> take :: Int -> [a] -> [a]
>If you want to define your own take
> mytake :: Show a => Int -> [a] -> [a]
>that's fine, but it's a different function.
>
>And for (head []) there's really no useful info to hand. So I'm stumped.
Although you can't show the value (because there isn't one), you can
at least show the type of the expression:
class ShowType a where
showType :: a -> String
instance ShowType a => ShowType [a] where
showType xs = "[" ++ showType x ++ "]"
where ~(x:_) = xs
Then
myhead :: ShowsType a => [a] -> a
myhead xs@[] = error ("myhead of empty list at type "++ showType xs)
myhead (x:_) = x
which, like "mytake", is a different function from the one defined
in the Prelude, but might give you enough info to help track down
the error.
Regards,
Malcolm
