- Beginners - Haskell.org

randomize the order of a list
by Gaius Hammond 28 Aug '10

28 Aug '10

Hi all, I am trying to randomly reorder a list (e.g. shuffle a deck of cards) . My initial approach is to treat it as an array, generate a list of unique random numbers between 0 and n - 1, then use those numbers as new indexes. I am using a function to generate random numbers in the State monad as follows: randInt∷ Int → State StdGen Int randInt x = do g ← get (v,g') ← return $ randomR (0, x) g put g' return v This is pretty much straight from the documentation. My function for the new indexes is: -- return a list of numbers 0 to x-1 in random order randIndex∷ Int → StdGen → ([Int], StdGen) randIndex x = runState $ do let randIndex' acc r | (length acc ≡ x) = acc | (r `elem` acc) ∨ (r ≡ (−1)) = do r' ← randInt (x − 1) randIndex' acc r' | otherwise = do r' ← randInt (x − 1) randIndex' r:acc r' in randIndex' [] (−1) This fails to compile on Couldn't match expected type `[a]' against inferred type `State StdGen b' In a stmt of a 'do' expression: r' <- randInt (x - 1) In the expression: do { r' <- randInt (x - 1); randIndex' acc r' } I can see what's happening here - it's treating randIndex' as the second argument to randInt instead of invisibly putting the State in there. Or am I going about this completely the wrong way? Thanks, G

4 4

Fwd: [Haskell-beginners] instances of different kinds
by Tobias Brandt 27 Aug '10

27 Aug '10

Forgot to reply to the list, sorry. ---------- Forwarded message ---------- From: Tobias Brandt <tob.brandt(a)googlemail.com> Date: 27 August 2010 11:14 Subject: Re: [Haskell-beginners] instances of different kinds To: Greg <greglists(a)me.com> On 27 August 2010 10:58, Greg <greglists(a)me.com> wrote: > data Foo a = Foo a > > class TwoPi a where > div2pi :: (Floating b) => a -> b > div2pi is polymorphic in a AND b. They are completely independent. > instance (Floating a) => TwoPi (Foo a) where > div2pi (Foo a) = a / (2*pi) > but here, div2pi has type Floating a => Foo a -> a. > > data Foo a = Foo a > > class TwoPi a where > div2pi :: a -> Float > > instance (Floating b) => TwoPi (Foo b) where > div2pi (Foo b) = b / (2*pi) > now div2pi has type Floating a :: Foo a -> a, but should have Floating a :: Foo a -> Float There are two possible solutions (I can think of). 1. make div2pi less polymorphic: class TwoPi a where div2pi :: a -> a then your first instance works 2. use associated types: class TwoPi a where type TwoPiRes a div2pi :: a -> TwoPiRes a instance Floating a => TwoPi (Foo a) where type TwoPiRes (Foo a) = a div2pi (Foo a) = a /(2*pi)

1 0

Encapsulation and Polymorphism
by Drew Haven 27 Aug '10

27 Aug '10

I thought I was getting pretty good at Haskell, until I ran into this problem. It's basically a way of maintaining encapsulation of data and functions while being able to have a container, such as a list, hold these items without worrying about their differences. In OO this would be pretty easy. You'd have an interface of some sort and the container would hold objects of the interface type. Then you could write the individual classes that implement that interface in their own files and the main program wouldn't have to know about the details. I'm trying to write a silly little life simulator. We've got a world where many different entities exist. These entities are all different, but for a start we'll use fish and shark. The fish like to swim around in the sea, and will manage to eat some amount of the overall food supply. The amount they consume from year to year varies and they have to consume a minimum amount every year or they die. When they've eaten enough food and have build up a surplus, they'll reproduce, but if they're starving they can't. Sharks are similar, except their food supply is the fish. The two are very similar, but do slightly different things. And every fish or shark has to keep track of its own amount of consumed food. If I wanted to define these separately, the best I've come up with is to create a data type that has all the information necessary, then somewhere maintain a master algebraic data type which wraps each of these in a type constructor. I can then write another head for a general function that will match that type constructor and describe the behavior. I tried something clever with closures, but it turned out to be too clever for me and I couldn't avoid infinite types. Thoughts? Drew Haven drew.haven(a)gmail.com

5 8

type variables
by Thomas 24 Aug '10

24 Aug '10

Hello! I have a question about type variables. The following works: type XMLCallback a = (XMLTreeNode -> a -> [Int] -> [XMLTreeNode] -> a) xmlTreeFold :: XMLCallback a -> a -> Maybe XMLTreeNode -> Maybe a xmlTreeFold _ _ Nothing = Nothing xmlTreeFold func acc (Just tree) = Just (xmlTreeWalkerWithContext func acc tree [] []) testFold :: XMLCallback [(XMLTreeNode, [Int], [XMLTreeNode])] testFold node a is ns = if (length is) > 1 then ((node, is, ns):a) else a => xmlTreeFold testFold [] tree But if I change the type declaration of 'testFold' to: testFold :: XMLCallback a it will not compile. I thought that 'a' is a type /variable/ thus able to hold any type, for example, but not limited to '[(XMLTreeNode, [Int], [XMLTreeNode])]'. Why do I need to be that much more specific in the declaration for 'testFold'? Especially since in the declaration of 'xmlTreeFold' the 'XMLCallback a' is well received. Thanks for any insights, Thomas

3 4

typeclass confusion
by Greg 24 Aug '10

24 Aug '10

Hi-- I'm back to trying to teach myself Haskell, and I've already got myself into a muddle again. I've begun playing a bit with the type system, and typeclasses and figured I'd build a type to hold angular values since it would be nice to force the type system to check whether I'm using degrees or radians: data Angle a = Radians a | Degrees a deriving (Eq, Show) Then I decided I wanted to create a (gratuitous) type class to abstract out the radian/degree conversions: class Angular a where rad :: a -> a rad x = pi * (deg x) / 180 deg :: a -> a deg x = 180 * (rad x) / pi Then tried to make Angle an instance of Angular (such as Float in an instance of Floating): instance Angular (Angle a) where rad (Radians x) = x deg (Degrees x) = x I've removed all the context clauses, rather than pick one of many equally broken implementations to share, because that seems to be a large part of where my problems are. Is this just an issue of syntax, or am I doing something fundamentally wrong? Thanks-- Greg

7 8

Classes in Polymorphic Constructors
by John Smith 23 Aug '10

23 Aug '10

Why can polymorphic data constructors not be instantiated as a class? (not sure if I used the right terminology there) For example, a = [1,True]::[Show] b = "foo":a c = map show b This would obviate the need for wrapper types

4 6

Offside rule for function arguments?
by John Smith 23 Aug '10

23 Aug '10

Why doesn't Haskell allow something like this? fac 0 = 0 1 = 1 x = x * fac (x-1) This would be clearer than repeating the function name each time, and follow the same pattern as guards and case.

6 9

Sections in Class definition
by John Smith 23 Aug '10

23 Aug '10

I tried to reproduce the definition of Eq with class Eq a where (==), (/=) :: a -> a -> Bool (/=) = not (==) (==) = not (/=) and got Couldn't match expected type `Bool' against inferred type `a1 -> a1 -> Bool' In the first argument of `not', namely `(==)' In the expression: not (==) In the definition of `/=': /= = not (==) Couldn't match expected type `Bool' against inferred type `a1 -> a1 -> Bool' In the first argument of `not', namely `(/=)' In the expression: not (/=) In the definition of `==': == = not (/=) What have I done wrong? Is it not possible to define Eq in point-free style?

2 1

hGetContents
by Luca Ciciriello 23 Aug '10

23 Aug '10

I all. I'm reading a txt file using the code inh <- openFile "myfile.txt" ReadMode contents <- hGetContents inh On Windows all works fine. I'm able to read the file without any problems. On Mac OS X (10.6.4), with the same txt file and the same code above, I get the exception: hGetContents: invalid argument (Illegal byte sequence). In both the systems (Mac and Windows) I've installed the same Haskell platform 2010.2.0.0 (GHC 6.12.3). Any idea why I've this behaviour on Mac system? Thanks in advance for any answer. Luca.

2 1

Hugs/Haskell Bug
by David Webster 22 Aug '10

22 Aug '10

This a test post to see if Imam doing this right. I am trying to boot my way into Haskell/Hugs by tgrying things to see what happens. Ehen I try the :load shown below, I get the resulting error. Am I doing something wrong , or is hee a bug? I am using Hugs Version 2006 David Webster Hugs> :load "C:\\Program Files\\WinHugs\\demos\\Cabal\\hapax.hs" ERROR file:{Hugs}\packages\base\Text\ParserCombinators\ReadP.hs:156 - Syntax error in type expression (unexpected `.')

2 1