Type signature inside an instance declaration

[Malcolm Wallace]

"Kwanghoon Choi" <lazyswamp at gmail.com> wrote:

> =====================================================================
> instance Arg a => Arg [a] where
>   pr _ = "[" ++ pr (undefined :: a) ++ "]"
> =====================================================================

You want to use `asTypeOf`, with a lazy pattern to name a value of type 'a'.

    pr xs = "[" ++ pr (undefined `asTypeOf` x) ++ "]"
            where (x:_) = xs

or
    pr ~(x:_) = "[" ++ pr (undefined `asTypeOf` x) ++ "]"

Regards,
    Malcolm