December 2012 - Glasgow-haskell-users

ANNOUNCE: GHC 7.6.2 Release Candidate 1
by Ian Lynagh 20 Mar '13

20 Mar '13

We are pleased to announce the first release candidate for GHC 7.6.2: http://www.haskell.org/ghc/dist/7.6.2-rc1/ This includes the source tarball, installers for Windows, and bindists for Windows, Linux, OS X and FreeBSD, on x86 and x86_64. We plan to make the 7.6.2 release early in 2013. Please test as much as possible; bugs are much cheaper if we find them before the release! Thanks Ian, on behalf of the GHC team

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Re: Fundeps and type equality
by Iavor Diatchki 11 Jan '13

11 Jan '13

Hello Conal, GHC implementation of functional dependencies is incomplete: it will use functional dependencies during type inference (i.e., to determine the values of free type variables), but it will not use them in proofs, which is what is needed in examples like the one you posted. The reason some proving is needed is that the compiler needs to figure out that for each instantiation of the type `ta` and `tb` will be the same (which, of course, follows directly from the FD on the class). As far as I understand, the reason that GHC does not construct such proofs is that it can't express them in its internal proof language (System FC). It seems to me that it should be fairly straight-forward to extend FC to support this sort of proof, but I have not been able to convince folks that this is the case. I could elaborate, if there's interest. In the mean time, the way forward would probably be to express the dependency using type families, which I find tends to be more verbose but, at present, is better supported in GHC. Cheers, -Iavor PS: cc-ing the GHC users' list, as there was some talk of closing the ghc-bugs list, but I am not sure if the transition happened yet. On Tue, Dec 25, 2012 at 6:15 PM, Conal Elliott <conal(a)conal.net> wrote: > I ran into a simple falure with functional dependencies (in GHC 7.4.1): > > > class Foo a ta | a -> ta > > > > foo :: (Foo a ta, Foo a tb, Eq ta) => ta -> tb -> Bool > > foo = (==) > > I expected that the `a -> ta` functional dependency would suffice to prove > that `ta ~ tb`, but > > Pixie/Bug1.hs:9:7: > Could not deduce (ta ~ tb) > from the context (Foo a ta, Foo a tb, Eq ta) > bound by the type signature for > foo :: (Foo a ta, Foo a tb, Eq ta) => ta -> tb -> Bool > at Pixie/Bug1.hs:9:1-10 > `ta' is a rigid type variable bound by > the type signature for > foo :: (Foo a ta, Foo a tb, Eq ta) => ta -> tb -> Bool > at Pixie/Bug1.hs:9:1 > `tb' is a rigid type variable bound by > the type signature for > foo :: (Foo a ta, Foo a tb, Eq ta) => ta -> tb -> Bool > at Pixie/Bug1.hs:9:1 > Expected type: ta -> tb -> Bool > Actual type: ta -> ta -> Bool > In the expression: (==) > In an equation for `foo': foo = (==) > Failed, modules loaded: none. > > Any insights about what's going wrong here? > > -- Conal > > _______________________________________________ > Glasgow-haskell-bugs mailing list > Glasgow-haskell-bugs(a)haskell.org > http://www.haskell.org/mailman/listinfo/glasgow-haskell-bugs > >

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Suppress "Duplicate constraints" warning?
by Conal Elliott 03 Jan '13

03 Jan '13

Is there a way to suppress GHC's "Duplicate constraints" warning? I'm auto-generating some code, and it's a lot more convenient for me to leave the duplicates than filter them out. -- Conal

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Is there a workaround for this bug?
by Dominic Steinitz 01 Jan '13

01 Jan '13

AFAICT this bug fix http://hackage.haskell.org/trac/ghc/ticket/7239 did not make it into 7.6.1. Also I am happily working on the Haskell Platform with 7.4.1 and I'd rather avoid upgrading if possible. Is there a workaround? I've attached my code below along with the error message (which is the same as in the above bug report). I'm rather hoping I won't have to build HEAD. Thanks, Dominic. bash-3.2$ ghc -fext-core --make Test.hs [1 of 1] Compiling Main ( Test.hs, Test.o ) ghc: panic! (the 'impossible' happened) (GHC version 7.4.1 for x86_64-apple-darwin): MkExternalCore died: make_lit {-# LANGUAGE FlexibleContexts #-} {-# OPTIONS_GHC -Wall -fno-warn-name-shadowing -fno-warn-type-defaults #-} import Data.Array.Repa as Repa import Data.Array.Repa.Eval import Control.Monad r, sigma, k, t, xMax, deltaX, deltaT :: Double m, n :: Int r = 0.05 sigma = 0.2 k = 50.0 t = 3.0 m = 80 xMax = 150 deltaX = xMax / (fromIntegral m) n = 800 deltaT = t / (fromIntegral n) data PointedArrayU a = PointedArrayU Int (Array U DIM1 a) deriving Show f :: PointedArrayU Double -> Double f (PointedArrayU j _x) | j == 0 = 0.0 f (PointedArrayU j _x) | j == m = xMax - k f (PointedArrayU j x) = a * x! (Z :. j-1) + b * x! (Z :. j) + c * x! (Z :. j+1) where a = deltaT * (sigma^2 * (fromIntegral j)^2 - r * (fromIntegral j)) / 2 b = 1 - deltaT * (r + sigma^2 * (fromIntegral j)^2) c = deltaT * (sigma^2 * (fromIntegral j)^2 + r * (fromIntegral j)) / 2 priceAtT :: PointedArrayU Double priceAtT = PointedArrayU 0 (fromListUnboxed (Z :. m+1) [ max 0 (deltaX * (fromIntegral j) - k) | j <- [0..m] ]) coBindU :: (Source U a, Source U b, Target U b, Monad m) => PointedArrayU a -> (PointedArrayU a -> b) -> m (PointedArrayU b) coBindU (PointedArrayU i a) f = computeP newArr >>= return . PointedArrayU i where newArr = traverse a id g where g _get (Z :. j) = f $ PointedArrayU j a testN :: Int -> IO (PointedArrayU Double) testN n = h priceAtT where h = foldr (>=>) return (take n $ Prelude.zipWith flip (repeat coBindU) (repeat f)) main :: IO () main = do r <- testN n putStrLn $ show r

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Pragmas in Lexer
by JP Moresmau 29 Dec '12

29 Dec '12

Hello, I'm using the GHC lexer to parse some haskell code containing a language pragma, in GHC 7.4.2 module Main where import GHC import GHC.Paths ( libdir ) import Lexer import qualified MonadUtils as GMU import StringBuffer import FastString (mkFastString) import SrcLoc import ErrUtils (mkPlainErrMsg) main::IO() main = do let contents="{-# LANGUAGE CPP #-}\nmodule Main where\nmain=undefined" runGhc (Just libdir) $ do flg <- getSessionDynFlags let sb=stringToStringBuffer contents let lexLoc = mkRealSrcLoc (mkFastString "<interactive>") 1 1 let prTS = lexTokenStream sb lexLoc flg case prTS of POk _ toks -> GMU.liftIO $ print $ map (show . unLoc) toks PFailed l msg -> GMU.liftIO $ print $ mkPlainErrMsg l msg This prints: ["ITblockComment \" CPP #\"","ITmodule","ITconid \"Main\"","ITwhere","ITvocurly","ITvarid \"main\"","ITequal","ITvarid \"undefined\""] Why is the first token ITblockComment and not ITlanguage_prag? Do I need to enable something special to get pragma tokens? Thanks! -- JP Moresmau http://jpmoresmau.blogspot.com/

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Updating commentary about PrimOps and codegen
by Jan Stolarek 23 Dec '12

23 Dec '12

Hi all, I've been reading Commentary about PrimOps and code generation and I noticed that it is partially outdated (e.g. references to old code gen files: compiler/codeGen/CgPrimOp.hs): http://hackage.haskell.org/trac/ghc/wiki/Commentary/PrimOps http://hackage.haskell.org/trac/ghc/wiki/Commentary/Compiler/CodeGen Would anyone be so kind to update these pages and possibly other pages about code generation, STG, Cmm and related stuff? I've been trying to figure out how to add new PrimOps and having up to date documentation would be most helpful. Janek

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Separating build tree from the source tree
by Jan Stolarek 21 Dec '12

21 Dec '12

Dear list, I have a GHC source tree in ghc-working. I would like to have a separate build tree in ghc-build, so I did something like this: mkdir ghc-build cd ghc-build lndir ../ghc-working Now, I have two different machines. On openSUSE with git 1.8.0 I get something like this: [killy@xerxes : /dane/uczelnia/projekty/ghc-build] ./configure checking for gfind... no checking for find... /usr/bin/find checking for GHC version date... configure: WARNING: cannot determine snapshot version: no .git or _darcs directory and no VERSION file Despite the warning configure succeeds. On Debian Squeeze with git 1.7.10 the same thing doesn't work: [killy@GLaDOS : /dane/uczelnia/projekty/ghc-build] ./configure checking for gfind... no checking for find... /usr/bin/find checking for GHC version date... fatal: Not a git repository (or any of the parent directories): .git configure: error: failed to detect version date: check that git is in your path Does anyone have a clue why this happens? There are probably some differences on these systems with the most obvious one being git version. On both systems however git doesn't recognize ghc-build as a valid git repository ('git status' failes), but on one of them this is only a warning while on another this is an error. Any advice will be appreciated. Janek

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Tagging subjects of emails sent from the list
by Jan Stolarek 20 Dec '12

20 Dec '12

Would it be possible to change mailing list settings so that topics of emails begin with "[glasgow-haskell-users]" (and for upcoming lists: "[ghc-devs]", "[ghc-commits]" and so on. No quotation marks of course). It would make filtering and searching in the mailbox easier. Janek

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SpecConstr and GADTs
by Nicolas Frisby 19 Dec '12

19 Dec '12

I suspect I've found a situation where, to a first approximation, I'd like SpecConstr to run more than once. I'm specializing on GADT constructors that contain coercions, so the constructor-specialized functions have RHSs with refined types. In my situation, those refined types enable simplifications/specializations that eventually result in additional calls that would lead to further constructor-specializations if the eventual calls were somehow already manifest. But it seems that the simplification/specialization only happens after the SpecConstr pass, so the resulting usages don't lead to further constructor-specialized functions. Ticket 2642 http://hackage.haskell.org/trac/ghc/ticket/2642 is the only mention I've seen of the SpecConstr pass being repeated. The more I think about my desired behavior of SpecConstr, the more I suspect that a failure to maximally share the constructor-specializations (leading to possible non-termination) might be a concern if the pass were simply repeated in my particular case. Regarding broader impact, I suspect this situation might arise with heavy uses of single types. Is there any known mechanisms to handle this interaction between SpecConstr and GADTs? I'll be able to provide concrete details about my situation in a few weeks, if anyone is interested. Thanks for your time.

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How to start with GHC development?
by Jan Stolarek 19 Dec '12

19 Dec '12

Dear list, I'm reposting my message from Haskell-cafe here since this seems like a more appropriate place to ask this question. I would like to learn about internals of GHC and contribute to its development in the future. I read a couple of papers that give a very general overview of GHC (chapter from AoS, papers about inliner and multicore support) and I'm thinking what direction should I pursue now. I got the GHC sources and started reading commentary on the wiki, but it seems that entry barrier is very high. Aside from the problem of understanding the whole project organization itself, I also don't have background on implementation of functional languages. In fact I have a rather basic knowledge of compilers in general - I only took Stanford's online course on Compilers at Coursera. I was thinking that perhaps I should read SPJs "Implementing Functional Languages: a tutorial" or "The Implementation Of Functional Languages" book to learn more about theory? On the other hand I don't know if getting stuck in theory for next couple of months is a good idea. I would greatly appreciate any advice and help. Janek

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