- Haskell-Cafe - Haskell.org

Newbie questions
by Crypt Master 02 Jul '04

02 Jul '04

Hi I consider myself a pretty good imperative programmer, been at it for decades, and am tryibng my hand at Haskell now. Unfrituantly I am not "getting" it all that quickly despite having the multimedia haskell book. To start ghetting some hands on, I thought iI owuld do something which i have done many times before in imperatives, a basic Genetic Algorithm Library. Unfortunatly I am comming unstuck straight away and was hoping some of you kind folk could point in the direction of dry land :-) iIf your not familiar with GAs its not too important, simply a GA searches over the whole solution space randomly. Well not quiet, its a guided search, its guided by evolution ie by the fitness of each indicidual of a random initial population. Evolution is simulated by 3 basic operators, selection, cross over and mutation. selection is which indiviuals get to breed, cross over is how they breed, and mutation is just things intetresting. So after reading the multimedia book, Iimmediately thought of a defining the solution space as a infinite list. I was hoping then I could do things like take 5 gaSolutionSpace to get 5 iterations or generations. My first attempt tied to use lis syntax [], but it wouldnt compile and after seeing "numsFrom" ina tutorial I redefined it as such. Here is what I have so far: -- gaSolutionSpace :: [a] -> [a] -- gaSolutionSpace [] = gaSolutionSpace createRandomPopulation -- recursive base case gaSolutionSpace x = x : gaSolutionSpace (evolvepopulation x) evolvepopulation :: a -> a evolvepopulation p = mutate cross select p -- createRandomPopulation :: [Num a] createRandomPopulation = [1,23,4,5,6] cross p = p mutate p = p select p = p ---- The take operator doesnt work on this. from hugs: HAGA> take 5 gaSolutionSpace [1,2,3,4,5] ERROR - Type error in application *** Expression : take 5 gaSolutionSpace [1,2,3,4,5] *** Term : take *** Type : Int -> [e] -> [e] *** Does not match : a -> b -> c -> d I am not sure I follow this. I assume its cause I didnt use the list notation in definaing gaSolutionSpace. Any ideas on how to do that. ? What about using map. It occurs to me that you can define gaSolutionspace as a map of the evolvolepopuloation function acrossan infinite solution space, but I dont know how to makethis infinte list to map over ...? There ar 2 papaers on GAs in haskell, but they use monads. I realise for performance I will probably have to use them too, but for now I would liek to do it without mondas even if performance isnt optimal. Sorry again for the newbie questions, but any help is appreciated. Stephen _________________________________________________________________ Add photos to your e-mail with MSN 8. Get 2 months FREE*. http://join.msn.com/?page=features/featuredemail

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RE: [Haskell-cafe] some newbie FFI questions
by Simon Marlow 01 Jul '04

01 Jul '04

On 01 July 2004 11:53, John Kozak wrote: > I'm trying to experiment with image processing in haskell (with which > I haven't much experience). I've written some FFI code to talk to > the ImageMagick library which provokes a few questions (environment is > ghc 6.2.1 on debian): > > 1. Speed: I'm reading in a 2000x1500 pixel image, and have defined a > Pixel type like this: > > data Pixel a = Pixel !a !a !a deriving Show > > I use ImageMagick to load the image, then build an Array of Pixel > Floats. Building the array takes 45 seconds on a 2.5Ghz P4 with > code compiled -O2, which seems slow to me - are my expectations > unrealistic? I've tried various UNPACK things which didn't make > much difference. It depends how you want to use the image data, but this is almost certainly not the best representation. The fastest choice would be simply 'Ptr CFloat' (i.e. do no conversion or marshalling at all). This is essentially what your C program is doing. In Haskell you can allocate the memory using allocaBytes or mallocForeignPtr (no free required). Your other choices are: - IOUArray Int Float Fast access, but you need to convert the entire array into one of these, and back again if you need to pass it back to C. - (IOUArray Int Float, IOUArray Int Float, IOUArray Int Float) i.e. split the planes. Might be useful, depending on how you access the array. Conversion required again. - StorableArray Int CFloat You can convert the 'Ptr CFloat' you get back from C directly into a StorableArray, which gives you easy access using the mutable array primitives in Haskell, and an easy way to pass the array back to C using withStorableArray. Unfortunately accessing StorableArrays isn't as fast as it could be. And there ought to be a way to convert between StorableArray and IOUArray. This is one area that could do with some attention. - Array Pixel, and data Pixel = Pixel {-# UNPACK #-}!Float ... Not too great, because you have a few extra layers of indirection compared to IOUArrays, even with the UNPACKs. - Array (Float,Float,Float) Terrible :-) > 2. How do I convert a CFloat into a Float? Using realToFrac, I guess. > 3. I get the wrong answer ;-) I expect the C and haskell code below > to produce the same pixel data, but they don't (the C code is > right). I suggest fixing a more efficient representation first. This will reduce the amount of manipulation of the data, and either make the bug go away or make it easier to find. Cheers, Simon

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Re: [Haskell-cafe] some newbie FFI questions
by MR K P SCHUPKE 01 Jul '04

01 Jul '04

Yes, deriving unboxable would be nice... I think for images though it is not too much of a problem to work with a record of arrays rather that an array of records. Keean.

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Re: [Haskell-cafe] some newbie FFI questions
by MR K P SCHUPKE 01 Jul '04

01 Jul '04

Oh... also when using unboxed arrays you may well have to have a separate array for each colour plane (r,g,b,a) as I think unboxed arrays can only contain primitive types - although I am not certain about this - it may be enough to have the contents strict. Keean.

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Re: [Haskell-cafe] some newbie FFI questions
by MR K P SCHUPKE 01 Jul '04

01 Jul '04

If you are using GHC the fastest way I know to do this is to build the array using a mutable-unboxed array in the state-monad. Then freeze the array and return an unboxed immutable array. This is a lot faster than building the array by copying (every time you set a value in an immutable array the whole array much be copied - for a sizeable image this is quite a lot of work!) Keean.

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Re: [Haskell] Help in understanding a type error involving forall and class constraints
by Duncan Coutts 30 Jun '04

30 Jun '04

On Wed, 2004-06-30 at 12:00, MR K P SCHUPKE wrote: > Ahh.. I see whats happening: > > MArray (STUArray s) a (ST s) => String -> String -> ST s (UArray (Int,Int) a) > > IArray UArray a => String -> String -> a > > nothing here is specifying a... you cannot leave a polymorphic in this case. > > You need to supply the constraint "MArray (STUArray s) a (ST s)" to distString > but "s" is not in context... Right, because the return type of distString Int is an instance of MArray. But if we try to make 'a' polymorphic then we can't specify the constraint because it involves 's'. So the obvious follow-on question, is is there any intermediate point where 's' is in scope where we can supply a type annotation to supply the context or is it simply impossible? To restate the question for Haskell-Cafe readers: Is it possible to return an arbitrary unboxed array that was constructed in the ST monad (as an STUArray)? The issue is that you end up with a MArray class constraint that involves the state thread's 's' parameter, but this type variable gets 'hidden' by runST which universally quantifies over it. runST :: forall a. (forall s. ST s a) -> a Duncan

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Switching monadic encapsulations
by Graham Klyne 29 Jun '04

29 Jun '04

Is there a function that switches monadic layering? f :: (Monad m1,Monad m2) => m1 (m2 a) -> m2 (m1 a) Does this even make sense in the general case? I'm thinking along the lines of a generalization of sequence to non-list monads. This question arises from a desire to generalize some error handling code, starting with: liftES :: (a->Either String [a]) -> Either String [a] -> Either String [a] where (Either String) as used as an error monad. A simple implementation of this might be: liftES _ (Left er) = Left er liftES f (Right as) = sequence $ map f as but I notice that liftES looks a little bit like liftM, and sequence does for the List monad what I am seeking to generalize: sequence :: [m a] -> m [a] and map looks like a version of liftM specialized to lists: liftM f = \a -> do { a1 <- a ; return (f a1) } = \a -> ( a >>= \a1 -> return (f a1) ) when m is a list monad: = \a -> concat (map (\a1 -> [f a1]) a) Similarly, concat appears to be a monadic join on lists. My listES might be partially generalized to something like: liftX :: (r -> m [r]) -> m [r] -> m [r] liftX mf = \mb -> do { b1 <- mb ; liftM concat $ sequence $ map mf b } the liftM here being used to apply 'concat' to achieve m [[r]] -> m [r]. I can see a possible replacement of concat with join and map with liftM, but I don't see any purely monadic equivalent for 'sequence'. Hence the original question. #g ------------ Graham Klyne For email: http://www.ninebynine.org/#Contact

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RE: [Haskell-cafe] readEitherChan
by Simon Marlow 28 Jun '04

28 Jun '04

On 25 June 2004 21:20, S. Alexander Jacobson wrote: > On Fri, 25 Jun 2004, Simon Marlow wrote: >> It seems to me that with MVars you can implement pretty much any >> abstraction you need, including choice, and it's not clear that there >> would be any advantage to providing any more primitives. That's >> based on my experience so far, anyway - I know there are people out >> there that have made rather more heavy use of MVars than I have (eg. >> George Russell). > > Well we appear to be having a problem implementing > a takeEitherMVar or readEitherChan without adding > a new abstraction.... I strongly suspect that it can't be done, at least not if you want to have (a) non-busy waiting, and (b) async exception safety. My point was that I believe in order to support takeEitherMVar as a primitive, we would have to add essentially the same machinery to MVars that you would need if you were to implement it as an abstraction on top of MVars in Haskell. Cheers, Simon

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RE: [Haskell-cafe] need help on exercise
by Graham Klyne 28 Jun '04

28 Jun '04

Your problem seems to be related to the Num class, and the fact that the compiler can't figure out what '5' is -- see pp 223/224 of Simon Thompson's book. The code below offers two possible resolutions, one commented: [[ class Show a => Visible a where toString :: a -> String size :: a -> Int toString x = show x size x = 1 {- instance Visible Int where toString x = show x size _ = 1 -} instance Num a => Visible a where toString x = show x size _ = 1 {- t1 - toString (5::Int) -} t1 = toString 5 ]] #g -- At 23:58 27/06/04 -0400, kasager wrote: >Hi, in my last posting i asked for help with exercise 12.5 however, i forgot >to give my code... here is my code and the error messege that hugs gives when >i try to run it (it compiles fine, however when i test it, it gives an >error)... > >Code: > >class Show a => Visible a where > toString :: a -> String > size :: a -> Int > > toString x = show x > size x = 1 > >instance Visible Int where > toString x = show x > size _ = 1 > >-- i have tried writing this without the show function, and the same error >messege still is given... i also have other instances in the class, which >work >fine and should not effect this issue... > >Error: > >Prelude> :l "C:\\WorkArea\\Haskell\\homework\\ch12_5.hs" >Main> toString 5 >ERROR - Unresolved overloading >*** Type : (Num a, Visible a) => [Char] >*** Expression : toString 5 > > >If you can help in any way or give any suggestions its greatly appreciated... >once again thanks much. ------------ Graham Klyne For email: http://www.ninebynine.org/#Contact

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need help on exercise
by kasager 26 Jun '04

26 Jun '04

Hi, I am working on exercise 12.5 from "Haskell, The Craft of Functional Programming, second edition" by Simon Thompson. It asks to make Int an instance in the Visible class. I have tried multiple ways of doing this however I keep receiving the same error messege when I compile my program, saying that my types do not match and i have given a (Num a, Visible a) => String, when it is supposed to be an Int -> String. If anyone could help me with this issue it would be greatly appreciated. Thanks, Kelly

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