August 2007 - 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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ghci and ghc -threaded broken with pipes & forking
by John Goerzen 29 Dec '08

29 Dec '08

Hi, I've been hitting my head against a wall for the past couple of days trying to figure out why my shell-like pipeline code kept hanging. I found fd leakage (file descriptors not being closed), which disrupts EOF detection and can lead to deadlocks. I just couldn't find the problem. I finally tried compiling my test with ghc instead of running it in ghci. And poof, it worked fine the first time. I tried asking on #haskell, and got the suggestion that ghci uses -threaded. I tried compiling my test program with ghc -threaded, and again, same deadlock. My program never calls forkIO or forkOS or any other threading code. You can see my test case with: darcs get '--tag=glasgow ml' http://darcs.complete.org/hsh ghc -fglasgow-exts --make -o test2 test2.hs That'll run fine. If you add -threaded, it will hang. Ideas? Thanks, -- John

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Type Synonyms and termination checking
by Jim Apple 03 Sep '07

03 Sep '07

Regarding http://www.haskell.org/pipermail/cvs-ghc/2007-August/037655.html and Are type functions checked for termination? If so, where can I find the details? Jim

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:def and ghci command output
by Claus Reinke 31 Aug '07

31 Aug '07

ghci's :def takes a 'String -> IO String' and converts it into a ghci command. this makes it possible to pass a command parameter to a haskell function, and to interpret the result of the call as a ghci command. that is nice, but how do i get access to the command's output? that is, i'd like to run a ghci command, and capture its ouput as a haskell String, eg, to pass it on to another ghci command, or to a haskell function. perhaps something simple, like an additional ghci top-level binding: GHCi.cmd :: String -> IO String so that i could write things like let doSomething output = .. :def mymodules \_->GHCi.cmd ":show modules" >>= doSomething or a way to compose ghci commands, so that i could write :def doSomething \output->.. :def mymodules \_->return ":show modules :>>= :doSomething" would that be difficult to provide? claus

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Derived instances
by Fernand 31 Aug '07

31 Aug '07

Hi, I have just discovered Generics in Haskell and, writing some code with GHC, I wondered about the portability of my code. It is possible I overlooked something in the GHC documentation but I did not found a way to obtain, let say, the haskell code for the derived classes. I mean, having the following type declaration : > data Something = This | That deriving (Eq, Ord, Typeable, Data) I can use the -fgenerics to automatically derive instances for Something. But I could wish to have the generated source code for those instances (like, I suppose, the DrIft tool does), for instance, to compile my source with another compiler, which does not support Generics. Is that possible ? Thank you, Fernand

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Re: [Haskell] ghc as a library and profiling
by Simon Marlow 30 Aug '07

30 Aug '07

Sönke Hahn wrote: > I am working on a program that uses ghc as a library to typecheck code. I want > to profile the program (compiling with '-prof -auto-all' and running > with '+RTS -p'). Trying to compile, results in the following message: > > Could not find module `GHC': > Perhaps you haven't installed the profiling libraries for package > ghc-6.6.1? Yes, by default we don't build or supply the GHC package with profiling, because of the extra bloat it would add to the downloads and the time taken to build it. With the latest GHC sources from darcs you can add 'GhcCompilerWays=p' to your mk/build.mk and get the GHC package built for profiling too. > A second idea that comes to my mind is this: > The ghc package is hidden in the standard installation, so i unhided (unhid?) > it via 'ghc-pkg expose ghc'. Maybe it just exposed the non-profiling version > and kept the profiling version hidden? How would i expose both? You probably don't really want to expose the GHC package, because it doesn't use hierarchical modules and so pollutes the top-level module namespace. Cheers, Simon

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template haskell syntax
by Christian Maeder 24 Aug '07

24 Aug '07

Hi, for haskell prime I suggest to change the stolen syntax "[e|", "[p|", "[d|", "[t|" to [|<letter>| in order to avoid the confusion with list comprehensions. Cheers Christian

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Status of GHC runtime dependency on GNU multi precision arithmetic library
by Brian Hulley 22 Aug '07

22 Aug '07

Hi, I know this is a sensitive issue and I absolutely don't want to start any kind of discussion about the merits or otherwise of LGPL, but I was wondering if there are any plans to remove the GNU mp library from the runtime so that it would be possible to distribute native executables compiled with GHC without having to deal with the additional issues raised by the current inclusion of this LGPL component in the runtime. (Afaik everything else in ghc is under a BSD3 license.) It's less of an issue on Linux where libgmp is dynamically linked but when thinking about using Haskell ghc for creating Windows apps it is for me a real problem, because it would mean I'd have to distribute the code for my app as a library along with the code as an executable, thus doubling the download size for my apps as well as having to include a license that has to explain to a possibly non-technical user that although the program includes code that is libre/gratis free the program is not itself free etc etc... Regards, Brian.

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Re: Qualified identifiers opinion
by Christian Maeder 21 Aug '07

21 Aug '07

Hi Isaac, just to give you a reply at all, see below. I reply glasgow-haskell-users(a)haskell.org since I'm not subscribed to haskell-prime. And I don't want to subscribe, because I'm more interested that Haskell becomes more stable (and standard). So here is my opinion: 1. The lexer should recognize keywords. 2. I would not mind if Haskel98 rejected all keywords that are also rejected by extensions, so that the lexer is extension independent. (Starting with Haskell98, removing conflicting identifiers as soon as I switch on valuable extensions does not make sense.) 3. I'm against qualified identifiers, with the unqualified part being a keyword like "Foo.where". (The choice of qualification should be left to the user, usually one is not forced to used qualified names.) 4. However, "Foo.where" should always be rejected and not changed to "Foo.wher e"! (Longest matching, aka "maximal munch", must not consider keywords!) (see end of: http://www.haskell.org/onlinelibrary/lexemes.html#sect2.4) I would not mind if a name "F. " is plainly rejected. It only makes sense, when a data constructor is the first argument of the composition operator "(.)" Maybe "." and "$" as operators should require white spaces on both sides, since "$(" also indicates template haskell. Cheers Christian Isaac Dupree wrote: > > Especially after writing a partial lexer for Haskell, I opine that this > should be all legal: > > > > module Foo where > > --in case you didn't know, this is legal syntax: > Foo.f = undefined > > Foo.mdo = undefined > Foo.where = undefined > x Foo.! y = undefined > x Foo... y = undefined --remember ".." is reserved id, e.g. [2..5] > > > {-# LANGUAGE RecursiveDo, BangPatterns #-} module Bar where > import Foo > hello !x = mdo { y <- Foo.mdo Foo... ({-Foo.-}f x y); return y } > > {- Haskell 98 -} module Baz where > import Foo > goodbye x = x ! 12 > > > > (Foo.where) lexing as (Foo.wher e) or (Foo . where) does not make me > happy. (being a lexer error is a little less bad...) Especially not > when the set of keywords is flexible. I don't see any good reason to > forbid declaring keywords as identifiers/operators, since it is > completely unambiguous, removes an extension-dependence from the lexer > and simplifies it (at least the mental lexer); Also I hear that the > Haskell98 lexing is (Foo.wher e), which I'm sure no one relies on... > > Well, that's my humble opinion on what should go into Haskell' on this > issue. > > Isaac

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[GHC-as-a-library]getModuleInfo: instances for package module unimplemented
by kenny lu 21 Aug '07

21 Aug '07

Hi all, We've been using to GHC-as-a-library to implement some parts of our project. Our goal is to use ghc to extract necessary type information from Haskell modules. Everything works fine until we attempt to collect the declared type class instances from a packaged haskell module, like List, Monad, etc. The program terminates with an error as follows, getModuleInfo: instances for package module unimplemented We manually trace into the source file /ghc/compiler/main/GHC.hs and find the following snipet of code: getPackageModuleInfo :: HscEnv -> Module -> IO (Maybe ModuleInfo) getPackageModuleInfo hsc_env mdl = do #ifdef GHCI (_msgs, mb_avails) <- getModuleExports hsc_env mdl case mb_avails of Nothing -> return Nothing Just avails -> do eps <- readIORef (hsc_EPS hsc_env) let names = availsToNameSet avails pte = eps_PTE eps tys = [ ty | name <- concatMap availNames avails, Just ty <- [lookupTypeEnv pte name] ] -- return (Just (ModuleInfo { minf_type_env = mkTypeEnv tys, minf_exports = names, minf_rdr_env = Just $! nameSetToGlobalRdrEnv names (moduleName mdl), minf_instances = error "getModuleInfo: instances for package module unimplemented", minf_modBreaks = emptyModBreaks })) It is clearly that the field mind_instances is not assigned to a proper value. Can someone kindly advise on how to implement the missing bit? Or pointing out a workaround? Thank you very much. Best Regards, Kenny Zhuo Ming Lu

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