October 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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Non-exhaustive pattern match(es) warning in lambda functions
by Mitar 18 Nov '10

18 Nov '10

Hi! Why GHC does not warn (with -Wall) about non-exhaustive pattern match(es) warning in lambda functions? For example, this code: data Foo = Bar | Baz test1 :: Foo -> IO () test1 Bar = return () -- Pattern match(es) are non-exhaustive warning, OK test2 :: Foo -> IO () test2 = \Bar -> return () -- No pattern match(es) are non-exhaustive, BAD I think it would be quite useful to also catch such situations and issue a warning. Mitar

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Proposal: Require OverlappingInstances for the most specific instance only
by John Smith 16 Nov '10

16 Nov '10

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.

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Wadler space leak
by Jan Christiansen 09 Nov '10

09 Nov '10

Hi, I have a question regarding the famous Wadler space leak. The following program is a variation of Wadler's example. let (first,rest) = break (const False) input in print (length (first ++ rest)) When I compile this program using -O2 and use a large text file as input the code runs in constant space. If I understand correctly, the program runs in constant space because ghc uses an optimization similar to the one proposed by Wadler/Sparud. If I define the following function which is based on break splitBy :: (a -> Bool) -> [a] -> [[a]] splitBy _ [] = [] splitBy p xs = l : case ys of [] -> [] _:zs -> splitBy' p zs where (l,ys) = break p xs and compile the following program the behaviour is different. print (length (concat (splitBy (const False) input))) In this case the memory usage is not constant. However if I use a strict pattern matching instead of the lazy tuple matching the program runs in constant space. splitBy' :: (a -> Bool) -> [a] -> [[a]] splitBy' _ [] = [] splitBy' p xs = case break p xs of (l,ys) -> l : case ys of [] -> [] _:zs -> splitBy p zs To me this looks like another instance of the Wadler space leak. Is this correct? I do not understand why the Wadler example runs in constant space and this example does not. Can anyone explain me why these functions behave differently? I still get the same behaviour if I use the following simplified function. test :: (a -> Bool) -> [a] -> [[a]] test _ [] = [] test p xs = l : case ys of [] -> [] where (l,ys) = break p xs That is, the following program does not run in constant space. print (length (concat (test (const False) input))) On a related note if I do not use -O2 the following program runs in constant space let (first,rest) = break (const False) input in print (length (first ++ rest)) while it does not run in constant space if I add an application of id as follows. let (first,rest) = break (const False) input in print (length (first ++ id rest)) Thanks, Jan

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ANNOUNCE: GHC 7.0.1 Release Candidate 2
by Ian Lynagh 09 Nov '10

09 Nov '10

We are pleased to announce the second release candidate for GHC 7.0.1: http://new-www.haskell.org/ghc/dist/7.0.1-rc2/ This includes the source tarball, installers for OS X and Windows, and bindists for amd64/Linux, i386/Linux, amd64/FreeBSD and i386/FreeBSD. Amongst the changes since RC 1 are: * -fglasgow-exts now gives a deprecated warning, and no longer enables GADTs or TypeFamilies. * The haskell98 packge is no longer automatically linked, although this often won't make a difference as --make is now enabled by default. 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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new-www
by Serge D. Mechveliani 09 Nov '10

09 Nov '10

People, what is, in short, the relation between www.haskell.org and new-www.haskell.org ? Which one do I need to use for looking for the Haskell materials, for GHC materials? Regards, ----------------- Serge Mechveliani mechvel(a)botik.ru

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Fixing LDAP lib compilation on OpenBSD
by Julien Dessaux 03 Nov '10

03 Nov '10

Hi GHC folks, I'm using the LDAP lib for one of my projects and I found a problem while building it on an OpenBSD system. It wouldn't compile because there is a macro named differently in the ldap.h include file. Under linux, this macro is named LDAP_X_PROXY_AUTHZ_FAILURE but on OpenBSD (and probably other BSD flavours), it's named LDAP_PROXY_AUTHZ_FAILURE. I attached the diff I wrote in order to compile the lib on OpenBSD, but it's not a patch I can submit cause it now won't compile on Linux. How can I amend this in order to have a code that would compile on both systems? How is it possible to specify such conditional system dependent stuff for a C binding? Thanks in advance for your help, -- Julien Dessaux

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making 7.01-pre
by Serge D. Mechveliani 02 Nov '10

02 Nov '10

Dear GHC developers, I am testing this fresh ghc-7.0.0.20101028 on Debian Linux, i386-family. Making it from source by ghc-6.12.3 is all right. Then, making it from source by itself reports (here I abbreviate the messages by inserting `...') --------------------------------------------------------------------- + test -f mk/config.mk.old + cp -p mk/config.mk mk/config.mk.old ... ... "inplace/bin/mkdirhier" utils/ghc-cabal/dist/build/tmp//. "inplace/bin/mkdirhier" bootstrapping/. "/home/mechvel/ghc/7.01pre/inst1/bin/ghc" -H32m -O --make utils/ghc-cabal/ghc-cabal.hs -o utils/ghc-cabal/dist/build/tmp/ghc-cabal ... ... "rm" -f compiler/stage1/ghc_boot_platform.h Creating compiler/stage1/ghc_boot_platform.h... Done. "/usr/bin/gcc" -fno-stack-protector -DTABLES_NEXT_TO_CODE -Iincludes -Irts -DGEN_HASKELL -c includes/mkDerivedConstants.c -o includes/dist-ghcconstants/build/mkDerivedConstants.o ... ... ... utils/genprimopcode/dist/build/Lexer.o: In function `s2yT_info': (.text+0x1e1d): undefined reference to `__stginit_arrayzm0zi3zi0zi2_DataziArray_' ... ... utils/genprimopcode/dist/build/Parser.o: In function `s4pK_info': (.text+0x5691): undefined reference to `__stginit_arrayzm0zi3zi0zi2_DataziArray_' collect2: ld returned 1 exit status make[1]: *** [utils/genprimopcode/dist/build/tmp/genprimopcode] Error 1 make: *** [all] Error 2 --------------------------------------------------------------------- Why cannot it make itself? Regards, ----------------- Serge Mechveliani mechvel(a)botik.ru

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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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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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