Polymorphic Recursion / Rank-2 Confusion

[Dominic Steinitz]

Ross,

Thanks very much. I was going to use

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

but I prefer your definitions.

Dominic.

----- Original Message -----
From: "Ross Paterson" <ross at soi.city.ac.uk>
To: "Dominic Steinitz" <dominic.steinitz at blueyonder.co.uk>
Cc: <haskell at haskell.org>
Sent: Saturday, September 20, 2003 5:19 PM
Subject: Re: Polymorphic Recursion / Rank-2 Confusion


> 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 []) (:[])