June 2008 - Haskell-Cafe

Default definitions for associated type synonyms
by Dreixel 02 Jun '08

02 Jun '08

Hello, Does anyone know if it is possible to specify a default definition for an associated type synonym? When I tried: class A a where > type B a = a > GHC (version 6.9.20080309) told me: "Type declaration in a class must be a kind signature or synonym default". However, when I look at the parser I see no way to specify such synonym default. Is this possible? Thanks, Zé Pedro

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GHCi panic
by Andrew Coppin 02 Jun '08

02 Jun '08

I don't suppose this will surprise anybody greatly, but... Apparently if you write a Haskell module that is 400 KB in size and defines a single CAF consisting of a 45,000-element [String], GHCi panics when attempting to load it interpretted, and hits a stack overflow attempting to load it compiled. GHC also takes forever to compile it in the first place, and eventually spits out a 5 MB interface file later followed by a 16 MB object file. And attempting to compile a trivial module against it again causes a stack overflow. Presumably the designers of GHC just didn't expect anybody to try to do anything this weird? ;-) I was hoping that doing things this way round would be *more efficient*. But this is apparently not the case at all, so I'll just go back to reading the file at runtime instead... [Presumably if I was desparate I could convert the data into some kind of preinitialised C structure and manually link it in - if I was that determined.]

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Images and GUIs in Haskell
by Ronald Guida 02 Jun '08

02 Jun '08

Two questions: 1. In a Haskell program, if all I want to do is output an image, like a graph or chart, what is the simplest library to use? N.B. Simpler := easier to get minimal functionality. I really don't want to wade through a bunch of boilerplate or climb a steep learning curve just to be able to plot a few lines or circles. 2. Suppose I want interactivity. For example, I want to plot a line graph, and then let the user click and drag the data points. From what I understand about GUIs, I would need to track mouse buttons (up and down), mouse movements, and possibly keystrokes. (I know this is the complete opposite extreme from my first question) In this case, what would be the best (not necessarily simplest) library to use? What would you recommend?

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haskell compiler on NetBSD amd64
by Donn Cave 02 Jun '08

02 Jun '08

So here I am with 64 bit Athlon hardware, running amd64 NetBSD (a.k.a. x86_64), reasonably motivated to compile Haskell. From what I make of the porting instructions, my only hope is to install FreeBSD-amd64 on another partition and download the 6.6.1 ghc for that platform, use it to generate .hc files to build 6.6.1 on NetBSD, and then use 6.6.1 to build 6.8 or whatever the current version may be by the time I get there? -- Donn Cave <donn(a)avvanta.com>

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Ghc / cgi static linking
by Pieter Laeremans 02 Jun '08

02 Jun '08

Hello, I'm researching the use of Haskell to replace some perl scripts (in a web app). The app is deployed with a webhosting provider. CGI scripts can be executed => I can use Haskell. I've tried some hello world cgi scripts, compiled them on the same linux the hosting company uses and deployed them. It worked! But I would like to implement a search feature for the website. For Java/php/perl there 's lucene. For haskell there 's holumbus. Unfortunately, sqlite is a requirement for holumbus. It is not installed at the server of the hosting company. Is it possible to staticly link the sqlite3 library using ghc ? And are there other lucene like libs available like holumbus for haskell ? thanks in advance, Pieter -- Pieter Laeremans <pieter(a)laeremans.org>

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Re: [Haskell-cafe] hmatrix
by Thomas Hartman 01 Jun '08

01 Jun '08

what package do you install/import to get at Vector? 2008/5/31 Thomas Hartman <tphyahoo(a)gmail.com>: > what package do you install/import to get at Vector? > > 2008/5/31 Anatoly Yakovenko <aeyakovenko(a)gmail.com>: >> What is the most efficient way to update a position in a matrix or a >> vector? I came up with this: >> >> updateVector :: Vector Double -> Int -> Double -> Vector Double >> updateVector vec pos val = vec `add` v2 >> where >> v2 = fromList $ (replicate (pos) 0.0) ++ ((val - (vec @> >> pos)):(replicate ((dim vec)- pos - 1) 0.0)) >> >> but this seems pretty inefficient to me. >> >> thanks, >> Anatoly >> _______________________________________________ >> Haskell-Cafe mailing list >> Haskell-Cafe(a)haskell.org >> http://www.haskell.org/mailman/listinfo/haskell-cafe >> >

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hint / ghc api and reloading modules
by Evan Laforge 01 Jun '08

01 Jun '08

I'm using "hint", but since it's basically a thin wrapper around the GHC API, this is probably a GHC api question too. Maybe this should go to cvs-ghc? Let me know and I'll go subscribe over there. It's my impression from the documentation that I should be able to load a module interpreted, make changes to it, and then reload it. This is, after all what ghci does. It's also my impression that the other imported modules should be loaded as object files, if the .hi and .o exist, since this is also what ghci does. However, if I load a module and run code like so (using hint): GHC.loadModules ["Cmd.LanguageEnviron"] GHC.setTopLevelModules ["Cmd.LanguageEnviron"] GHC.setImports ["Prelude"] cmd_func <- GHC.interpret (mangle_code text) (GHC.as :: LangType) It works fine until I change LanguageEnviron. If I make a change to a function, I don't see my changes in the output, as if the session is only getting partially reset. If I insert a syntax error, then I do see it, so it is recompiling the file in some way. However, if I *rename* the function and call it with the new name, I get a GhcException: During interactive linking, GHCi couldn't find the following symbol: ... etc. So I examined the code in hint for loadModules and the code in ghci/InteractiveUI.hs:/loadModule, and they do look like they're doing basically the same things, except a call to rts_revertCAFs, which I called too just for good measure but it didn't help (I can't find its source anywhere, but the ghci docs imply it's optional, so I suspect it's a red herring). Here's a condensed summary of what hint is doing: -- reset GHC.setContext session [] [] GHC.setTargets session [] GHC.load session GHC.LoadAllTargets -- rts_revertCAFs -- load targets <- mapM (\f -> GHC.guessTarget f Nothing) fs GHC.setTargets session targets GHC.load session GHC.LoadAllTargets -- interpret let expr_typesig = "($expr) :: xyz" expr_val <- GHC.compileExpr session expr_typesig return (GHC.Exts.unsafeCorce# expr_val :: a) -- GHC.compileExpr maybe_stuff <- hscStmt hsc_env ("let __cmCompileExpr = "++expr) ([n],[hv]) <- (unsafeCoerce# hval) :: IO [HValue] return (Just hv) and then ghci does: -- load GHC.setTargets session [] GHC.load session LoadAllTargets targets <- io (mapM (uncurry GHC.guessTarget) files') GHC.setTargets session targets GHC.load session LoadAllTargets rts_revertCAFs putStrLn "Ok, modules loaded: $modules" -- interpret GHC.runStmt session stmt step -- GHC.runStmt Just (ids, hval) <- hscStmt hsc_env' expr coerce hval to (IO [HValue]) and run it carefully So it *looks* like I'm doing basically the same thing as ghci... except obviously I'm not because ghci reloads modules without any trouble. Before I go start trying to make hint even more identical to ghci, is there anything obviously wrong here that I'm doing?

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