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Using QuickCheck to verify that an expression can always be evaluated
by Amy de Buitléir 15 Apr '11

15 Apr '11

Occasionally I write a QuickCheck test where I want to evaluate something just to make sure it doesn't crash, but I don't care about the actual value. For example, in the test below, I just want to make sure that the expression "foldl' apply newWorkbench gs" doesn't crash. I don't really care whether or not "devotion w /= -1", that's just an expression (which is always true) that I wrote to force evaluation of w. But I assume there's a better way to do this? prop_all_gene_sequences_are_legal :: [Gene] -> Property prop_all_gene_sequences_are_legal gs = property $ devotion w /= -1 where w = foldl' apply newWorkbench gs Thank you in advance for any suggestions.

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please critique my first stab at systems programming
by Sean Perry 15 Apr '11

15 Apr '11

The idea is to walk the disk looking for a signature, say NTFS or EXT. Since we do not know where the block containing this identifier is, we read the blocks in one at a time. Long term I would like to support command line arguments for the name of the file and the offset to start looking. Comments on style, idioms, etc. welcomed. I am particularly interested in the hIsEOF check and if there is a better way to handle that. import qualified Data.ByteString as B import qualified Data.ByteString.Char8 as BC import IO import System.IO chunkSize = 512 searchForPattern handle pat = searchForPattern' 0 handle pat searchForPattern' index handle pat = do eof <- hIsEOF handle if eof then return Nothing else do bytes <- B.hGet handle chunkSize case BC.breakSubstring pat bytes of (x, y) | BC.null y -> searchForPattern' (index + 1) handle pat | otherwise -> return (Just index) main = do fromHandle <- openBinaryFile "blocks" ReadMode result <- searchForPattern fromHandle (BC.pack "PART") case result of Nothing -> putStr "Not Found.\n" Just n -> putStr $ "Found at " ++ show n ++ ".\n" hClose fromHandle putStr "Done.\n"

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How to extract from Maybe & Either in GHCi
by blackcat＠pro-ns.net 14 Apr '11

14 Apr '11

I've got some values in GHCi that look like Just (Left (Blah [stuff])) and I'd like to pull out the (Blah [stuff]) to operate on it. Is there a way to do this directly in GHCi without writing a helper function? thanks Lee Short

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Haskell Package for Mac issues
by Andrew n marshall 13 Apr '11

13 Apr '11

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Idiomatic way to avoid type class instance definitions for Int and Integer separately
by Amitava Shee 13 Apr '11

13 Apr '11

While reading "Learn You a Haskell for Great Good!" I came across the YesNo type class I tried a minimal version as below module Kind where class Yesno a where yesno :: a -> Bool instance Yesno Int where yesno 0 = False yesno _ = True I was surprised to get an error *Kind> :load kind.hs [1 of 1] Compiling Kind ( kind.hs, interpreted ) Ok, modules loaded: Kind. *Kind> yesno 10 <interactive>:1:6: Ambiguous type variable `t' in the constraints: `Num t' arising from the literal `10' at <interactive>:1:6-7 `Yesno t' arising from a use of `yesno' at <interactive>:1:0-7 Probable fix: add a type signature that fixes these type variable(s) Turns out 10 in this instance is an Integer and I have not defined Yesno over Integer Easy fix - just define an instance over Integer instance Yesno Integer where yesno 0 = False yesno _ = True My question - Is there a way to avoid this kind of boilerplate? What is the idiomatic way? Thanks & Regards, Amitava Shee

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[Slightly OT] Any Haskell hackers in Rhein-Neckar region?
by C Gosch 12 Apr '11

12 Apr '11

Hello everyone, sorry, this is slightly off topic; are there any Haskell hackers in the Rhein-Neckar region, Germany, on this list? I'm trying to find like-minded people around there, but it seems so far that this area is a Haskell blind spot. Cheers, Christian

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class (Monad m) => MonadState s m | m -> s where - What does m | m -> s construct do?
by Amitava Shee 11 Apr '11

11 Apr '11

I can across the following construct in ghc source class (Monad m) => MonadState s m | m -> s where get :: m s put :: s -> m () I am not aware of the syntactic construct | m -> s just before the where? Can someone please explain it? Thanks & Regards, Amitava Shee

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Where to discuss and ask questions on "Category Theory"?
by Amitava Shee 11 Apr '11

11 Apr '11

I am trying to understand Category theory and currently reading Conceptual Mathematics - A first introduction to categories by Lawvere and Schanuel Do you know of a forum to ask clarifications and discuss key ideas? I am assuming this is not that forum. Thanks & Regards, Amitava Shee

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what does (# a, s #) mean?
by Amitava Shee 11 Apr '11

11 Apr '11

I came across this definition in ghc source $GHC/compiler/utils/State.hs ======================= newtype State s a = State { runState' :: s -> (# a, s #) } What does the construct (# a, s #) mean? Thanks & Regards, Amitava Shee

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Real programmers don't eat quiche.
by Sean Charles 10 Apr '11

10 Apr '11

LMVAO. Laughed my vegetarian etc. yadda yadda yadda Where on earth did you find that ?? My local chinese supermarket has some awesomely badly named stuff. funnyengrish.com is good for that kind of thing.

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