hi all,

I'm new to Haskell.

I wrote a permutation prog.

permutation [] = [[]]
permutation seq = foldr (++) [] (map appendEachTo seq)
where appendEachTo i = (map (i:) (permutation (filter (i/=) seq)))
It works.

but I don't know how to give the type of 'permutation'.

I wrote 'permutation::[a]->[[a]]', Hugs complain that

*** Expression : permutation
*** Type : [a] -> [[a]]
*** Given context : ()
*** Constraints : Eq a

I think I must miss something when reading the tutorial.

any help will be appreciated.

Now I know, it should be 'permutation :: Eq a => [a]->[[a]]'.

Xiaobin Huang wrote:
> hi all,
>
> I'm new to Haskell.
>
> I wrote a permutation prog.
>
> permutation [] = [[]]
> permutation seq = foldr (++) [] (map appendEachTo seq)
> where appendEachTo i = (map (i:) (permutation (filter (i/=) seq)))
> It works.
>
> but I don't know how to give the type of 'permutation'.
>
> I wrote 'permutation::[a]- >[[a]]', Hugs complain that
>
> *** Expression : permutation
> *** Type : [a] - > [[a]]
> *** Given context : ()
> *** Constraints : Eq a
>
> I think I must miss something when reading the tutorial.
>
> any help will be appreciated.