June 2008 - Haskell-Cafe

Haskell Related Reading
by Darrin Thompson 29 Jun '08

29 Jun '08

I have a trip coming up and might have some reading time. I was hoping to get through some of the classics, bananas and lenses, the essence, etc. So I have a few questions: Bananas and lenses et. al. uses some notation that I don't understand right out of the gate. Is there a good primer on whatever that brand of double bars and arrows means? The essense of functional programming looks good, I could understand it when I skimmed it but can I print it out on US letter? The PDF at citeseer was aligned badly. (Essece seemed like a fabulous intro or chapter 2 on getting used to monads. Better than most stuff on the web. Funny that...) I'm also interested in FRP as it might relate to web programming. Anyone have a recommendation? -- Darrin

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Ptr Word8 display question
by Galchin, Vasili 29 Jun '08

29 Jun '08

Hi, Suppose I have a value of type Ptr Word8 and also an Int which is the length of the Ptr Word8(sorry if I am thinking too much in a C string frame of mind). How can I display the Ptr Word8 value of the given length? Vasili

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Learning GADT types to simulate dependent types
by Paul Johnson 28 Jun '08

28 Jun '08

I'm trying to understand how to use GADT types to simulate dependent types. I'm trying to write a version of list that uses Peano numbers in the types to keep track of how many elements are in the list. Like this: {-# OPTIONS -fglasgow-exts -fallow-undecidable-instances #-} module Plist where infixr 3 :| data Zero data S a class Add n1 n2 t | n1 n2 -> t, n1 t -> n2 instance Add Zero n n instance Add (S n1) n2 (S t) data Plist n a where Nil :: Plist Zero a (:|) :: a -> Plist n a -> Plist (S n) a instance (Show a) => Show (Plist n a) where show Nil = "Nil" show (x :| xs) = show x ++ " :| " ++ show xs pHead :: Plist (S n) a -> a pHead (x :| _) = x pTail :: Plist (S n) a -> Plist n a pTail (_ :| xs) = xs pConcat Nil ys = ys pConcat (x :| xs) ys = x :| pConcat xs ys Everything works except the last function (pConcat). I figured that it should add the lengths of its arguments together, so I created a class Add as shown in the Haskell Wiki at http://www.haskell.org/haskellwiki/Type_arithmetic. But now I'm stuck. When I try to load this module I get: Plist.hs:32:8: GADT pattern match in non-rigid context for `Nil' Tell GHC HQ if you'd like this to unify the context In the pattern: Nil In the definition of `pConcat': pConcat Nil ys = ys Failed, modules loaded: none. (Line 32 is "pConcat Nil ys = ys") So how do I do this? Am I on the right track? Can someone help improve my Oleg rating? Thanks, Paul.

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Galois move complete, Hackage back online
by Don Stewart 28 Jun '08

28 Jun '08

Galois has completed the move of its data center, and the services: hackage.haskell.org darcs.haskell.org are now back online. Thanks for your patience, everyone! Enjoy the new bandwidth. -- Don

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Hackage
by Cetin Sert 28 Jun '08

28 Jun '08

Hi, Hackage seems to have been down for a few days now. When is it going to be back online? It is not possible to darcs-get any packages either. Best Regards, Cetin Sert

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blas bindings, why are they so much slower the C?
by Anatoly Yakovenko 28 Jun '08

28 Jun '08

here is the C: #include <cblas.h> #include <stdlib.h> int main() { int size = 1024; int ii = 0; double* v1 = malloc(sizeof(double) * (size)); double* v2 = malloc(sizeof(double) * (size)); for(ii = 0; ii < size*size; ++ii) { double _dd = cblas_ddot(0, v1, size, v2, size); } free(v1); free(v2); } this is the haskell: module Main where import Data.Vector.Dense.IO main = do let size = 1024 v1::IOVector Int Double <- newListVector size [0..] v2::IOVector Int Double <- newListVector size [0..] mapM_ (\ ii -> do v1 `getDot` v2) [0..size*size] time ./testdot real 0m0.017s user 0m0.010s sys 0m0.010s time ./htestdot real 0m4.692s user 0m4.670s sys 0m0.030s so like 250x difference htestdot.prof is no help Tue Jun 17 20:46 2008 Time and Allocation Profiling Report (Final) htestdot +RTS -p -RTS total time = 3.92 secs (196 ticks @ 20 ms) total alloc = 419,653,032 bytes (excludes profiling overheads) COST CENTRE MODULE %time %alloc main Main 88.3 83.0 CAF Main 11.7 17.0 individual inherited COST CENTRE MODULE no. entries %time %alloc %time %alloc MAIN MAIN 1 0 0.0 0.0 100.0 100.0 CAF Main 216 7 11.7 17.0 100.0 100.0 main Main 222 1 88.3 83.0 88.3 83.0 CAF GHC.Float 187 1 0.0 0.0 0.0 0.0 CAF GHC.Handle 168 3 0.0 0.0 0.0 0.0

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ghci vi mode?
by Adam Smyczek 27 Jun '08

27 Jun '08

If I remember correctly, ghci uses readline framework on Linux/OS X. Is it possible to change ghci editing mode to vi? Thanks, Adam

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Error in configure script for GHC
by Jefferson Heard 27 Jun '08

27 Jun '08

I've tried all the 6.6 and 6.8 versions, trying to compile them on a RHEL-based supercomputer, and I always see this error when I run ./configure: -bash-3.00$ ./configure checking build system type... x86_64-unknown-linux-gnu checking host system type... x86_64-unknown-linux-gnu checking target system type... x86_64-unknown-linux-gnu Which we'll further canonicalise into: x86_64-unknown-linux checking for path to top of build tree... pwd: timer_create: Invalid argument configure: error: cannot determine current directory Is this a known bug with GHC, or is it my supercomputer? We have several supercomputers here running in production, so I can't really ask them to change the configuration just for me, but nonetheless I'd like to run on them.

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Call Graph Tool?
by C.M.Brown 27 Jun '08

27 Jun '08

Hi, I have approx. 100+ source files and I was wondering if anyone has a tool that would let me see a visual call graph for the source files; i.e. a visual hierarchy of which module is imported by what, and so forth. Kind regards, Chris.

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what's up with hackage.haskell.org?
by Martin DeMello 27 Jun '08

27 Jun '08

haven't been able to get to it in a couple of days (at least) martin

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