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Referential Transparency and Monads
by Mark Spezzano 09 Apr '09

09 Apr '09

Hi, How exactly do monads “solve” the problem of referential transparency? I understand RT to be such that a function can be replaced with a actual value. Since a monad could potentially encapsulate any other value—say, data read from a keyboard—doesn’t that violate the assumption of RT on monads? Or does RT ignore the encapsulated data and just view the “action” performed by the monad as the “value” of the monad? Just curious as to the rationale behind referential transparency and how it applies to monads. Cheers, Mark Spezzano No virus found in this outgoing message. Checked by AVG. Version: 7.5.557 / Virus Database: 270.11.48/2048 - Release Date: 8/04/2009 7:02 PM

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Problems getting errors with HXT
by rodrigo.bonifacio 09 Apr '09

09 Apr '09

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Debugging compile times (Template Haskell)
by Henning Günther 09 Apr '09

09 Apr '09

Hi, as the author of the encoding package[1] I'm quite annoyed by the extreeme long time it takes to compile a certain module (JISX0208). This module uses template haskell to generate both a Map (Map Char (Word8,Word8)) and an Array (UArray (Word8,Word8) Int) for character conversion. The file has about 6000 entries and the generated array has 8649 entries. Generating the syntax for the array and map is very fast (under a second) but compiling the code seems to take forever. Is there any way to find out why that is the case? I've tried pinpointing the problem but couldn't... [1] http://hackage.haskell.org/cgi-bin/hackage-scripts/package/encoding

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OFX libs available ?
by Simon Michael 09 Apr '09

09 Apr '09

Has anyone got code for OFX financial data handling they could share ? Best - Simon

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ANN: tree-monad 0.2
by Sebastian Fischer 08 Apr '09

08 Apr '09

I am pleased to announce version 0.2 of the package tree-monad that provides instances of MonadPlus that represent the search space of non- deterministic computations as a tree. In version 0.2 I have implemented an optimized CPS version of the tree monad based on the idea described in "Asymptotic Improvement of Computations over Free Monads" [^1]. The package is on Hackage: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/tree-monad The sources are on Github: http://github.com/sebfisch/tree-monad Installation and usage instructions are at: http://sebfisch.github.com/tree-monad A possible application of search trees is parallel search as described in the blog post: http://www-ps.informatik.uni-kiel.de/~sebf/haskell/speedup-search-with-para… The package parallel-tree-search by Fabian Reck and myself http://hackage.haskell.org/cgi-bin/hackage-scripts/package/parallel-tree-se… implements the search described in the above blog post based on the hereby announced package tree-monad. Cheers, Sebastian [^1]: http://wwwtcs.inf.tu-dresden.de/~voigt/mpc08.pdf

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Ghc: -O harmful?
by Petr Skovron 08 Apr '09

08 Apr '09

Hello all, I have a haskell program that runs an order of magnitude slower when compiled with optimisations turned on. This happens on 6.8.2 as well as 6.10.1: petr@r4at184:/tmp[1]% ghc --make -fforce-recomp -o out buga.hs [1 of 1] Compiling Main ( buga.hs, buga.o ) Linking out ... petr@r4at184:/tmp% time ./out < 3-med.in >|/dev/null ./out < 3-med.in >| /dev/null 0,03s user 0,00s system 99% cpu 0,038 total petr@r4at184:/tmp% ghc --make -fforce-recomp -o out buga.hs -O [1 of 1] Compiling Main ( buga.hs, buga.o ) Linking out ... petr@r4at184:/tmp% time ./out < 3-med.in >|/dev/null ./out < 3-med.in >| /dev/null 0,99s user 0,01s system 99% cpu 1,001 total petr@r4at184:/tmp[1]% ghc --make -fforce-recomp -o out buga.hs -O2 [1 of 1] Compiling Main ( buga.hs, buga.o ) Linking out ... petr@r4at184:/tmp% time ./out < 3-med.in >|/dev/null ./out < 3-med.in >| /dev/null 0,99s user 0,01s system 99% cpu 1,004 total petr@r4at184:/tmp% ghc --version The Glorious Glasgow Haskell Compilation System, version 6.8.2 psxxxxxx:/tmp$ ghc --make -fforce-recomp -o out buga.hs [1 of 1] Compiling Main ( buga.hs, buga.o ) Linking out ... psxxxxxx:/tmp$ time ./out < 3-med.in >|/dev/null real 0m0.028s user 0m0.011s sys 0m0.007s psxxxxxx:/tmp$ ghc --make -fforce-recomp -o out buga.hs -O [1 of 1] Compiling Main ( buga.hs, buga.o ) Linking out ... psxxxxxx:/tmp$ time ./out < 3-med.in >|/dev/null real 0m0.252s user 0m0.225s sys 0m0.011s psxxxxxx:/tmp$ ghc --make -fforce-recomp -o out buga.hs -O2 [1 of 1] Compiling Main ( buga.hs, buga.o ) Linking out ... Vazeny pane, cekam na Vase rozkazy. psxxxxxx:/tmp$ time ./out < 3-med.in >|/dev/null real 0m0.239s user 0m0.225s sys 0m0.010s psxxxxxx:/tmp$ ghc --version The Glorious Glasgow Haskell Compilation System, version 6.10.1 The GHC documentation states that > -O2: > Means: "Apply every non-dangerous optimisation, even if it means > significantly longer compile times." The avoided "dangerous" > optimisations are those that can make runtime or space worse if > you're unlucky. They are normally turned on or off individually. If I understand this correctly, the dangerous optimisations should not be performed with -O. Am I doing anything wrong? Am I hitting a bug in GHC optimiser? A known one? The program source: -------------- petr@r4at184:/tmp$ cat buga.hs {-# LANGUAGE FlexibleContexts #-} import Control.Monad import Control.Monad.State data Predpoc a = Nic | Kus { delka :: Int, levy :: Predpoc a, pravy :: Predpoc a } deriving Show gList i = Kus 1 Nic Nic gPredpoc kus1 kus2 = Kus (delka kus1 + delka kus2) kus1 kus2 pop :: MonadState [a] m => m a pop = get >>= \(x:xs) -> put xs >> return x mbuild :: MonadState [Int] m => Int -> m (Predpoc Int) mbuild 1 = gList `liftM` pop mbuild n = liftM2 gPredpoc (mbuild n1) (mbuild n2) where n1 = n`div`2 n2 = n - n1 build n li = evalState (mbuild n) li best :: Predpoc Int -> Int -> Int -> Int best kus i j | i == 1 && j == delka kus = delka kus | j <= del1 = best (levy kus) i j | i > del1 = best (pravy kus) (i-del1) (j-del1) | otherwise = best (levy kus) i del1 + best (pravy kus) 1 (j-del1) where del1 = delka (levy kus) main = do n <- read `liftM` getLine pole <- liftM (build n . map read . words) getLine replicateM_ 100 $ do getLine print $ best pole 42 420 -------------- The sample input file (30k) can be downloaded from http://pikomat.mff.cuni.cz/petr/3-med.in I suspect that the main's variable "pole" (which is a large binary tree of type Predpoc Int) may be built from scratch in each iteration of the replicateM_ loop. Petr

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Functions from Data.Hashtable
by Peter Verswyvelen 08 Apr '09

08 Apr '09

These functions have their arguments reversed when compare to e.g. Map For example Data.HashTable.lookup<http://haskell.org/ghc/docs/latest/html/libraries/base/Data-HashTable.html#…> :: HashTable<http://haskell.org/ghc/docs/latest/html/libraries/base/Data-HashTable.html#…> key val -> key -> IO<http://haskell.org/ghc/docs/latest/html/libraries/base/System-IO.html#t%3AIO> (Maybe<http://haskell.org/ghc/docs/latest/html/libraries/base/Data-Maybe.html#t%3A…> val) Data.Map.lookup<http://hackage.haskell.org/packages/archive/containers/0.2.0.0/doc/html/Dat…> :: Ord<http://hackage.haskell.org/packages/archive/base/4.0.0.0/doc/html/Data-Ord.…> key => key -> Map<http://hackage.haskell.org/packages/archive/containers/0.2.0.0/doc/html/Dat…> key val -> Maybe<http://hackage.haskell.org/packages/archive/base/4.0.0.0/doc/html/Data-Mayb…> val I find this a bit odd. Is this by design?

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A challenge
by Thomas Davie 08 Apr '09

08 Apr '09

We have two possible definitions of an "iterateM" function: iterateM 0 _ _ = return [] iterateM n f i = (i:) <$> (iterateM (n-1) f =<< f i) iterateM n f i = sequence . scanl (>>=) (return i) $ replicate n f The former uses primitive recursion, and I get the feeling it should be better written without it. The latter is quadratic time – it builds up a list of monadic actions, and then runs them each in turn. Can anyone think of a version that combines the benefits of the two? Bob

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stream fusion -- multi-module programs
by Jason Dusek 08 Apr '09

08 Apr '09

In the paper "Stream Fusion: From Streams To Lists To Nothing At All", we find: For large multi-module programs, the results are less clear, with just as many programs speeding up as slowing down. We ﬁnd that for larger programs, GHC has a tendency to miss optimisation op- portunities for stream fusible functions across module boundaries, which is the subject of further investigation. What's the state of these things? Is this still an issue for stream fusion? -- Jason Dusek

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Re: A challenge
by Thomas Davie 08 Apr '09

08 Apr '09

Woops, the second version there applies it one too many times, it should have been replicate (n-1) f. Bob

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