December 2007 - Haskell-Cafe

A Random Question
by Dominic Steinitz 15 Dec '07

15 Dec '07

I need to generate distinct arbitrary values for my quickcheck tests and they don't have to be arbitrary (although that doesn't matter). No problem I thought, I'll create my own random number generator (which will not be random at all) and use > choose :: forall a. (Random a) => (a, a) -> Gen a Here's my code: > import Test.QuickCheck > import System.Random > > data MyGen = MyGen Int > deriving (Eq, Show) > > myNext :: MyGen -> (Int, MyGen) > myNext (MyGen s1) = > (s1, MyGen (s1 + 1)) > > -- Assume we will never need this > mySplit :: MyGen -> (MyGen, MyGen) > mySplit = error "No split for predictable random generator" > > myGenRange :: MyGen -> (Int, Int) > myGenRange (MyGen s1) = (s1, s1) > > instance RandomGen MyGen where > next = myNext > split = mySplit > genRange = myGenRange > > data Foo = Foo Int > deriving (Eq, Show) > > myRandomR :: (Foo, Foo) -> MyGen -> (Foo, MyGen) > myRandomR (Foo lo, Foo hi) g = > let (n, g') = next g > in (Foo n, g') > > instance Random Foo where > randomR = myRandomR > random = undefined But I get > Supply.hs:33:13: > Couldn't match expected type `g' against inferred type `MyGen' > `g' is a rigid type variable bound by > the type signature for `randomR' at <no location info> > Expected type: (Foo, Foo) -> g -> (Foo, g) > Inferred type: (Foo, Foo) -> MyGen -> (Foo, MyGen) > In the expression: myRandomR > In the definition of `randomR': randomR = myRandomR > Failed, modules loaded: none. I have two questions: 1. Why can't I instantiate a type class with any function I like provided it fits the type signature? 2. Is this the right approach to generating predictable arbitrary values? Are there others? Thanks, Dominic.

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Fw: hdbc odbc also crashes on 6.8.2 Re: [Haskell-cafe] HDBC-ODBC crashes on ghc 6.8
by Thomas Hartman 15 Dec '07

15 Dec '07

I just tried HDBC-ODBC on 6.8.2, but it still crashes. Works on 6.6.1. thomas. Thomas Hartman <thomas.hartman+external(a)db.com> Sent by: haskell-cafe-bounces(a)haskell.org 11/19/2007 12:05 PM To haskell-cafe(a)haskell.org cc Subject [Haskell-cafe] HDBC-ODBC crashes on ghc 6.8 This minimal program below fails in 6.8.1, works for 6.6.1, for up to date HDBC-ODBC-1.1.3. (I tried installing both from package and from darcs head) Installed and running from cygwin. When I run it as runghc fail.hs I get a windows popup error "ghc.exe has encountered a problem and needs to close, sorry for the inconvenience". ghci, same behavior. When I run it as ghc -e 'main' fail.hs, I don't get a popup window, and no error message is output, but it fails all the same. (I know this because more complicated, non-minimal programs, fail.) So this would seem to be a problem with ghc 6.8.1. $ cat fail.hs import Database.HDBC import Database.HDBC.ODBC main = connectODBC "some valid connect string, works when run from 6.6.1, not from 6.8.1" --- This e-mail may contain confidential and/or privileged information. If you are not the intended recipient (or have received this e-mail in error) please notify the sender immediately and destroy this e-mail. Any unauthorized copying, disclosure or distribution of the material in this e-mail is strictly forbidden. _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe(a)haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe --- This e-mail may contain confidential and/or privileged information. If you are not the intended recipient (or have received this e-mail in error) please notify the sender immediately and destroy this e-mail. Any unauthorized copying, disclosure or distribution of the material in this e-mail is strictly forbidden.

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New slogan for haskell.org
by gwern0＠gmail.com 15 Dec '07

15 Dec '07

FWIW to the discussion about changing the main page, I was reading the CUFP paper and I saw some germane comments (and the writer is apparently one Noel Welsh, whose name I don't see in the thread); the context is a discussion (pg 17) of various members or potential members of the Haskell community and how supported they are: "What are the needs of the potential programmer? People program to solve prob- lems; so there had better be a clear statement of what kinds of problem the language is good for. The Python community does a good job of this on python.org: "Python is a dynamic object-oriented programming language that can be used for many kinds of software development. It offers strong support for integration with other languages and tools, comes with extensive standard libraries, and can be learned in a few days." Compare this with the equivalent from haskell.org: "Haskell is a general purpose, purely functional programming language featuring static typing, higher-order functions, polymorphism, type classes, and monadic effects. Haskell compilers are freely available for almost any computer." If you understand all that, you don't need to be here: you're already a Haskell programmer." -- gwern

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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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