), intToDigit (ord c `quot` 8 `mod` 8), intToDigit (ord c `mod` 8)] which quite obviously fails if c is a code point with a large ord value (like japanese text for example). This affects at least HsOpenSSL. - Taru Karttunen

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Can't infer type (type family + "element" type)
by Sean Leather 19 Jan '10

19 Jan '10

Suppose I have a class C, > class C a where > type E a > c :: E a -> a -> a a datatype T, > data T a = T a and an instance of C for T > instance C (T a) where > type E (T a) = a > c x (T _) = T x I would like to write a function such as f > f t@(T x) = c x t without a type signature. Unfortunately, I can't because GHC tells me Couldn't match expected type `t' against inferred type `T (E t)' In the second argument of `c', namely `t' In the expression: c x t In the definition of `f': f (t@(T x)) = c x t There are at least three possible ways to write the above code such that it works. (1) Give a type signature for f > f :: T a -> T a (2) Define the class C using an equality constraint > class C t where > type E t > c :: (E t ~ e) => e -> t -> t (3) Define the class C using functional dependencies > class C t e | t -> e where > c :: e -> t -> t But the real question is why don't I get a type for f? This has been tested in GHC 6.10.1 and 6.12.1. Thanks, Sean

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RE: [Template-haskell] Querying the instances a data type implements
by Simon Peyton-Jones 18 Jan '10

18 Jan '10

All: as Robert says, I keep meaning to close the template-haskell list altogether; but I am not sure how to do so. Jonathan: where did you find a pointer to the mailing list? I should remove those too. Returning to the question, sadly there's no way to do what you want right now. There's an open feature request: http://hackage.haskell.org/trac/ghc/ticket/1835 Would anyone like to undertake it? It's not hard. Simon From: template-haskell-bounces(a)haskell.org [mailto:template-haskell-bounces@haskell.org] On Behalf Of jonathanGfischoff(a)gmail.com Sent: 17 January 2010 23:42 To: template-haskell(a)haskell.org Subject: [Template-haskell] Querying the instances a data type implements Hi, I am trying query the instances that a data type implements. Calling reify on the data type doesn't appear to return anything relating to type classes. I am very new to Template Haskell, so I apologize if there is a easy way to do this that I missed. -Jonathan

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Poll: Error message spans
by Ian Lynagh 15 Jan '10

15 Jan '10

Hi all, Currently, when GHC reports an error it only gives a source position, not a source span. For example, with this module: main = print (f (const 'x' 'x') 'y') f xs y = xs ++ [y] you get an error starting: u.hs:1:18: Couldn't match expected type `[a]' against inferred type `Char' There's a (probably little-known) flag -ferror-spans with which you get this error instead: u.hs:1:18-30: Couldn't match expected type `[a]' against inferred type `Char' i.e. rather than just giving the start column number 18, it gives you the span 18-30 (the "const 'x' 'x'"). Would you find the extra information useful, or just noise? i.e. should we show error spans by default? Thanks Ian

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Type families and type inference - a question
by Dmitry Tsygankov 11 Jan '10

11 Jan '10

Dear all, I was playing around recently with translating the dependency injection idea (http://martinfowler.com/articles/injection.html) into Haskell, and got to the following code: {-# LANGUAGE TypeFamilies, FlexibleContexts #-} data Movie = Movie { getDirector :: String } data (MovieFinder f) => MovieLister f = MovieLister { getFinder :: f } -- Cannot remove the type signature here createLister :: (MovieFinder f) => (FinderResultMonad f) (MovieLister f) createLister = fmap MovieLister createFinder class (Monad (FinderResultMonad f), Functor (FinderResultMonad f)) => MovieFinder f where type FinderResultMonad f :: * -> * createFinder :: (FinderResultMonad f) f findAll :: f -> (FinderResultMonad f) [Movie] It may be dumb (well, the Java version isn't particularly useful either), but the thing I really do not understand is the type signature - why can't I simply remove it? Some output from GHCi: GHCi, version 6.12.1: http://www.haskell.org/ghc/ :? for help *IfaceInj> :t fmap MovieLister fmap MovieLister :: (MovieFinder a, Functor f) => f a -> f (MovieLister a) *IfaceInj> :t createFinder createFinder :: (MovieFinder f) => FinderResultMonad f f Looks reasonable so far... *IfaceInj> :t fmap MovieLister createFinder fmap MovieLister createFinder :: (f ~ FinderResultMonad a, MovieFinder a, Functor f) => f (MovieLister a) Here's the first WTF. If the type inference engine knows that f ~ FinderResultMonad a, it can 'guess' the type (MovieFinder a, Functor (FinderResultMonad a)) => (FinderResultMonad a) (MovieLister a) , can't it? And since there's a constraint on the MovieFinder type class, it can further 'guess' (MovieFinder a) => (FinderResultMonad a) (MovieLister a) , which is exactly the type signature I have written by hand, but it doesn't. Is it a bug, a missing feature, or just my lack of knowledge? OK, so far, so good, let's call it a missing feature or something that is impossible to implement. *IfaceInj> let q = fmap MovieLister createFinder <interactive>:1:25: Couldn't match expected type `FinderResultMonad a' against inferred type `f' NB: `FinderResultMonad' is a type function, and may not be injective In the second argument of `fmap', namely `createFinder' In the expression: fmap MovieLister createFinder In the definition of `q': q = fmap MovieLister createFinder Here's the second WTF. It seems like the type inference engine CAN infer the type of (fmap MovieLister createFinder). If I manually enter the type inferred by ':t fmap MovieLister createFinder' to the signature of createLister, everything compiles OK. But if I remove the type signature from createLister completely, I get the same error: *IfaceInj> :load "/home/dima/projects/IfaceInj.hs" [1 of 1] Compiling IfaceInj ( /home/dima/projects/IfaceInj.hs, interpreted ) /home/dima/projects/IfaceInj.hs:9:32: Couldn't match expected type `FinderResultMonad a' against inferred type `f' NB: `FinderResultMonad' is a type function, and may not be injective In the second argument of `fmap', namely `createFinder' In the expression: fmap MovieLister createFinder In the definition of `createLister': createLister = fmap MovieLister createFinder Failed, modules loaded: none. That looks like a bug to me, but I can't be sure since I have no real experience in Haskell. Any ideas? Regards, Dmitry.

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