- Beginners - Haskell.org

monads and mutants
by Amy de Buitléir 21 Aug '10

21 Aug '10

I'm dipping a toe in the shallow end of the monad pool. I would appreciate any feedback on the first function here, "mutateList". It seems to be working fine, but I have this feeling that I'm doing some unnecessary mucking about with state and I don't know how to fix it. Since both randomListSelection and mutateGene update the state, it seems unnecessary for mutateList to get and put the state as well. What do ye think? I have a list of 16-bit unsigned words. I want to randomly select an element in the list. In the selected element, I want to flip a bit at random. ---------- The code ---------- {-# LANGUAGE PackageImports #-} import "mtl" Control.Monad.State import System.Random import Data.Word import Data.Bits -- | ***** LOOK HERE ***** -- | Flip a random bit in a random element in the list. mutateList :: [Word16] -> State StdGen [Word16] mutateList xs = do g <- get let ((i, x), g') = runState (randomListSelection xs) g let (x', g'') = runState (mutateGene x) g' put g'' return (replaceElement xs i x') -- | ***** LOOK HERE ***** -- | Choose an element at random from a list and return the element and its index randomListSelection :: [a] -> State StdGen (Int, a) randomListSelection xs = do g <- get let s = length xs let (i, g') = randomR (0,s-1) g put g' return (i, xs !! i) -- | Randomly flip a bit in this gene. mutateGene :: Word16 -> State StdGen Word16 mutateGene x = do g <- get let (i, g') = randomR (0,16) g put g' return (x `complementBit` i) -- | Replace an element in a list with a new element. replaceElement -- | The list :: [a] -- | Index of the element to replace. -> Int -- | The new element. -> a -- | The updated list. -> [a] replaceElement xs i x = fore ++ (x : aft) where fore = take i xs aft = drop (i+1) xs

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Data Class problem
by Michael Bradley 20 Aug '10

20 Aug '10

Hello all! I have been experimenting with data and class declarations and came across a problem. I have been trying to do this: Code: data Fire = Burn | Ember data Water = Bubble | WaterGun class Elements a instance Elements Fire instance Elements Water data Elemental = Elemental { name :: String, move :: (Elements a) => a } So, the idea was that "move" in the data constructor "Elemental" would be able to take any value from either the type Fire or Water. However, the error message tells me this is an illegal polymorphic type. Therefore, I tried creating a function that could read my value for me after "show" was applied to the move. Hence, the data declaration for Elemental could now assign "move" to a String. The function looked like this: Code: getMove :: (Elements b) => String -> b getMove x = read x :: (Elements a) => a This will not work either, as the function "read" complains of ambiguity in the letter a. I also tried this (amongst other attempts) Code: getMove :: (Elements b) => String -> b getMove "Burn" = Burn getMove "Ember" = Ember getMove "Bubble" = Bubble getMove "WaterGun" = WaterGun getMove _ = error "Unknown move!" The above caused the function to infer the type Fire, and then complain about the type Water. So, how can I either create a function that can return multiple types like I am trying to above, or is there a way to adjust the data declaration for Elemental? Also, I have noticed that 3 :: (Num a) => a will work but Burn :: (Elements a) => a causes an ambiguity error. Why is this the case? Please help! Mike

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Re: [Haskell-beginners] Data Class problem
by Markus Läll 20 Aug '10

20 Aug '10

I think the problem is, that in the datatype Elemental the compiler can't tell, what 'a' is. Both of these compile: {-# LANGUAGE ExistentialQuantification #-} ...code... data Elemental = forall a. (Elements a) => Elemental { name :: String, move :: a } or ...code... data Elemental a = Elemental { name :: String, move :: a } For the first, Elemental is an existentially quantified type over a, which takes any 'a' that is in the typeclass Elements. The second is a polymorphic type with a parameter 'a'. On 8/20/10, Jonas Almström Duregård <jonas.duregard(a)chalmers.se> wrote: > Not sure if this helps, but when you have > > move :: Elements a => a > > It doesn't mean move will return some unknown Elements type, it means you > can choose any type which is a member of elements and move will return that > type. > > This means that > > Burn :: Elements a => a > > implies > > Burn :: Water > > Which is wrong. > > 3 :: Double > 3 :: Integer > > are both correct on the other hand. > > /J > > > On 20 August 2010 10:58, Michael Bradley > <michael.bradley(a)hotmail.co.uk>wrote: > >> Hello all! >> >> I have been experimenting with data and class declarations and came across >> a problem. I have been trying to do this: >> >> Code: >> >> data Fire = Burn | Ember >> data Water = Bubble | WaterGun >> >> class Elements a >> >> instance Elements Fire >> instance Elements Water >> >> data Elemental = Elemental { name :: String, >> move :: (Elements a) => a >> } >> >> So, the idea was that "move" in the data constructor "Elemental" would be >> able to take any value from either the type Fire or Water. However, the >> error message tells me this is an illegal polymorphic type. >> >> Therefore, I tried creating a function that could read my value for me >> after "show" was applied to the move. Hence, the data declaration for >> Elemental could now assign "move" to a String. The function looked like >> this: >> >> Code: >> >> getMove :: (Elements b) => String -> b >> getMove x = read x :: (Elements a) => a >> >> This will not work either, as the function "read" complains of ambiguity >> in >> the letter a. I also tried this (amongst other attempts) >> >> Code: >> >> getMove :: (Elements b) => String -> b >> getMove "Burn" = Burn >> getMove "Ember" = Ember >> getMove "Bubble" = Bubble >> getMove "WaterGun" = WaterGun >> getMove _ = error "Unknown move!" >> >> The above caused the function to infer the type Fire, and then complain >> about the type Water. So, how can I either create a function that can >> return >> multiple types like I am trying to above, or is there a way to adjust the >> data declaration for Elemental? >> >> Also, I have noticed that 3 :: (Num a) => a will work but Burn :: >> (Elements a) => a causes an ambiguity error. Why is this the case? >> >> Please help! >> >> Mike >> >> >> _______________________________________________ >> Beginners mailing list >> Beginners(a)haskell.org >> http://www.haskell.org/mailman/listinfo/beginners >> >> >

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Argument order and partial function application
by Tom Murphy 19 Aug '10

19 Aug '10

Hi all, Thanks for this great resource. I've got a basic question about partial function application: Let's say I have a very basic function: volumeFunction length width height = length * width * height Its type signature is: volumeFunction :: Int -> Int -> Int -> Int This implies that I first pass it the length, *then* the width, *then* the height. So while it is very easy for me to write volumeWithLengthTen = volumeFunction 10 , it's not possible to create a volumeWithWidthTen or volumeWithHeightTen. I can see how I might be able to "hack" my way into accessing the "width" argument with infix functions, but there's no way at all that I can see to access the height. This seems arbitrary to me: my volumeFunction doesn't need them in the order they're given. Is there a way around this? Could I somehow combine three smaller functions? I appreciate any insight. Tom

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Restoring interleaved lists?
by Patrick LeBoutillier 19 Aug '10

19 Aug '10

Hi all, I'm parsing a file that represents a type of musical notation (Guitar Pro file format). In the format there are t tracks, each composed of m measures. In the file the measures are all stored sequentially, i.e. from measure 1 to measure t*m, in the following fashion: - measure 1 of track 1 ; - measure 1 of track 2 ; - ... - measure 1 of track t ; - measure 2 of track 1 ; - measure 2 of track 2 ; - ... - measure 2 of track t ; - ... - measure m of track 1 ; - measure m of track 2 ; - ... - measure m of track t ; I need a function that, given t and the list of t*m measures, can spilt the measures by track, returning a list of t lists, each containing m measures. I cannot figure out how to do this, even though it seems to me like it shouldn't be too hard... I can't figure out how to "update" the lists of lists when I want to add a new element. Do anyone have any ideas? Thanks, Patrick -- ===================== Patrick LeBoutillier Rosemère, Québec, Canada

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Question about ghc and cabal: finding packages
by C Gosch 19 Aug '10

19 Aug '10

Hi everyone, I have installed ghc 6.12.3 and the Haskell platform; now, when I install packages from hackage using cabal install <package-name>, they get installed in ~/.cabal, which is fair enough. However, when I call a runhaskell Setup.lhs configure that used to work on a different computer with the same software setup, ghc (or is it cabal?) complains that it cannot find the packages I have just installed. Checking again revealed that they are indeed installed, just not in the ghc installation directory, but under ~/.cabal. How can I tell cabal and/or ghc where to look for libraries? Thanks, again, for any help! Christian

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forM vs mapM
by Gaius Hammond 19 Aug '10

19 Aug '10

Hi all, Can anyone give me an example of a situation where you would use forM but *not* mapM or vice versa? As far as I can see one is simply a flip of the other. Does forM make any guarantees about order of execution? Thanks, G ------------------

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2 Questions: mutable data and copying results
by C Gosch 18 Aug '10

18 Aug '10

Hello to all Haskell experts! I have two more beginner questions: I ran into texts mentioning types of "mutable" data. Does that mean the data is actually changed in place? How would I go about implementing such a data type, as I expect much higher performance in some applications? Do I do that with the Foreign module and Ptr types? The second question is this: say I have some functions working with a data type, say a graph. Some of them might want to change the graph and return a new graph as a result. If they change say only one graph node, it would be extremely bad for performance if the whole graph got copied any time such a change takes place. Is the compiler smart enough to make the code only copy the parts that have changed (copying lazily, so to speak)? Is that even possible? Thank you in advance for any hint! Christian

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monad nomad gonad gomad
by prad 18 Aug '10

18 Aug '10

ok so taking MAN's suggestion: "I think It's time for you to get serious with the monads" that's just what i'm going to do! i found the following references: You Could Have Invented Monads! http://blog.sigfpe.com/2006/08/you-could-have-invented-monads-and.html an interesting way to become familiar with the idea Monads for the Working Haskell Programmer http://www.engr.mun.ca/~theo/Misc/haskell_and_monads.htm seems to have several practical ideas there (though i don't understand them yet) A tour of the Haskell Monad functions http://members.chello.nl/hjgtuyl/tourdemonad.html a nice reference with brief explanations Explaining Haskell IO without Monads http://neilmitchell.blogspot.com/2010/01/haskell-io-without-monads.html possibly a good conceptual aid for background All About Monads http://www.haskell.org/all_about_monads/html/index.html this seems to have everything though it'll take some work to dig in in any case, monads seem to be a rather important concept to getting anything done in haskell. for instance, i have a program which is generating the output it is supposed to (i can print it), but i can't seem to get it into another function and keep getting the error i've seen so many times: Couldn't match expected type `[String]' against inferred type `IO [String]' so it's time to understand them. besides, the stuff looks rather intriguing and certainly appears to take one into computer language design theory. any sources others have found useful would be appreciated as well as suggestions on how to proceed through the above. in friendship, prad -- In friendship, prad ... with you on your journey Towards Freedom http://www.towardsfreedom.com (website) Information, Inspiration, Imagination - truly a site for soaring I's

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hClose: illegal operation (handle is finalized) and/or <<loop>>
by Travis Erdman 18 Aug '10

18 Aug '10

> Perhaps in your {BOOLEANS} you are referring to > 'recurseChildren', which in turn needs the {BOOLEANS}? Uggh! Indirectly, yes, I am. OK, back to the drawing board for me on this one. Thanks!

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