June 2003 - Glasgow-haskell-users

Dynamic Compilation
by Carsten Schultz 12 Jun '03

12 Jun '03

Hallo, Ghci and Template Haskell show that it is possible to compile and load a module into a running program. Is there a user interface to do that? Would one be possible? Would that make a Haskell analog of Yaws (yaws.hyber.org) possible? For those who don't know Yaws: It is a web server written in Erlang that can compile and load Erlang code embedded in HTML files to produce dynamic pages. I have played with it a bit, and it seems useful to me, but I would prefer Haskell over Erlang. Greetings, Carsten -- Carsten Schultz (2:40, 33:47), FB Mathematik, FU Berlin http://www.math.fu-berlin.de/~cschultz/ PGP/GPG key on the pgp.net key servers, fingerprint on my home page.

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RE: type error
by Simon Peyton-Jones 12 Jun '03

12 Jun '03

You have two instance declarations: instance SubType a (Either a x) where instance (SubType a b) => SubType a (Either x b) where Which one should GHC use to resolve the constraint? (SubType x Value) It can't make an unambiguous choice, hence the error message. Simon | -----Original Message----- | From: glasgow-haskell-users-admin(a)haskell.org [mailto:glasgow-haskell-users-admin@haskell.org] | On Behalf Of Ed Komp | Sent: 02 June 2003 15:43 | To: glasgow-haskell-users(a)haskell.org | Subject: type error | | | I have encountered an unexpected type error of the form: | | >> Could not unambiguously deduce <type> from the context <context> | | I was able to generate a quite simple test case to demonstrate | the problem, that I include at the end of this message. | | I am using Version 5.04.2 of the Glasgow Compiler, with options: | | -fglasgow-exts -fallow-overlapping-instances -fallow-undecidable-instances | | Here is the output generated when I attempt to load the file, test.hs, | (test.hs appears at the end of this message): | | >> fleck [61] % ghci --version | >> The Glorious Glasgow Haskell Compilation System, version 5.04.2 | >> fleck [62] % ghci -fglasgow-exts -fallow-overlapping-instances | -fallow-undecidable-instances test.hs | >> ___ ___ _ | >> / _ \ /\ /\/ __(_) | >> / /_\// /_/ / / | | GHC Interactive, version 5.04.2, for Haskell 98. | >> / /_\\/ __ / /___| | http://www.haskell.org/ghc/ | >> \____/\/ /_/\____/|_| Type :? for help. | >> | >> Loading package base ... linking ... done. | >> Loading package haskell98 ... linking ... done. | >> Compiling Test ( test.hs, interpreted ) | >> | >> test.hs:26: | >> Could not unambiguously deduce (SubType x Value) | >> from the context (SubType x BaseType) | >> The choice of (overlapping) instance declaration | >> depends on the instantiation of `x' | >> Probable fix: | >> Add (SubType x Value) | >> to the existential context of a data constructor | >> Or add an instance declaration for (SubType x Value) | >> arising from use of `inj' at test.hs:26 | >> In the definition of `test': inj x | >> Failed, modules loaded: none. | >> Prelude> | | I do not understand why the compiler cannot deduce | (SubType x Value) from (SubType x BaseType) | | since: | | type Value = (Either Double BaseType) | | and | instance (SubType a b) => SubType a (Either x b) where | | The message actually says it cannot "unambiguously" make the | deduction. | | Can someone explain to me why this is, and more importantly | how I can resolve the issue? | | The suggestion, | >> Add (SubType x Value) | >> to the existential context of a data constructor | | will work in this simple example (I tested this); | but in the code where I originally encountered the problem | this is not a viable alternative since it would generate | circular dependencies. | | Thanks in advance for your help ! | | Ed Komp | komp(a)ittc.ku.edu | | -- ----------------------------------------------------------- | -- test.hs | -- ----------------------------------------------------------- | module Test where | | import Control.Monad.Error | | class SubType a b where | inj :: a -> b | prj :: b -> Maybe a | | instance SubType a (Either a x) where | inj = Left | prj = either Just (const Nothing) | | instance (SubType a b) => SubType a (Either x b) where | inj = Right . inj | prj = either (const Nothing) prj | | -- ----------------------------------------------------------- | | type BaseType = Either Integer ( Either Bool () ) | | type Value = (Either Double BaseType) | | data Foo = forall x. (SubType x BaseType) => MkFoo x | | test :: Foo -> Value | test (MkFoo x) = inj x | | _______________________________________________ | Glasgow-haskell-users mailing list | Glasgow-haskell-users(a)haskell.org | http://www.haskell.org/mailman/listinfo/glasgow-haskell-users

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RE: Conditions on deriving Typeable
by Simon Peyton-Jones 12 Jun '03

12 Jun '03

It's a bug. Thank you Simon | -----Original Message----- | From: glasgow-haskell-users-admin(a)haskell.org [mailto:glasgow-haskell-users-admin@haskell.org] | On Behalf Of George Russell | Sent: 11 June 2003 13:07 | To: glasgow-haskell-users(a)haskell.org | Subject: Conditions on deriving Typeable | | Please satisfy my curiosity, what is it about | | data WrappedDisplayType = forall displayType . | DisplayType displayType => WrappedDisplayType displayType | deriving (Typeable) | | that causes GHC 6.0 to utter | | DisplayTypes.hs:135: | Can't make a derived instance of `Typeable WrappedDisplayType' | (`WrappedDisplayType' has existentially-quantified constructor(s)) | When deriving instances for type `WrappedDisplayType' | | Obviously it doesn't like the existential quantifier, but why should | that be a problem for Typeable? | | _______________________________________________ | Glasgow-haskell-users mailing list | Glasgow-haskell-users(a)haskell.org | http://www.haskell.org/mailman/listinfo/glasgow-haskell-users

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RE: bug with definition at top-level:
by Simon Peyton-Jones 12 Jun '03

12 Jun '03

This embarrassing bug in GHC 6.0 has been reported several times now. There's a good workaround: just say :t GHC.Err.error as your first command in GHCi. Then it'll be happy with your subsequent definition. Simon | -----Original Message----- | From: glasgow-haskell-bugs-admin(a)haskell.org [mailto:glasgow-haskell-bugs-admin@haskell.org] On | Behalf Of Hal Daume III | Sent: 11 June 2003 23:56 | To: glasgow-haskell-bugs(a)haskell.org | Subject: bug with definition at top-level: | | 3:54pm moussor:handalign/ ghci | ___ ___ _ | / _ \ /\ /\/ __(_) | / /_\// /_/ / / | | GHC Interactive, version 6.0, for Haskell 98. | / /_\\/ __ / /___| | http://www.haskell.org/ghc/ | \____/\/ /_/\____/|_| Type :? for help. | | Loading package base ... linking ... done. | Loading package lang ... linking ... done. | Prelude> let f s = let (p1:_:p2:_:r1:_:f2:_) = map read (words s) in show | ((2*p1*r1)/(p1+r1)) ++ " & " ++ show ((2*p2*f2)/(p2+f2)) | ghc-6.0: panic! (the `impossible' happened, GHC version 6.0): | getLinkDeps No iface for [<pkg>]GHCziErr | | Please report it as a compiler bug to glasgow-haskell-bugs(a)haskell.org, | or http://sourceforge.net/projects/ghc/. | | | | -- | Hal Daume III | hdaume(a)isi.edu | "Arrest this man, he talks in maths." | www.isi.edu/~hdaume | | _______________________________________________ | Glasgow-haskell-bugs mailing list | Glasgow-haskell-bugs(a)haskell.org | http://www.haskell.org/mailman/listinfo/glasgow-haskell-bugs

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Conditions on deriving Typeable
by George Russell 11 Jun '03

11 Jun '03

Please satisfy my curiosity, what is it about data WrappedDisplayType = forall displayType . DisplayType displayType => WrappedDisplayType displayType deriving (Typeable) that causes GHC 6.0 to utter DisplayTypes.hs:135: Can't make a derived instance of `Typeable WrappedDisplayType' (`WrappedDisplayType' has existentially-quantified constructor(s)) When deriving instances for type `WrappedDisplayType' Obviously it doesn't like the existential quantifier, but why should that be a problem for Typeable?

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RE: Problems building GHC 6.0 with gcc 3.3
by Simon Marlow 10 Jun '03

10 Jun '03

> I'm having problems building GHC 6.0 with GCC 3.3. I've fixed a couple > of problems with comments in CPP macros as seen elsewhere, and then > everything seemed to be going fine until it came to create > stage2/ghc-6.0 and it failed with a huge number of things like > > stage2/utils/Util.o(.text+0xb8): In function `r4c8_entry': > : undefined reference to `GHCziBase_True_closure' > stage2/utils/Util.o(.text+0xc8): In function `r4c8_entry': > : undefined reference to `GHCziBase_False_closure' Fixed in CVS, thanks for the report. Workaround: don't use GCC 3.3 (yet). Cheers, Simon

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RE: directories not closed?
by Simon Marlow 10 Jun '03

10 Jun '03

> I regularly call getDirectoryContents on a certain directory > looking for > changes, when I 'lsof' my process (list open files) it has an absurd > number of open descriptors to said directory open. I didn't see an > obvious bug in getDirectoryContents, does anyone know what is > going on? Hmm. It looks fine to me: I just tried a test doing getDirectoryContents repeatedly on the same directory and it re-uses the same descriptor each time. An strace might be useful... > Also, did the fixes for sockets not getting properly closed all the > time make it into 6.00? I am trying to track down some out of > descriptor problems. Yes, all the fixes to Network are in 6.0. Cheers, Simon

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RE: happy, ghc and {# OPTIONS #} pragma
by Simon Marlow 10 Jun '03

10 Jun '03

> Martin Norbäck <d95mback(a)dtek.chalmers.se> writes: > > > {-# OPTIONS -fglasgow-exts -cpp #-} > > -- parser produced by Happy Version 1.13 > > {-# OPTIONS -fno-warn-unused-matches #-} > > Generally, it'd be nice to be able to occasionally suppress warnings > for sections of code (with better than module granularity), where you > know the code is safe. > > Any chance of something like that? Yes, it would indeed be useful. I doubt we'll get around to it in the near future, though. Cheers, Simon

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Bom dia glasgow-haskell-users
by kandrak 10 Jun '03

10 Jun '03

Message to glasgow-haskell-users(a)haskell.org Ola!!!! glasgow-haskell-users GANHAMOS O PRÊMIO DE MELHOR SITE DO RAMO ======Estamos operando em Novo Formato====== Confira em: escuta21.kit.net ou http://www.escuta21.kit.net ei.... glasgow-haskell-users Cuidado com o que fala ao Celular... ele tb tem ouvidos... para remover o glasgow-haskell-users(a)haskell.org de nossa lista responda este e-mail e coloque remover nos perdoe o transtorno...ok? kandrak

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happy, ghc and {# OPTIONS #} pragma
by Martin Norbäck 10 Jun '03

10 Jun '03

I am trying to get an {-# OPTIONS #-} pragma to appear in my Happy generated file, since I use warnings and want to disable them for this file. I'm putting the pragma first in the module header section of my Happy file. My problem is that the generated file starts like this (the first pragma is generated by Happy): {-# OPTIONS -fglasgow-exts -cpp #-} -- parser produced by Happy Version 1.13 {-# OPTIONS -fno-warn-unused-matches #-} This means that GHC will only see the first one. Is there a way to make Happy put the pragma above the comment, or not produce the comment at all? Or is there a way to make GHC recognise the pragma even though it is after a comment? Regards, Martin -- Martin Norbäck d95mback(a)dtek.chalmers.se Kapplandsgatan 40 +46 (0)708 26 33 60 S-414 78 GÖTEBORG http://www.dtek.chalmers.se/~d95mback/ SWEDEN OpenPGP ID: 3FA8580B

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