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