[Haskell-beginners] how to specify type class in type of function?

[Daniel Fischer]

Am Donnerstag 28 Januar 2010 22:06:36 schrieb george young:
> [complete newbie using ghci 6.10.4
> I understand that I should indicate that 'a' is in class Ord, since I
> use lessthan.  But what is the syntax?  Other style suggestions are
> welcome.
>
> mysort :: [a] -> [a]

mysort :: Ord a => [a] -> [a]

Generally,

function :: (Class1 a, Class2 b) => a -> b -> c

The part of the type signature before the '=>' is called the context, if it 
consits of only one constraint, the parentheses can be omitted.

> mysort [] = []
> mysort [x] = [x]
> mysort [x, y] | (x <= y) = [x, y]
>
>               | otherwise = [y, x]
>
> mysort l =
>   (mysort s1) ++ (mysort s2)

Doesn't work,

mysort [3,2,1,4] ~> [2,3,1,4]

you want merge here instead of (++)

(how to write merge is left as an exercise).

>     where
>       (s1, s2) = splitAt (div (length l) 2) l

If you have a long list, all those calls to length will take a loong time.
Consider passing it to a helper function which takes the length of the list 
as a parameter.

>
> Could not deduce (Ord a) from the context ()
>       arising from a use of `<=' at /home/gry/foo.hs:53:17-22
>     Possible fix:
>       add (Ord a) to the context of the type signature for `mysort'
>     In the expression: (x <= y)
>     In a stmt of a pattern guard for
>                  the definition of `mysort':
>         (x <= y)
>     In the definition of `mysort':
>         mysort [x, y]
>
>                  | (x <= y) = [x, y]
>                  | otherwise = [y, x]