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Very basic question on monads
by blackcat＠pro-ns.net 23 Feb '11

23 Feb '11

I'm working through the "Write Yourself a Scheme" wikibook and having trouble with one of the exercises. For the function parseNumber :: Parser LispVal, both parseNumber = liftM (Number . read) $ many1 digit parseNumber' = do digits <- many1 digit return $ (Number . read) digits work. But parseNumber'' = many1 digit >>= liftM read >>= liftM Number complains "couldn't match expected type Char against inferred type [Char]". Is liftM engaging the list monad? How do I fix this?

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Having trouble defining type correctly
by Chris Bolton 22 Feb '11

22 Feb '11

Ok, so here's my current code: http://paste.pocoo.org/show/342949/ My basic problem is I'm not quite sure how to define Command. When a user calls the script they can either call add, remove, update, or query, which all have different type signatures, which is why I need Command so it'll pass through haskell's type checking. If I get rid of query, A definition of { data Command m = Save m | Rem m } seemed to work, from what I can tell, but query screws it up, so I'm trying to define exactly what Save, Rem, etc. should be, which I've taken directly from the MongoDB API, but it just keeps throwing errors, as seen here: http://paste.pocoo.org/show/342924/ Also, I'm very new to haskell and programming in general, so sorry for the possibly messy code / noobish question, but anyone have any ideas here? :P

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Cabal dependencies problem
by Jun HU 22 Feb '11

22 Feb '11

Hello everyone, I've some problems with cabal dependency mechanism. For example, when i try to install package "funsat", :> cabal install funsat it shows ====================== MSG =================== BGN Registering parsec-3.1.1... Installing library in /home/jhu/.cabal/lib/parsec-3.1.1/ghc-6.12.1 Registering parsec-3.1.1... cabal: Error: some packages failed to install: array-0.2.0.0 failed during the building phase. The exception was: ExitFailure 1 bimap-0.2.4 depends on array-0.2.0.0 which failed to install. containers-0.2.0.1 depends on array-0.2.0.0 which failed to install. fgl-5.4.2.2 depends on array-0.2.0.0 which failed to install. funsat-0.6.1 depends on array-0.2.0.0 which failed to install. parse-dimacs-1.2 depends on array-0.2.0.0 which failed to install. ====================== MSG =================== END while the current array package is on version array-0.3.0.2. The package has back compatibility problem. Any advice is appreciated... thank you in advance. Sincerely, Jun

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Parallel builds with Cabal
by João Paulo Pizani Flor 21 Feb '11

21 Feb '11

Does anybody know if this is even possible? I want to do with cabal something similar to what "make -j N" does. Uses N threads to do a parallel build. From what I've read unitl know it seems Cabal doesn't (still) support this feature... Can I do this? "cabal install gtk2hs -j 4"?? It would be SO awesome :) João Paulo Pizani Flor joaopizani(a)gmail.com Computer Science B.Sc Federal University of Santa Catarina - Brazil

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Tail recursion problem
by Sebastian Arndt 20 Feb '11

20 Feb '11

Hi everyone, i'm trying to understand tail recursion, but im stuck when it comes to tail recursion with tuples. Look at the following source code: module Main(main) where main = do putStrLn "tail recursion" print $ countOdd2 [0..10000000] {- Tail recursion doesnt work -} countOdd :: [Int] -> (Int, Int) countOdd lst = countOdd' lst (0,0) countOdd' :: [Int] -> (Int, Int) -> (Int, Int) countOdd' (x:xs) last = if odd x then countOdd' xs $! (fst last, snd last + 1) else countOdd' xs $! (fst last + 1, snd last) countOdd' [] last = last {- Working tail recursion -} countOdd2 :: [Int] -> Int countOdd2 lst = countOdd2' lst 0 countOdd2' :: [Int] -> Int -> Int countOdd2' (x:xs) last = if odd x then countOdd2' xs $! last + 1 else countOdd2' xs last countOdd2' [] last = last The tail recursion in the function *countOdd2 *works fine. But it seems not to work in *countOdd *because i get a*Stack space overflow *error. Can anyone rewrite *countOdd *so that it works without a Stack space overflow? Thank you. Cheers Sebastian Arndt

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can't build haskell-crypto
by Chris Bolton 20 Feb '11

20 Feb '11

http://paste.pocoo.org/show/341187/ any ideas?

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Monadic Project Euler 1
by Javier M Mora 18 Feb '11

18 Feb '11

Hi, I'm trying to improve my skills with monads. I'm started with project Euler problems but creating/using Monads. I know that can be an overkill approach but, they are easy enough to focus in monad only. First Step: What I want? ------------------------ In this problem: I think monads as a DSL (Domain Specific Language) main = do print $ sumM $ do makeList 10 -- create candidates list multiples 3 -- choose multiples of 3 multiples 5 -- choose multiples of 5 (not choosed yet) Data under de monad is a pair of lists: (validValues, CandidatesNonValidYet) so makeList 10 = MyState ([],[1,2,3,4,5,6,7,8,9]) after multiples 3 -> MyState ([3,6,9],[1,2,4,5,7,8]) after multiples 5 -> MyState ([3,5,6,9],[1,2,4,7,8]) Second Step: What I have? ------------------------- newType MyState a = MyState {execMyState :: ([a],[a])} sumM :: (Integral a) => MyState a -> a sumM = sum $ fst $ execMyState makeList:: (Integral a) => a -> MyState a makeList max = MyState ([],[1..max-1]) -- maybe: makeList max = return [1..max-1] Third Step: function prototypes ------------------------------- ideal: multiple :: (Integral a) => a -> [a] -> MyState a less ideal multiple :: (Integral a) => a -> ([a],[a]) -> MyState a Fourth Step: Instanciate Monad ------------------------------ instance Monad MyState where return = error "no implemented" --(>>=) :: (Monad m) => m a -> (a -> m b) -> m b m >>= k = let (v, c) = execMyState m n = k c (nv, nc) = execMyState n in MyState (v++nv, nc) with this instanciation: k :: a -> m b but (multiple 3) :: [a] -> MyState a in one the function ask for a value "a" type and in the other case for a list. So, doesn't compile :-( Second option: newType MyState a = MyState {execMyState :: (a,a)} I like more the other option, because when you say "MyState Int" or "MyState float" you're saying than the possibilities are type Int or float or whatever. With this option (the second) you have to coerce that a type have to be a container in other part. if I can force (MonadPlus a) : instance Monad MyState where return a = MyState (a,mzero) --(>>=) :: (Monad m) => m a -> (a -> m b) -> m b m >>= k = let (v, c) = execMyState m n = k c (nv, nc) = execMyState n in MyState (v `mplus` nv, nc) or maybe I must use Monoid... but I don't know how force that and here I'm stuck Any hints?

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Understanding Haskell fcn calls
by blackcat＠pro-ns.net 17 Feb '11

17 Feb '11

When I learn a new language, I like to look at the assembly code for some simple fcn calls. I tried GHC -S on the following program: add' x y = x + y main = do print $ add' 1 2 The result of ghc -S add.hs was 301 lines of assembly code. Adding an explicit type annotation for add' brought it down to 252 lines, and the code was still beyond my meager ability to take in assembly. Is there a good reference that gives a simple explanation of how Haskell function calls work? This seems crucial to understanding how to efficiently use the language, given what I've read about tail call optimization. thanks Lee Short

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Monad Transformer Type Signatures
by Britt Anderson 15 Feb '11

15 Feb '11

I am looking at code from a tutorial that has the following type signature for a function: getScreen :: MonadReader AppConfig m => m Surface AppConfig is a data structure that was defined previously. My question is how am I to interpret this type signature? Is it any m belonging to the MonadReader AppConfig class? or any AppConfig m belonging to the MonadReader class? Since I can't see any instance of the MonadReader Class being defined for MonadReader AppConfig I am puzzled. I would appreciate some clarification or a pointer to a prior discussion of this issue. I have tried to look on my own. Thank you.

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State monad
by Britt Anderson 15 Feb '11

15 Feb '11

Many of the tutorials on the state monad seem to suggest that you can use State as a data constructor, but I can't get this to work. E.g. in ghci after :m Control.Monad.State I can let f = (\x -> (x,x)) let y = state f but if I, let z = State f I get an error message: Not in scope: data constructor `State'. Can someone please explain? Thanks, Britt

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