On 12/07/07, Crediné Menezes
I have written this code in Haskell which gives an unresolved overloading error. g x = [2] ++ [3,5..truncate(sqrt x)] p n = fp n (g n) fp n [ ] = True fp n (x:xs) = if (mod n x) == 0 then False else fp n xs
when I submit g 103 I get: [2,3,5,7,9] :: [Integer]
when I submit: fp 103 (g 103) I get True :: Bool
But when I submit : p 103 I get ERROR - Unresolved overloading *** Type : (RealFrac a, Floating a, Integral a) => Bool *** Expression : p 103
I know why, there is no type that is at the same time: RealFrac, Floating and Integral; but I don´t know how to solve.
What kind of type casting or type definition can I use to fix the error?
This one's quite subtle, but as usual getting the inferred types from GHCi helps immensely: Prelude> :t p p :: (Integral a, Floating a, RealFrac a) => a -> Bool Prelude> :t fp fp :: (Integral a) => a -> [a] -> Bool Prelude> :t g g :: (Integral t, RealFrac a, Floating a) => a -> [t] The function that doesn't work is the one that calls the other two, namely p. It doesn't work, but the separate invocation of
fp 103 (g 103)
does work. So let's look at that one further: Prelude> :t \x y -> fp x (g y) \x y -> fp x (g y) :: (RealFrac a1, Floating a1, Integral a) => a -> a1 -> Bool Prelude> :t \x -> fp x (g x) \x -> fp x (g x) :: (RealFrac a, Floating a, Integral a) => a -> Bool The first type signature is for the one that worked, and the second is for the definition used in the function p. They're different. So the problem is turning one into the other. In fact, turning (RealFrac a, Floating a) into (Integral a). Which is what truncate should do: Prelude> :t \x -> fp (truncate x) (g x) \x -> fp (truncate x) (g x) :: (RealFrac a, Floating a) => a -> Bool I hope that helps! ;-) Cheers, Dougal.