- Haskell-Cafe - Haskell.org

Problem with Gtk2hs
by Andrew Coppin 15 Dec '07

15 Dec '07

I just spent the evening writing a library that's a thin layer over Gtk2hs. It took an age to get it to compile, but eventually it worked. Yay! When I ran it, I got this: Test2: gtk/Graphics/UI/Gtk/Gdk/PixbufData.hs.pp:58:0: No instance nor default method for class operation Data.Array.Base.getNumElements Er... wow. OK, at this point, I am completely stumped. Any hints? (Also, the documentation for Graphics.UI.Gtk.Misc.DrawingArea suggests that you may want to draw a Pixbuf using the pixbufRenderToDrawable function - but GHC complains that this function doesn't exist. I had to use drawPixbuf instead. Wow that has a lot of parameters... In particular, the final two don't seem to be documented. Interesting. They're both 0 in the "fastdraw" demo.)

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Strict String IO now packaged
by Don Stewart 14 Dec '07

14 Dec '07

Here at Haskell.org we know you have a choice in programming languages, and a choice in evaluation strategies. To help make your decision easier, I'm pleased to announce an update to the 'strict' package providing strict file IO for Strings. You can now choose when and where to evaluate your Strings, meaning more predictable resource usage for readFile-like Strict IO. And of course you can still use lazy IO when appropriate. Find the strict IO package in System.IO.Strict, on hackage: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/strict Cheers, Don

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GUI
by Miguel Mitrofanov 14 Dec '07

14 Dec '07

Is there any really cross-platform GUI library for Haskell? Gtk2Hs is good (I suppose), but it requires X. OK, I have X, but it's not "native" on my Mac; some Mac users don't install it and almost all Mac users don't always run it. I was able to install wxHaskell (after some hacking - this was really painful); and Blobs editor compiled successfully, but then resisted to run. Tk-based libraries seem to be good, and Tk can be run "natively" on Mac (i.e., without X), but none of them seem to compile. Sorry if this message seems like I'm angry; I am, but that's only for a moment.

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Data.Generics: how do I get the type name of a value?
by Alistair Bayley 14 Dec '07

14 Dec '07

I've been learning/playing with Data.Generics a bit, and have a how-to question... If I say > dataTypeOf "" then I get > DataType {tycon = "Prelude.[]", datarep = AlgRep [[],(:)]} No surprises there. But I'd really like to know that I have a String, or [Char]. How do I get the name of the concrete type that the list contains? Is that a reasonable thing to ask for? I can say: > gmapQ dataTypeOf "a" to get: > [ DataType {tycon = "Prelude.Char", datarep = StringRep} > , DataType {tycon = "Prelude.[]", datarep = AlgRep [[],(:)]} > ] But if I say: > gmapQ dataTypeOf "" I get: > [] which makes sense when you consider the stucture of the List ADT, but doesn't help me determine the type of the value. Alistair

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type classes
by Peter Padawitz 14 Dec '07

14 Dec '07

I'd like to define several instances of the same type class with the same type variable instance. Only method instances differ. How can I do this without writing copies of the type class?

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-threaded
by Maurício 14 Dec '07

14 Dec '07

Hi, I see in the documentation and in many messages in this list that, if you want multithreading in your program, you need to use -threaded in ghc. Why isn't that option default? Does it imply some kind of overhead? Thanks, Maurício

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Problems with split-objs
by Magnus Therning 14 Dec '07

14 Dec '07

I've followed the instructions at [1] to create a .deb of vty[2]. It seems the helper scripts for Debian passes `--enable-split-obj' when running `./Setup.lhs configure'. This results in numerous multiple definitions of stuff. I have a few questions regarding this... I think I understand what --split-objs does, but why doesn't it work for vty? I've debianised other haskell libs using the same method without problems. The only difference I can see is that vty uses FFI. Is that what causes the problems? How do I fix this? I'd rather not go in and manually change any automatically generated files, and the file that passes `--enable-split-objs' is automatically generated :( /M [1]: http://haskell.org/haskellwiki/Creating_Debian_packages_from_Cabal_package [2]: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/vty-3.0.0 -- Magnus Therning (OpenPGP: 0xAB4DFBA4) magnus＠therning．org Jabber: magnus．therning＠gmail．com http://therning.org/magnus What if I don't want to obey the laws? Do they throw me in jail with the other bad monads? -- Daveman

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Examples of useful functions of order >= 3?
by Ben Lippmeier 14 Dec '07

14 Dec '07

Hi all, I'm working on a type-based side effect analysis for a Haskell-like language, and need to test it against some more higher order functions. The situation is a bit icky because it uses bounded quantification and various related issues (ie covariance vs contravariance) only come into play when dealing with functions of order >= 3. Trouble is, the vast majority of useful higher order functions that I can think of are of order 2. Oh, I can certainly invent toys like: order3 f = f succ order4 b f = if b then f else order3 .. and so on, but I'm left feeling unsatisfied. I vaguely remember a paper called something like "Is there any use for a seventh order function?", but I forget why they were used, and I can't find it on Google, or ACM or any of the other likely places. Does anyone have any examples of code (in whatever language) which usefully uses functions of order >= 3?? Preferably up to 5? Thanks, Ben.

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Hoogle error
by jim burton 14 Dec '07

14 Dec '07

Hoogling (->) (>>=) gives Error, your search was invalid: Parse Error: Unexpected character '>>=)' Is there a way to escape the input so it would work? (I wasn't really expecting the right results BTW as I think hoogle searches type signatures not patterns in definitions, right?) Thanks, Jim

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Execution of external command
by Bulat Ziganshin 14 Dec '07

14 Dec '07

Hello haskell-cafe, please help me with selection of proper function to use i need to run external command with parameter and get its stdout, smth like this: output <- system "cmd param" the code should be compatible with unix and windows, and it should be possible to execute scripts (so afaiu it should execute command via cmd/sh). i use ghc 6.6.1 and it will be great if this function will not require any non-bundled libs and be compatible with later ghc versions -- Best regards, Bulat mailto:Bulat.Ziganshin@gmail.com

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