- Glasgow-haskell-users

RE: Type error in GHC-7 but not in GHC-6.12.3
by Simon Peyton-Jones 01 Nov '10

01 Nov '10

| foo :: (forall s. ST s a) -> a | foo st = ($) runST st This is a motivating example for type inference that can deal with impredicative types. Consider the type of ($): ($) :: forall p q. (p->q) -> p -> q In the example we need to instantiate 'p' with (forall s. ST s a), and that's what impredicative polymorphism means: instantiating a type variable with a polymorphic type. Sadly, I know of no system of reasonable complexity that can typecheck 'foo' unaided. There are plenty of complicated systems, and I have been a co-author on papers on at least two, but they are all Too Jolly Complicated to live in GHC. We did have an implementation of boxy types, but I took it out when implementing the new typechecker. Nobody understood it. However, people so often write runST $ do ... that in GHC 7 I implemented a special typing rule, just for infix uses of ($). Just think of (f $ x) as a new syntactic form, with the obvious typing rule, and away you go. It's very ad hoc, because it's absolutely specific to ($), and I'll take it out if you all hate it, but it's in GHC 7 for now. Anyway, that's why you encountered the puzzling behaviour you describe below. Simon | -----Original Message----- | From: Bas van Dijk [mailto:v.dijk.bas@gmail.com] | Sent: 30 October 2010 21:14 | To: glasgow-haskell-users(a)haskell.org | Cc: Simon Peyton-Jones | Subject: Re: Type error in GHC-7 but not in GHC-6.12.3 | | On Sat, Oct 30, 2010 at 1:57 AM, Bas van Dijk <v.dijk.bas(a)gmail.com> wrote: | > I could isolate it a bit more if you want. | | And so I did. The following is another instance of the problem I'm | having but set in a more familiar setting: | | {-# LANGUAGE RankNTypes #-} | | import Control.Monad.ST | | foo :: (forall s. ST s a) -> a | foo st = ($) runST st | | Couldn't match expected type `forall s. ST s a' | with actual type `ST s a' | In the second argument of `($)', namely `st' | | Note that: 'foo st = runST st' type checks as expected. | | Surprisingly 'foo st = runST $ st' also type checks! | | I find the latter surprising because according to the report[1]: e1 op | e2 = (op) e1 e2. So either both should type check or both should fail. | | I guess that a RULE somewhere eliminates the ($) before the | type-checker kicks in. I do find that a little strange because I | thought RULES where applied after type checking. | | Regards, | | Bas | | [1] http://www.haskell.org/onlinereport/exps.html#operators

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Re: [Haskell-cafe] Equivalent of withForeignPtr for System.Mem.Weak?
by Antoine Latter 01 Nov '10

01 Nov '10

On Sun, Oct 31, 2010 at 10:14 PM, Matthew Steele <mdsteele(a)alum.mit.edu> wrote: > I have an object to which I have added one or more finalizers via > addFinalizer from System.Mem.Weak. I would like to have a function that > allows me to make use of the object within a block of IO code, and guarantee > that the finalizer(s) will not be called during the code block -- sort of > like withForeignPtr, but for arbitrary objects. Does such a function exist, > and if not, how could I write one? > > I can imagine writing something like: > > \begin{code} > withObject :: a -> IO b -> IO b > withObject obj action = do > result <- action > touchObject obj -- touchObject :: a -> IO () > return result > \end{code} > CC'ing GHC-users list. The 'primitive' package has a general purpose 'touch' function: http://hackage.haskell.org/packages/archive/primitive/0.3.1/doc/html/Contro… But it doesn't state what guarantees are provided by the function. Antoine

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network programming with GHC 7
by Kazu Yamamoto 31 Oct '10

31 Oct '10

Hello, When I compiled a network server with GHC 7 without the "-threaded" option and ran it, I got the following error. file descriptor 5496824 out of range for select (0--1024). I have read the paper "Scalable Event Handling for GHC" and understand that this new IO manger assumes the "-threaded" option. Does this mean that we need the "-threaded" option to GHC 7 for networking programming and GHC 7 does not maintain backward compatibility? If so, this should be documented. --Kazu

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newbie: how to expose a package?
by Larry Evans 28 Oct '10

28 Oct '10

With the following .hs: ==<--TestPrettyPrintLeijen.hs--> module TestPrettyPrintLeijen where -- import Text.PrettyPrint.Leijen {- Above import causes error message: TestPrettyPrintLeijen.hs:3:7: Could not find module `Text.PrettyPrint.Leijen': Use -v to see a list of the files searched for. -} import Debug.QuickCheck.Batch {- Above import based on exposed-modules: section of: /usr/lib/ghc-6.12.1/package.conf.d/QuickCheck-1.2.0.0.conf However: ghci TestPrettyPrintLeijen.hs produces error: TestPrettyPrintLeijen.hs:9:7: Could not find module `Debug.QuickCheck.Batch': It is a member of the hidden package `QuickCheck-1.2.0.0'. Use -v to see a list of the files searched for. -} import GHC.Paths {- Above import based on exposed-modules: section of: http://hackage.haskell.org/packages/archive/ghc-paths/0.1.0.8/ghc-paths.cab… -} ==</--TestPrettyPrintLeijen.hs--> ghc still fails to find the QuickCheck as shown by: ==<--ghci run--> $ sudo ghc-pkg expose QuickCheck-1.2.0.0 $ ghci TestPrettyPrintLeijen.hs GHCi, version 6.12.1: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer-gmp ... linking ... done. Loading package base ... linking ... done. TestPrettyPrintLeijen.hs:9:7: Could not find module `Debug.QuickCheck.Batch': It is a member of the hidden package `QuickCheck-1.2.0.0'. Use -v to see a list of the files searched for. Failed, modules loaded: none. Prelude> ==</--ghci run--> How does one enable import of a module from some package? TIA. -regards, Larry

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Unaligned word-sized reads on ByteArray#
by Johan Tibell 27 Oct '10

27 Oct '10

Hi, I noticed that indexWordArray# only allows for aligned reads (by forcing the offset to be in words, rather than in bytes.) Is it possible to perform unaligned reads on a ByteArray# e.g. going via Addr#? There's the byteArrayContent# primitive but I don't know how to force the ByteArray# to stay pinned in memory while performing my reads. The ByteArray# is not guaranteed to be pinned in my case. A somewhat related question: are there any defines (in e.g. MachDeps.h) that I could use to detect whether the machine supports unaligned reads? For example, if I knew the processor architecture, I could figure that out. Johan

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Re: Proposal: Require OverlappingInstances for the most specific instance only
by Christian Höner zu Siederdissen 26 Oct '10

26 Oct '10

Hi, can't remember to have been hit by it; but changing would mean that library maintainers can act more conservatively. Otherwise one needs to be able to guess if an instance might be overlapped. The new version wouldn't require that, yes? Gruss, Christian * John Smith <voldermort(a)hotmail.com> [26.10.2010 21:45]: > In the case of overlapping instance declarations, GHC currently > requires the less specific instance to be compiled with > OverlappingInstances for the more specific instance to be usable. > This means that, for example, if you write > > type ChessBoard = Array (Int,Int) Piece > > there is no way to > > instance Show ChessBoard where > show board = ... > > because Array is already an instance of Show, and was not compiled with OverlappingInstances. > > http://hackage.haskell.org/trac/ghc/ticket/3877 requests that > OverlappingInstances be required for only the most specific instance > declarations, as suggested in the documentation. "Perhaps the rule > should instead say that the overlapping instance declaration should > be compiled in this way, rather than the overlapped one ... We are > interested to receive feedback on these points." > > SPJ offered to make this change, if the list users agree. (See > comment 6.) Proposed deadline for discussion is 16th November. > > _______________________________________________ > Glasgow-haskell-users mailing list > Glasgow-haskell-users(a)haskell.org > http://www.haskell.org/mailman/listinfo/glasgow-haskell-users

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Implicit 'forall' in data declarations
by Sebastian Fischer 25 Oct '10

25 Oct '10

Hello, GHC 6.12.3 allows to omit the explicit quantification of higher-rank type variables using 'forall' in data types if they appear in a type class context {-# LANGUAGE RankNTypes #-} data Foo = Foo (Eq a => a) Is this implicit introduction of 'forall' intended? If it is, why does it not work in function types? The following is not accepted by my GHC: bar :: Eq b => (Eq a => a) -> b bar x = x The error message is All of the type variables in the constraint `Eq a' are already in scope (at least one must be universally quantified here) (Use -XFlexibleContexts to lift this restriction) Using `FlexibleContexts` the signature of `bar` seems to be interpreted as bar :: (Eq b, Eq a) => a -> b because then the error becomes Couldn't match expected type `b' against inferred type `a' So unlike in data-type declarations, a 'forall' in a function type must be written explicitly even if the quantified variable appears in a local type class constraint. bar :: Eq b => (forall a . Eq a => a) -> b bar x = x I have not yet installed GHC 7. Is this inconsistency between data and function declarations intended or has it been changed in the new type checker? Sebastian

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ghc7 typing problem
by Christian Maeder 25 Oct '10

25 Oct '10

Hi, I've looked at: http://hackage.haskell.org/trac/ghc/blog/LetGeneralisationInGhc7 but still have a problem with the attached program that fails with: GHCi, version 7.1.20101010: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer-gmp ... linking ... done. Loading package base ... linking ... done. Loading package ffi-1.0 ... linking ... done. [1 of 1] Compiling Main ( HasKey.hs, interpreted ) HasKey.hs:17:10: Couldn't match type `key' with `key1' because this skolem type variable would escape: `key1' This skolem is bound by the instance declaration In the instance declaration for `Ord (Keyed x)' at the final instance for Ord. The original code comes from http://hackage.haskell.org/packages/archive/uni-util/2.2.0.0/doc/html/Util-… Cheers Christian

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ANNOUNCE: GHC 7.0.1 Release Candidate 1
by Ian Lynagh 25 Oct '10

25 Oct '10

We are pleased to announce the first release candidate for GHC 7.0.1: http://new-www.haskell.org/ghc/dist/7.0.1-rc1/ This includes the source tarball, installers for OS X and Windows, and bindists for amd64/Linux and i386/Linux. 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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GHC static binaries with glibc-2.12?
by Alexander Dunlap 23 Oct '10

23 Oct '10

Hi all, I recently upgraded my Arch Linux system to glibc 2.12 and static binaries compiled with GHC 6.12.3 all fail with the message "mkTextEncoding: invalid argument (Invalid argument)". This did not happen with glibc 2.11. Has anyone else had this problem? Does anyone have any advice for debugging it? Thanks, Alex

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