- Glasgow-haskell-users

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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is GHC developers wiki down?
by Valery V. Vorotyntsev 11 Jan '10

11 Jan '10

Clicking ``Developers (Wiki)'' link [http://hackage.haskell.org/trac/ghc] from GHC page [http://www.haskell.org/ghc/] returns Environment not found Neither the link is accessible from command line: $ curl -I 'http://hackage.haskell.org/trac/ghc' HTTP/1.0 404 Not Found Date: Mon, 11 Jan 2010 10:00:36 GMT Server: Apache/2.2.3 (Debian) Content-Type: text/plain; charset=UTF-8 Age: 38 Content-Length: 21 X-Cache: HIT from xproxy.foo.bar.ua X-Cache-Lookup: HIT from xproxy.foo.bar.ua:3128 Via: 1.1 xproxy.foo.bar.ua:3128 (squid/2.7.STABLE6) Connection: close -- Best regards, vvv

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hptSomeThingsBelowUs missing module
by Evan Laforge 11 Jan '10

11 Jan '10

Ever since upgrading to 6.12.1, I've been getting a new warning. I compile with --make, e.g. (clipping out irrelevant stuff): ghc-6.12.1 -threaded -W -debug -package ghc --make \ -main-is App.Main App/Main.hs \ -o build/seq If I remove all imports to a module (say Util.Debug), compiling other modules prints this warning: WARNING in hptSomeThingsBelowUs missing module Util.Debug Probable cause: out-of-date interface files I searched on trac but didn't see anything with this name. Is this a known issue? It doesn't seem to cause any harm in the eventual binary, but is somewhat alarming to see a bunch of them scroll past. Manually wiping out a bunch of .hi files causes recompilation and the warning goes away.

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Ray tracer language comparison
by Jon Harrop 11 Jan '10

11 Jan '10

I recently published some performance results using GHC 6.12: http://flyingfrogblog.blogspot.com/2010/01/hlvm-on-ray-tracer-language-comp… However, I just noticed that the Haskell is producing garbage output with GHC 6.12 and not 6.10 or 6.8. You may like to check this with a recent build or put the program in the GHC test suite. -- Dr Jon Harrop, Flying Frog Consultancy Ltd. http://www.ffconsultancy.com/?e

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haddock source link are wrong for haskell98 modules
by Christian Maeder 07 Jan '10

07 Jan '10

Hi, is this a known issue? All source links in haskell98 modules are dead I.e. in http://www.haskell.org/ghc/docs/latest/html/libraries/haskell98-1.0.1.1/Arr… The link under the first "Source" (at the right margin) points to http://www.haskell.org/ghc/docs/latest/html/libraries/haskell98-1.0.1.1/src… which does not exist. Can this be fixed easily? Cheers Christian

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Associativity of ViewPatterns
by Bas van Dijk 07 Jan '10

07 Jan '10

Hello, Why is the following a syntax error: f (view1 -> view2 -> pattern) = ... and the following isn't: f (view1 -> (view2 -> pattern)) = ... I would prefer the first version. regards, Bas

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