September 2012 - Beginners

Problem building monte-carlo package.
by Libor Wagner 14 Sep '12

14 Sep '12

Hi, I have problem building the monte-carlo package (http://hackage.haskell.org/package/monte-carlo/) Using both the hackage and github (https://github.com/patperry/hs-monte-carlo) version, I have got the following error: lib/Control/Monad/MC/Base.hs:23:24: Expecting one more argument to `m' In the class declaration for `MonadMC' I'm using ghc 7.4.2. Can someone see the what parameter is missing, because I can't. Thanks, Libor Here is the Base.hs: {-# LANGUAGE TypeFamilies #-} ----------------------------------------------------------------------------- -- | -- Module : Control.Monad.MC.Base -- Copyright : Copyright (c) 2010, Patrick Perry <patperry(a)gmail.com> -- License : BSD3 -- Maintainer : Patrick Perry <patperry(a)gmail.com> -- Stability : experimental -- module Control.Monad.MC.Base where import Control.Monad import qualified Control.Monad.MC.GSLBase as GSL import qualified Data.Vector.Storable as VS class HasRNG m where -- | The random number generator type for the monad. type RNG m class (Monad m, HasRNG m) => MonadMC m where -- | Get the current random number generator. getRNG :: m (RNG m) -- | Set the current random number generator. setRNG :: RNG m -> m () -- | @uniform a b@ generates a value uniformly distributed in @[a,b)@. uniform :: Double -> Double -> m Double -- | @uniformInt n@ generates an integer uniformly in the range @[0,n-1]@. -- It is an error to call this function with a non-positive value. uniformInt :: Int -> m Int -- | @normal mu sigma@ generates a Normal random variable with mean -- @mu@ and standard deviation @sigma@. normal :: Double -> Double -> m Double -- | @exponential mu@ generates an Exponential variate with mean @mu@. exponential :: Double -> m Double -- | @levy c alpha@ gets a Levy alpha-stable variate with scale @c@ and -- exponent @alpha@. The algorithm only works for @0 < alpha <= 2@. levy :: Double -> Double -> m Double -- | @levySkew c alpha beta @ gets a skew Levy alpha-stable variate -- with scale @c@, exponent @alpha@, and skewness @beta@. The skew -- parameter must lie in the range @[-1,1]@. The algorithm only works -- for @0 < alpha <= 2@. levySkew :: Double -> Double -> Double -> m Double -- | @poisson mu@ generates a Poisson random variable with mean @mu@. poisson :: Double -> m Int -- | @cauchy a@ generates a Cauchy random variable with scale -- parameter @a@. cauchy :: Double -> m Double -- | @beta a b@ generates a beta random variable with -- parameters @a@ and @b@. beta :: Double -> Double -> m Double -- | @logistic a@ generates a logistic random variable with -- parameter @a@. logistic :: Double -> m Double -- | @pareto a b@ generates a Pareto random variable with -- exponent @a@ and scale @b@. pareto :: Double -> Double -> m Double -- | @weibull a b@ generates a Weibull random variable with -- scale @a@ and exponent @b@. weibull :: Double -> Double -> m Double -- | @gamma a b@ generates a gamma random variable with -- parameters @a@ and @b@. gamma :: Double -> Double -> m Double -- | @multinomial n ps@ generates a multinomial random -- variable with parameters @ps@ formed by @n@ trials. multinomial :: Int -> VS.Vector Double -> m (VS.Vector Int) -- | @dirichlet alphas@ generates a Dirichlet random variable -- with parameters @alphas@. dirichlet :: VS.Vector Double -> m (VS.Vector Double) -- | Get the baton from the Monte Carlo monad without performing any -- computations. Useful but dangerous. unsafeInterleaveMC :: m a -> m a -- | Generate 'True' events with the given probability bernoulli :: (MonadMC m) => Double -> m Bool bernoulli p = liftM (< p) $ uniform 0 1 {-# INLINE bernoulli #-} ------------------------------- Instances ----------------------------------- instance HasRNG GSL.MC where type RNG GSL.MC = GSL.RNG instance MonadMC GSL.MC where getRNG = GSL.getRNG {-# INLINE getRNG #-} setRNG = GSL.setRNG {-# INLINE setRNG #-} uniform = GSL.uniform {-# INLINE uniform #-} uniformInt = GSL.uniformInt {-# INLINE uniformInt #-} normal = GSL.normal {-# INLINE normal #-} exponential = GSL.exponential {-# INLINE exponential #-} levy = GSL.levy {-# INLINE levy #-} levySkew = GSL.levySkew {-# INLINE levySkew #-} poisson = GSL.poisson {-# INLINE poisson #-} cauchy = GSL.cauchy {-# INLINE cauchy #-} beta = GSL.beta {-# INLINE beta #-} logistic = GSL.logistic {-# INLINE logistic #-} pareto = GSL.pareto {-# INLINE pareto #-} weibull = GSL.weibull {-# INLINE weibull #-} gamma = GSL.gamma {-# INLINE gamma #-} multinomial = GSL.multinomial {-# INLINE multinomial #-} dirichlet = GSL.dirichlet {-# INLINE dirichlet #-} unsafeInterleaveMC = GSL.unsafeInterleaveMC {-# INLINE unsafeInterleaveMC #-} instance (Monad m) => HasRNG (GSL.MCT m) where type RNG (GSL.MCT m) = GSL.RNG instance (Monad m) => MonadMC (GSL.MCT m) where getRNG = GSL.liftMCT GSL.getRNG {-# INLINE getRNG #-} setRNG r = GSL.liftMCT $ GSL.setRNG r {-# INLINE setRNG #-} uniform a b = GSL.liftMCT $ GSL.uniform a b {-# INLINE uniform #-} uniformInt n = GSL.liftMCT $ GSL.uniformInt n {-# INLINE uniformInt #-} normal mu sigma = GSL.liftMCT $ GSL.normal mu sigma {-# INLINE normal #-} exponential mu = GSL.liftMCT $ GSL.exponential mu {-# INLINE exponential #-} levy c alpha = GSL.liftMCT $ GSL.levy c alpha {-# INLINE levy #-} levySkew c alpha beta_ = GSL.liftMCT $ GSL.levySkew c alpha beta_ {-# INLINE levySkew #-} poisson mu = GSL.liftMCT $ GSL.poisson mu {-# INLINE poisson #-} cauchy a = GSL.liftMCT $ GSL.cauchy a {-# INLINE cauchy #-} beta a b = GSL.liftMCT $ GSL.beta a b {-# INLINE beta #-} logistic a = GSL.liftMCT $ GSL.logistic a {-# INLINE logistic #-} pareto a b = GSL.liftMCT $ GSL.pareto a b {-# INLINE pareto #-} weibull a b = GSL.liftMCT $ GSL.weibull a b {-# INLINE weibull #-} gamma a b = GSL.liftMCT $ GSL.gamma a b {-# INLINE gamma #-} multinomial n ps = GSL.liftMCT $ GSL.multinomial n ps {-# INLINE multinomial #-} dirichlet alphas = GSL.liftMCT $ GSL.dirichlet alphas {-# INLINE dirichlet #-} unsafeInterleaveMC = GSL.unsafeInterleaveMCT {-# INLINE unsafeInterleaveMC #-}

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type class functions as function arguments
by Christopher Howard 14 Sep '12

14 Sep '12

In my app, I am trying to save a whole bunch of code duplication by leverage the fact that functions can be passed in to other functions as arguments. However, I run into a kink when I try to use functions from type classes, and then apply those functions to different types of arguments after the functions have been passed in. My actual code is rather long and somewhat complicated, so I will use this simpler example: Obviously both the following functions work fine: code: -------- g :: (Integer, Double) g = ((+) 1 2, (+) 2.0 3.0) g' :: (Integer, Double) g' = ((-) 1 2, (-) 2.0 3.0) *Main> g (3,5.0) *Main> g' (-1,-1.0) -------- However, what if I want to create a single function, where I just pass in the (+) or the (-) function? I not sure the right way to do that, exactly, but I tried something like this: code: -------- gCore :: Num a => (a -> a -> a) -> (Integer, Double) gCore f = (f 1 2, f 2.0 3.0) -------- The resulting error is: code: -------- Couldn't match type `Double' with `Integer' In the return type of a call of `f' In the expression: f 1 2 In the expression: (f 1 2, f 2.0 3.0) -------- It is as though the function "forgot" that it could be used on different types, and just picked one, and now is complaining that is being used with the wrong type in the other case. -- frigidcode.com indicium.us

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cyclic imports
by Christopher Howard 13 Sep '12

13 Sep '12

In my app, I have a class definition and a data type definition that depend on each other: code: -------- class (InternallyUpdating a) => Transient a where {- Whether or not the object should be removed from existence -} expired' :: a -> Maybe [AfterEffect] data AfterEffect = forall a. ( Animation a , Transient a ) => VisualEffect a -- to be extended algebraically... -------- It's written in this somewhat recursive manner so that one effect can give rise to another effect when it is completed. This code compiles just fine when the two definitions are in the same file. However, if I separate them out to different modules, each module most import the other, resulting in this error: code: -------- Module imports form a cycle: module `AfterEffect' (AfterEffect.hs) imports `Updating' (./Updating.hs) which imports `AfterEffect' (AfterEffect.hs) -------- As a point of curiosity, at least, I'm wondering why GHC does not allow mutally-dependent imports, as recursive dependencies doesn't seem to be a problem in any other area of Haskell. And I'm wondering if there is some kind of trick to get around this and allow the cyclic dependency. -- frigidcode.com indicium.us

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Question about time consume when calculate prime numbers
by Yi Cheng 13 Sep '12

13 Sep '12

Recently, I'm trying to solve some problems in project euler using haskell. When it came to problem 10, calculating the sum of all primes below 20000000, I try to write a program which can generate primes. In my memory Eratosthenes is faster than just whether a number can be divided by the number less then the square root of it. Firstly, I wrote the following programs: module Main where isPrime x = isPrime' 3 x (round . sqrt. fromIntegral $ x) isPrime' d target maxd | d > maxd = True | mod target d == 0 = False | otherwise = isPrime' (d + 2) target maxd main = print $ (sum (filter isPrime [3,5..2000000]) + 2) And it consume about 11s in my computer. Then, I tried to figure out how to solve the problem by Eratosthenes, but failed. Later, I find a program implemented by others, meeting my purpose and I've used it to solve the problem: primes :: [Int] primes = primes' [2..] primes' :: [Int] -> [Int] primes' [] = [] primes' (n:ns) = n : primes' (filter (\v -> v `mod` n /= 0) ns) solve x = sum $ primes' [2..x] main = print $ solve 2000000 Well, although the code is beautiful, it is slow. Even waiting for a minute, no answer was printed. In C version, Eratosthenes is faster than the method implemented in my earlier code, which only consume 0.3s(the earlier method consume 1.6s). So I want to know, why Eratosthenes implemented in Haskell is slow than the ugly code implemented by me. Could anyone tell me? Thank you Yi Cheng

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Haskell for Android
by Miroslav Karpis 13 Sep '12

13 Sep '12

Hi haskellers, please can you help me with this? I read several posts about it, but unfortunately it is still not clear. What is (if there is) the most straight way to port haskell code to android? Translate haskell to c or javascript? Not sure how debugging will work this way…. ? thanks, m.

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sometimes Haskell isn't what you want
by Dennis Raddle 12 Sep '12

12 Sep '12

Sadly, I've decided Haskell is not the right language for my current project. Python is better. I need to hack together data, and strict typing is getting in the way. Most of my algorithms are better served with imperative/mutable-data. I learned a lot about Haskell trying to do it, but my knowledge of the language is not quiet good enough and I feel like I'm fighting the language. Python is better. For now.

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Playing and combining audio files
by Tobias 12 Sep '12

12 Sep '12

Hello, I'm searching for a library with which I can process audio files (.wav or .mp3). All I need is functions with which I can - play audio files - concat several audio files to one audio file Can you suggest a library for this task? I looked through http://hackage.haskell.org/packages/archive/pkg-list.html#cat:sound but I didn't manage to find ways to do this respectively sample code for this. Any hints are welcome- thanks, Tobias

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Data.Map: fromList
by Stayvoid 12 Sep '12

12 Sep '12

Hello, I'm reading learnyouahaskell.com. There is an example: [1] ghci> Map.fromList [("betty","555-2938"),("bonnie","452-2928"),("lucille","205-2928")] fromList [("betty","555-2938"),("bonnie","452-2928"),("lucille","205-2928")] Why does Map.fromList represent its output like this? Could you explain me the following definition? data Map.Map k a = Data.Map.Tip | Data.Map.Bin !Data.Map.Size !k a !(Map.Map k a) !(Map.Map k a) I guess that this fromList example is a bit premature. The only parameterised type I know is this one: data Maybe a = Nothing | Just a [1] http://learnyouahaskell.com/modules#data-map Thanks

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Data.Char: isAlpha vs. isLetter
by Stayvoid 12 Sep '12

12 Sep '12

Hi, What's the reason to have both isAlpha and isLetter? Looks like it's just an alias. I don't understand the purpose. Could you explain? Thanks

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Who discovered map reduce?
by Bryce 11 Sep '12

11 Sep '12

In the same vein as the "who discovered the fold operation" thread from last week, I would like to know the same answer for "map". I tried looking it up on wikipedia, but the best line I could find from it is this: "The map function originated in functional programming languages but is today supported (or may be defined) in many procedural, object oriented, and multi-paradigm languages as well: In C++'s Standard Template Library, it is called transform, in C# (3.0)'s LINQ library, it is provided as an extension method called Select." Does anyone have an answer or idea on where I could look? Thanks in advance and sorry if it it's a redundant question. Bryce

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