Polymorphic Recursion / Rank-2 Confusion

Sat Sep 20 13:57:09 EDT 2003

On Sat, 20 Sep 2003, Ross Paterson wrote:

> 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 []) (:[])
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