Ambiguous types

[Ashley Yakeley]

At 2002-01-21 01:15, Koen Claessen wrote:

> | But this gives an error:
> |
> |   emptyList :: (Ord a) => [a]
> |   emptyList = []
> |
> |   isempty :: Bool
> |   isempty = null emptyList
>
>Here is another (very similar) case:
>
>  emptyList :: [a]
>  emptyList = []
>
>  showEmptyList :: String
>  showEmptyList = show emptyList

>Now, you might argue that the type error resulting from this
>program is too restrictive, since the empty list can always
>be shown as: "[]".

Of course not, show doesn't know how to show things of arbitrary type 
[a]. But null is supposed to work on _any_ list. And just from the type 
of emptyList, Haskell knows that it is a list of something.


-- 
Ashley Yakeley, Seattle WA