July 2010 - Glasgow-haskell-users

partially applied type synonyms
by Arie Peterson 05 Mar '12

05 Mar '12

Dear list, Is there a good reason that partially applied type synonyms cannot be made instances of classes? - A simple example would be making the identity type 'Id' ("type Id x = x") instance of 'Monad'. This would eliminate the need for some silly occurrences of 'runIdentity', and perhaps narrow the gap between monads and monad transformers. - The reason I complain about this now is a more involved example, where introducing a newtype to work around the restriction is not an option. Well, not as far as I can see now, anyway. Partially applied type synonyms are, in a sense, the anonymous functions at the type level, and one might argue that they deserve the same 'first-class member status' as the lambda at the value level. Kind regards, Arie Peterson

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FFI, signals and exceptions
by Edward Z. Yang 02 Sep '10

02 Sep '10

Hello all, I am currently investigating techniques to nicely handle SIGINTs when FFI code is running. Yes, I know it sounds kind of crazy. Ignoring the problems of cleaning up the unceremoniously terminated C computation, I'm having difficulty getting the FFI to /stop/ running when I get the signal. I currently have some code like this: http://hpaste.org/fastcgi/hpaste.fcgi/view?id=28422#a28422 which doesn't work, and which you guys can probably tell me doesn't work. :-) Throwing exceptions from the signal handler also doesn't work. Any suggestions for GHC source code to look at / techniques to try? Cheers, Edward

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Re: [Haskell-cafe] Weird behavior with arrow commands
by Jürgen Doser 02 Aug '10

02 Aug '10

El vie, 23-07-2010 a las 23:27 -0400, Ronald Guida escribió: > I am trying to figure out how to use GHC's arrow commands, and I found > some extremely weird behavior. CC'ed to ghc-users, because this may be a ghc bug. > In GHC's manual, there is a description of arrow commands, which I > don't really understand. > http://www.haskell.org/ghc/docs/latest/html/users_guide/arrow-notation.html… > (Primitive Constructs) > > I have two questions: > 1. What are arrow commands supposed to do? > 2. What is this code supposed to do? > > -- start of code -- > > {-# LANGUAGE Arrows #-} > module Main where > > import Control.Arrow > > foo :: (b -> String) -> ((((b, Int), Float), Double) -> String) -> (b -> String) > foo f g b = f b ++ " " ++ g (((b, 8), 1.0), 6.0) > > bar :: (t -> String) -> ((Double, Int) -> String) -> t -> String > bar f g = proc x -> do > (f -< x) `foo` \n m -> g -< (n) > > main = do > putStrLn $ foo show show 17 > putStrLn $ bar show show 17 > putStrLn $ foo show show 42 > putStrLn $ bar show show 42 > > -- end of code -- > > Output from GHCi: > > 17 (((17,8),1.0),6.0) > 17 (6.730326920298707e-306,0) > 42 (((42,8),1.0),6.0) > 42 (6.730326920298707e-306,0) > > Output after compiling with GHC: > > 17 (((17,8),1.0),6.0) > 17 (5.858736684536801e-270,0) > 42 (((42,8),1.0),6.0) > 42 (5.858736684536801e-270,0) > > GHC Version: > The Glorious Glasgow Haskell Compilation System, version 6.12.3 This seems to be a bug in ghc. First, let's fix bar to give the full three arguments (Int, Float, Double) to g: bar f g = proc x -> do (f -< x) `foo` (\n m k -> g -< (n,m,k)) ghc infers the type: bar :: (t -> String) -> ((Double, Float, Int) -> String) -> t -> String and we see that the argument order in the second argument to bar is reversed. But the arguments are still given to bar in the order (Int, Float, Double). For example, the 6.0 in foo is interpreted as an Int and outputs a 0 (the first 32 bits in such a small double are zeros). When one varies the numbers in foo, one can see the effects in bar. Can someone from GHC HQ confirm my understanding, or is this just not supposed to work with multiple arguments? Jürgen

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Deadlock with Repa
by Jean-Marie Gaillourdet 29 Jul '10

29 Jul '10

Hi, I've been trying to use repa and stumbled into rather strange behavior of my program. Sometimes, there seems to be a deadlock at other times it seems there is a livelock. Now, I've managed to prepare a version which consistently generates the following output: $ ./MDim MDim: thread blocked indefinitely in an MVar operation $ But I don't use any MVar directly. And the only used library which is not part of ghc are repa and repa-algorithms. To state it clearly I don't use any MVars, par, pseq, forkIO nor any other parallelism or cuncurrency functionality. The only thing my program uses is repa which is supposed to use some kind of parallelism as far as the documentation says. So I am wondering whether this is a problem with my code or with repa or with ghc. The same behaviour occurs with ghc-6.12.1 and ghc-6.13.20100726. This is on MacOSX 10.6.4. In order to reproduce take the attached file and compile it with: $ ghc --make -package dph-prim-par MDim.hs -main-is MDim -fforce-recomp [1 of 1] Compiling MDim ( MDim.hs, MDim.o ) Linking MDim ... $ ./MDim MDim: thread blocked indefinitely in an MVar operation $ I haven't been able to make the program smaller without removing the blockage. But, essentially it's just pure number crunching you can ignore. I'd be really happy if anyone could give me a hint how to debug this, or whether I am able to do anything about it, at all. Best regards, Jean-Marie -- Dipl.-Inf. Jean-Marie Gaillourdet Software Technology Group University of Kaiserslautern, Department of Computer Science Gebäude 32, Raum 406 Gottlieb-Daimler-Str. D-67653 Kaiserslautern Tel: 06 31 - 205 - 32 72 Fax: 06 31 - 205 - 34 20

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SPECIALIZE function for type defined elsewhere
by Sebastian Fischer 28 Jul '10

28 Jul '10

Dear GHC experts, say I have module A where class C a where ... f :: C a => String -> a module B where import A data T = ... instance C T where ... g :: String -> SomeOtherType g s = doSomethingWith (f s) Is it possible to SPECIALIZE `f` for the type `T`? If I put the pragma {-# SPECIALIZE f :: String -> T #-} in module A, GHC complains that `T` is not in scope. If I put it in module B GHC complains that there is no accompanying binding for `f`. In my case, I don't want to put everything in a single module because I cannot know what other B-like modules people will implement. Are they bound to use `f` unspecialized for their types? Why? Cheers, Sebastian -- Underestimating the novelty of the future is a time-honored tradition. (D.G.)

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Allowing Instances to Unify Types
by Matt Brown 27 Jul '10

27 Jul '10

Hi all, I've been hacking on GHC for a couple months now, experimenting with some different ideas I find interesting. One thing I'm trying to do is allow instance unifs (when there's an unambiguous choice, a question which is simplified in this case by there being only one), and force the required unification. Here's a simple example: {-# LANGUAGE MultiParamTypeClasses, FunctionalDependencies, FlexibleInstances, UndecidableInstances #-} class Apply a b c | a b -> c where applyInst :: a -> b -> c instance (Monad m) => Apply (a -> m b) (m a) (m b) where applyInst = (=<<) apply :: (Monad m) => (a -> m b) -> (m a) -> (m b) apply = (=<<) ioStr :: IO String ioStr = return "foo" printStr :: String -> IO () printStr = print main = do print `apply` (return "foo") printStr `applyInst` ioStr print `applyInst` (return "bar") -- this fails With my code to use the unif instance enabled, I get Ambiguous type variable errors for "Show a" (from print) and "Monad m" (from return). My question is: in the case of apply (which isn't implemented by a class), how does the typechecker propagate "a ~ String" and "m ~ IO" to the predicates for print and return? If someone (such as myself) had sufficient time and energy to spend trying to achieve similar behavior for applyInst, where might he/I start? Thanks and Regards, -matt

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Error building head on OS X
by Johan Tibell 21 Jul '10

21 Jul '10

Building from a clean checkout of HEAD (from today) I get: "inplace/bin/ghc-stage2" -H32m -O -package-name dph-seq-0.4.0 -hide-all-packages -i -ilibraries/dph/dph-seq/../dph-common -ilibraries/dph/dph-seq/dist-install/build -ilibraries/dph/dph-seq/dist-install/build/autogen -Ilibraries/dph/dph-seq/dist-install/build -Ilibraries/dph/dph-seq/dist-install/build/autogen -Ilibraries/dph/dph-seq/. -optP-include -optPlibraries/dph/dph-seq/dist-install/build/autogen/cabal_macros.h -package array-0.3.0.0 -package base-4.3.0.0 -package dph-base-0.4.0 -package dph-prim-seq-0.4.0 -package ghc-6.13.20100720 -package ghc-prim-0.2.0.0 -package random-1.0.0.2 -package template-haskell-2.4.0.0 -Odph -funbox-strict-fields -fcpr-off -fdph-this -package-name dph-seq -XTypeFamilies -XGADTs -XRankNTypes -XBangPatterns -XMagicHash -XUnboxedTuples -XTypeOperators -no-user-package-conf -rtsopts -O2 -XGenerics -fno-warn-deprecated-flags -odir libraries/dph/dph-seq/dist-install/build -hidir libraries/dph/dph-seq/dist-install/build -stubdir libraries/dph/dph-seq/dist-install/build -hisuf hi -osuf o -hcsuf hc -c libraries/dph/dph-seq/../dph-common/Data/Array/Parallel/Lifted/PArray.hs -o libraries/dph/dph-seq/dist-install/build/Data/Array/Parallel/Lifted/PArray.o Loading package ghc-prim ... linking ... done. Loading package integer-gmp ... linking ... done. Loading package base ... linking ... done. Loading package array-0.3.0.0 ... linking ... done. Loading package containers-0.3.0.0 ... linking ... done. Loading package filepath-1.2.0.0 ... linking ... done. Loading package old-locale-1.0.0.2 ... linking ... done. Loading package old-time-1.0.0.5 ... linking ... done. Loading package unix-2.4.0.1 ... linking ... done. Loading package directory-1.0.1.2 ... linking ... done. Loading package pretty-1.0.1.1 ... linking ... done. Loading package process-1.0.1.3 ... linking ... done. ghc-stage2: internal error: Invalid Mach-O file:Address out of bounds while relocating object file (GHC version 6.13.20100720 for i386_apple_darwin) Please report this as a GHC bug: http://www.haskell.org/ghc/reportabug make[1]: *** [libraries/dph/dph-seq/dist-install/build/Data/Array/Parallel/Lifted/PArray.o] Abort trap make: *** [all] Error 2 P.S. I'm running OS X 10.5.8 on a Intel Core 2 Duo Mac Book Pro.

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ghc 6.12.3: Data.Array.Diff and package diffarray
by Rene de Visser 20 Jul '10

20 Jul '10

Hello, According to the release notes there is a new package that now contains the module Data.Array.Diff. According to Ian's commit this package is called diffarray. This package does not seem to be available on Hackage. Could someone upload it? Rene.

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hot-refreshing a package (same version number, new content)
by Albert Y. C. Lai 19 Jul '10

19 Jul '10

Suppose I start ghci, use a value from a package, causing it to be linked, then I modify the package and reinstall, same version number, without quitting my ghci session. Is there a way to tell the ghci session to refresh and use the modified package? Suppose I do something similar with the GHC API (6.12.3, linux, x86, 32 bit). The attachment g.hs is my program to get a value from the package (called watermelon-0.1), print it, wait for me to press return, (at this point I modify the source code of the package, rebuild, reinstall), then loop back to get a value from the package again... how should I refresh so I get the new value? Or I am supposed to forget this and just bump the version number of the package? I attach my program g.hs and the package tarball for watermelon-0.1.

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new qualified operators
by Christian Maeder 16 Jul '10

16 Jul '10

Hi, seeing http://www.haskell.org/ghc/docs/6.12.2/html/users_guide/syntax-extns.html#n… Could you add: "Spaces are not allowed in new qualified operators!" For {-# LANGUAGE NewQualifiedOperators #-} f1 = (+) f2 = ( +) f3 = (+ ) f4 = Prelude.(+) f5 = Prelude.( +) f6 = Prelude.(+ ) f1 - f4 are ok, but f5 and f6 are rejected by ghc-6.12.3 with: Not in scope: data constructor `Prelude' So ghc seems to see a composition and: data T = T (Int -> Int) f = T.( +) goes through, whereas f = T.(+) reports: Not in scope: `T.+' Note the old qualifiation in the error message. Christian

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