Polymorphic Recursion / Rank-2 Confusion

[Ross Paterson]

On Sat, Sep 20, 2003 at 12:01:32PM +0100, Dominic Steinitz wrote:
> Can anyone tell me why the following doesn't work (and what I have to do to
> fix it)? I thought by specifying the type of coalw as rank-2 would allow it
> to be used both at a and (a,b).

Change the signature to

	coal_ :: (v -> [a]) -> (w -> [a]) -> Vector_ v w -> [a]

Then you can define

	type Vector = Vector_ ()

	coal :: Vector a -> [a]
	coal = coal_ (const []) (:[])