[Haskell] How to zip folds: A library of fold transformers

Dylan Thurston dthurston at barnard.edu
Wed Oct 12 18:40:50 EDT 2005

On Tue, Oct 11, 2005 at 05:25:24PM -0700, oleg at pobox.com wrote:
> First we define the representation of a list as a fold:
> > newtype FR a = FR (forall ans. (a -> ans -> ans) -> ans -> ans)
> > unFR (FR x) = x
> It has a rank-2 type. The defining equations are: if flst is a value
> of a type |FR a|, then
> 	unFR flst f z = z if flst represents an empty list
> 	unFR flst f z = f e (unFR flst' f z)
> 			if flst represents the list with the head 'e'
> 			and flst' represents the rest of that list

Presumably you noticed that this is isomorphic to the representation
of a list in lambda calculus, right?

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