- Glasgow-haskell-users

Local evidence and type class instances
by Max Bolingbroke 02 Nov '10

02 Nov '10

Hi GHC users, Now that the Glorious New type checker can handle local evidence seamlessly, is it a big implementation burden to extend it to deal with local *type class instances* in addition to local *equality constraints*? For example, you could write this: """ f :: Bool f = id < id where instance Ord (a -> b) where _ `compare` _ = LT """ Not only would this prevent instances from "leaking" into all importing modules, but you could refer to *lexically scoped variables* in the type class instance. This would let programmers implement e.g. implicit configurations in the style of Kieslyov and Shan (http://okmij.org/ftp/Haskell/types.html#Prepose) without any mad hackery to tunnel pointers through the type system. Of course, it could lead to problems. For example: """ newtype MyInt = MyInt { unMyInt :: Int } f :: [Int] -> S.Set Int f xs = S.map unMyInt $ S.toList (map MyInt xs) where instance Ord MyInt where x `compare` y = unMyInt x `compare` unMyInt y g :: [Int] -> S.Set Int g xs = S.map unMyInt $ S.toList (map MyInt xs) where instance Ord MyInt where x `compare` y = unMyInt y `compare` unMyInt x main = do let s1 = f [1, 2] s2 = g [1, 2] print $ s1 == s2 -- prints False! print $ S.toList s1 == s2 -- prints True! """ I don't think this is a big problem. It would be up to the users of this extension to do the Right Thing. As Kieslyov and Shan point out, this extension also kills the principal types property - but so do GADTs. Just add more type signatures! Are there big theoretical problems with this extension, or is it just a lack of engineering effort that has prevented its implementation? Max

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GHC.Types consturctors with #
by Larry Evans 01 Nov '10

01 Nov '10

http://www.haskell.org/ghc/docs/6.10.2/html/libraries/ghc-prim/GHC-Types.ht… contains: data Int = I# Int# What does I# Int# mean? I've tried a simple interpretation: Prelude GHC.Types> I# 5# <interactive>:1:5: parse error (possibly incorrect indentation) Prelude GHC.Types> but obviously that failed :( TIA. -Larry

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Forall and type synonyms in GHC 7.0
by Mario Blažević 01 Nov '10

01 Nov '10

Before uploading a new version of my project on Hackage, I decided to future-proof it against GHC 7.0. I ran into several compile errors caused by the changes in let generalization, but these were easy to fix by adding extra type annotations. But then I ran into another problem that I can't fix so easily. Here is its trimmed-down reproduction: > {-# LANGUAGE RankNTypes #-} > > module Test where > > data Component c = Component {with :: c} > > pair1 :: (Bool -> c1 -> c2 -> c3) -> Component c1 -> Component c2 -> Component c3 > pair1 combinator (Component c1) (Component c2) = Component (combinator True c1 c2) > > type PairBinder m = forall x y r. (x -> y -> m r) -> m x -> m y -> m r > > pair2 :: Monad m => (PairBinder m -> c1 -> c2 -> c3) -> Component c1 -> Component c2 -> Component c3 > pair2 combinator = pair1 (combinator . chooseBinder) > > chooseBinder :: Monad m => Bool -> PairBinder m > chooseBinder right = if right then rightBinder else leftBinder > > leftBinder :: Monad m => PairBinder m > leftBinder f mx my = do {x <- mx; y <- my; f x y} > > rightBinder :: Monad m => PairBinder m > rightBinder f mx my = do {y <- my; x <- mx; f x y} The general idea here, if you're intrigued, is that pair1 belongs to a generic module that packages things it knows nothing about into Components. The remaining definitions belong to a client of the generic module, and pair2 is a specialization of pair1 to components that have something to do with monads. Now this little test compiles fine with GHC 6.12.1, but GHC 7.0.0.20101029 reports the following error in the pair2 definition: TestForall.lhs:13:42: Couldn't match expected type `forall x y r. (x -> y -> m r) -> m x -> m y -> m r' with actual type `(x -> y -> m1 r) -> m1 x -> m1 y -> m1 r' Expected type: Bool -> PairBinder m Actual type: Bool -> (x -> y -> m1 r) -> m1 x -> m1 y -> m1 r In the second argument of `(.)', namely `chooseBinder' In the first argument of `pair1', namely `(combinator . chooseBinder)' I've tried adding extra type annotations without making any progress. At this point I'm beginning to suspect I ran into a bug in GHC 7.0, but I can't find it in GHC Trac; the only ticket that looks similar is #4347, but that one works for me. Is this a bug? If not, how do I make my code compile?

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Type error in GHC-7 but not in GHC-6.12.3
by Bas van Dijk 01 Nov '10

01 Nov '10

Hello, I'm updating my usb-safe package for GHC-7: darcs get http://code.haskell.org/~basvandijk/code/usb-safe It depends on the HEAD version of regions: darcs get http://code.haskell.org/~basvandijk/code/regions I think I'm suffering from the new implied MonoLocalBinds extension (I'm using GADTs) as described in: http://hackage.haskell.org/trac/ghc/blog/LetGeneralisationInGhc7 However, I'm not sure this is the problem because I'm not using local bindings and use explicit type signatures everywhere. I try to make a small isolated example when I have time but for now let's use the actual definitions: The following function type-checked fine in GHC-6.12.3 but fails in GHC-7.1.20101027: withDeviceWhich ∷ ∀ pr α . MonadCatchIO pr ⇒ USB.Ctx → (USB.DeviceDesc → Bool) → (∀ s. RegionalDeviceHandle (RegionT s pr) → RegionT s pr α) → pr α withDeviceWhich ctx p f = do devs ← liftIO $ USB.getDevices ctx useWhich devs withDevice p f The error I get is: Couldn't match expected type `forall s. RegionalDeviceHandle (RegionT s pr) -> RegionT s pr α' with actual type `RegionalDeviceHandle (RegionT s pr) -> RegionT s pr α' In the fourth argument of `useWhich', namely `f' These are the types and definitions of the other functions involved: useWhich ∷ ∀ k desc e (m ∷ * → *) α . (GetDescriptor e desc, MonadIO m) ⇒ [e] → (e → k → m α) → (desc → Bool) → k → m α useWhich ds w p f = case find (p ∘ getDesc) ds of Nothing → throw USB.NotFoundException Just d → w d f withDevice ∷ MonadCatchIO pr ⇒ USB.Device → (∀ s. RegionalDeviceHandle (RegionT s pr) → RegionT s pr α) → pr α withDevice dev f = runRegionT $ openDevice dev >>= f Note that when I inline the definition of useWhich it type-checks fine: withDeviceWhich ctx p f = do devs ← liftIO $ USB.getDevices ctx case find (p ∘ getDesc) devs of Nothing → throw USB.NotFoundException Just d → withDevice d f Since I'm not using local bindings and use explicit type signatures everywhere, I'm not sure MonoLocalBinds is the problem. Note that other applications of useWhich which also use RankNTypes type-check just fine in both GHC-6.12.3 and GHC-7.1.20101027: ------------------------------------------------------------------------------- setConfigWhich ∷ ∀ pr cr α . (pr `AncestorRegion` cr, MonadCatchIO cr) ⇒ RegionalDeviceHandle pr → (USB.ConfigDesc → Bool) → (∀ sCfg. ConfigHandle sCfg → cr α) → cr α setConfigWhich h = useWhich (getConfigs h) setConfig getConfigs ∷ RegionalDeviceHandle r → [Config r] setConfig ∷ ∀ pr cr α . (pr `AncestorRegion` cr, MonadCatchIO cr) ⇒ Config pr → (∀ sCfg. ConfigHandle sCfg → cr α) → cr α ------------------------------------------------------------------------------- withInterfaceWhich ∷ ∀ pr sCfg α . MonadCatchIO pr ⇒ ConfigHandle sCfg → (USB.Interface → Bool) → (∀ s. RegionalInterfaceHandle sCfg (RegionT s pr) → RegionT s pr α) → pr α withInterfaceWhich h = useWhich (getInterfaces h) withInterface getInterfaces ∷ ConfigHandle sCfg → [Interface sCfg] withInterface ∷ ∀ pr sCfg α . MonadCatchIO pr ⇒ Interface sCfg → (∀ s. RegionalInterfaceHandle sCfg (RegionT s pr) → RegionT s pr α) → pr α ------------------------------------------------------------------------------- setAlternateWhich ∷ ∀ pr cr sCfg α . (pr `AncestorRegion` cr, MonadCatchIO cr) ⇒ RegionalInterfaceHandle sCfg pr → (USB.InterfaceDesc → Bool) → (∀ sAlt. AlternateHandle sAlt pr → cr α) → cr α setAlternateWhich h = useWhich (getAlternates h) setAlternate getAlternates ∷ RegionalInterfaceHandle sCfg r → [Alternate sCfg r] setAlternate ∷ ∀ pr cr sCfg α . (pr `AncestorRegion` cr, MonadCatchIO cr) ⇒ Alternate sCfg pr → (∀ sAlt. AlternateHandle sAlt pr → cr α) → cr α ------------------------------------------------------------------------------- I'm happy to provide more info when needed. Regards, Bas

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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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