April 2009 - Haskell-Cafe

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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Parallel combinator, performance advice
by Neil Mitchell 08 Apr '09

08 Apr '09

Hi, I've written a parallel_ function, code attached. I'm looking for criticism, suggestions etc on how to improve the performance and fairness of this parallel construct. (If it turns out this construct is already in a library somewhere, I'd be interested in that too!) The problem I'm trying to solve is running system commands in parallel. Importantly (unlike other Haskell parallel stuff) I'm not expecting computationally heavy Haskell to be running in the threads, and only want a maximum of n commands to fire at a time. The way I'm trying to implement this is with a parallel_ function: parallel_ :: [IO a] -> IO () The semantics are that after parallel_ returns each action will have been executed exactly once. The implementation (attached) creates a thread pool of numCapabililties-1 threads, each of which reads from a task pool and attempts to do some useful work. I use an idempotent function to ensure that all work is done at most one, and a sequence_ to ensure all work is done at least once. Running a benchmark of issuing 1 million trivial tasks (create, modify, read and IO ref) the version without any parallelism is really fast (< 0.1 sec), and the version with parallelism is slow (> 10 sec). This could be entirely due to space leaks etc when queueing many tasks. I'm useful for any thoughts people might have! Thanks in advance, Neil

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Monad transformer to consume a list
by Stephan Friedrichs 08 Apr '09

08 Apr '09

Hello, is there a monad transformer to consume an input list? I've got external events "streaming into the monad" that are consumed on demand and I'm not sure if there's something better than a StateT. //Stephan -- Früher hieß es ja: Ich denke, also bin ich. Heute weiß man: Es geht auch so. - Dieter Nuhr

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How to cut a file effciently?
by Magicloud Magiclouds 08 Apr '09

08 Apr '09

Hi, Let us say I have a text file of a million lines, and I want to cut it into smaller (10K lines) ones. How to do this? I have tried a few ways, none I think is lazy (I mean not reading the file all at the start). -- 竹密岂妨流水过山高哪阻野云飞

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Painful Data Abstraction
by Brad Larsen 08 Apr '09

08 Apr '09

I asked this earlier on #haskell, but it was quiet hour. Here goes. Suppose I have an abstract data type for prefix dictionaries of bytestrings. I define a typeclass in a module PrefixDict.Class: module PrefixDict.Class where import qualified Data.ByteString.Char8 as B type Word = B.ByteString class PrefixDict d where empty :: d isEmpty :: d -> Bool insert :: Word -> d -> d delete :: d -> Word -> d hasWord :: d -> Word -> Bool hasPrefix :: d -> Word -> Bool I then write several implementations, including a naive list implementation, and a trie implementation, in the modules PrefixDict.ListDict and PrefixDict.TrieDict, both looking something like this: module PrefixDict.ListDict where import PrefixDict.Class data ListDict = ListDict [Word] instance PrefixDict ListDict where ... Now, there are many properties that should be satisfied by any implementation of PrefixDict, that are not easily captured in the type system. Trying to be a good coder, I write up some generic QuickCheck properties, such as prop_insert_idempotent :: (Eq d, PrefixDict d) => d -> Word -> Bool prop_insert_idempotent d w = insert w d' == d' where d' = insert w d The problem is this: I can't think of a non-kludged way of checking that TrieDict and ListDict satisfy this abstract property without having to add explicit type signatures, e.g. quickCheck (prop_insert_idempotent :: TrieDict -> Word -> Bool) I have many abstract properties, and I *really* don't like having to put an explicit signature on each one. I came up with a hackishway that minimizes the number of signatures required, by checking all abstract properties in one, generic function, reflecting a type as a value, and using scoped type variables: testAbstractProperties :: forall d. (PrefixDict d) => d -> IO () testAbstractProperties _ = do quickCheck (prop_insert_idempotent :: d -> Word -> Bool) quickCheck (prop_delete_works :: d -> Word -> Property) ... I then call this function from a "test suite" function written for each module, e.g. testTrieDict :: IO () testTrieDict = do testAbstractProperties (undefined :: TrieDict) testTrieDictSpecificProperties This means I only have to write the type signatures on the properties in one place, but it is still less than ideal. Is there a better way of doing this, or is Haskell simply not good at this kind of data abstraction? Best, Brad Larsen

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cabal on mac os x (gmp problem)
by brian 08 Apr '09

08 Apr '09

For future reference: Solved, I just created a symlink to libgmp.dylib in /usr/local, and the zlib package was able to build.

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Generating arbitrary function in QuickCheck
by Yusaku Hashimoto 07 Apr '09

07 Apr '09

Hi, today, I want to ask about QuickCheck. I have known recently that arbitrary works on functions as well. I played on this for a while, and met a strange behavior. I tried to write property for sortBy. import Test.QuickCheck import Data.List instance Show (a -> b) where show _ = "<<function>>" instance Arbitrary Ordering where arbitrary = elements [LT,EQ,GT] mySortBy :: (Ord a) => (a -> a -> Ordering) -> [a] -> [a] mySortBy = sortBy prop_mysortby_sorted :: (Int -> Int -> Ordering) -> [Int] -> Property prop_mysortby_sorted cmp ls = null ls `trivial` all eq_or_lt (zipWith cmp sorted (tail sorted)) where sorted = mySortBy cmp ls eq_or_lt ord = ord == EQ || ord == LT I had thought Arbitrary instance for Ord and property for sortBy both fine. But checking fails: ghci> quickCheck prop_mysortby_sorted Falsifiable, after 2 tests: <<function>> [2,3] I guess arbitrary for (Int -> Int -> Ordering) generates a non-sense function like this: -- let (arb_compare :: Int -> Int -> Ordering) generated function. arb_compare 0 1 = GT arb_compare 1 0 = GT arb_compare _ _ = EQ Then, I want to ask two questions. 1. Is my guessing in function generated by arbitrary right? 2. If so, How do I generate right function? Thanks, Yusaku Hashimoto

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ANN: HCard -- A library for implementing card-like structures.
by Joe Fredette 07 Apr '09

07 Apr '09

HCard, is a library which supports a card-like data structures, it uses associated types [1] to provide shuffling, dealing, and other facilities. It's general enough to support many different types of playing cards, it currently comes with the common "French Deck" (4-suit, 13 card deck that is very common in the US) implemented and an example (though untested) cribbage scoring application. You can find HCard here [2,3], and a post about it's use of associated types here [1]. On another note, I can't think of a good reason I should be using associated types, as I explain in [1]. Well, no good reason other than the "cool factor" -- can anyone enlighten me on a possible practical benefit of using Assoc types as opposed to a simple indexed record? Anyway, enjoy HCard! /Joe Fredette [1] http://lowlymath.net/2009/card-games-scorekeeping-and-associated-datatypes/ [2] darcs get http://patch-tag.com/publicrepos/HCard [3] http://hackage.haskell.org/cgi-bin/hackage-scripts/package/HCard

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