On Sun, Jul 01, 2007 at 05:06:10PM +0100, Andrew Coppin wrote:
Stefan O'Rear wrote:
This is *much* easier expressed as a bottom-up traversal.
compile = transform optimize . transform eliminate
eliminate (Lam v e) = transform (abstract v) e eliminate x = x
abstract v (Var v') | v == v' = I abstract v (a :@ b) = S :@ a :@ b abstract v x = x
optimize (S :@ (K :@ x) :@ (K :@ y)) = K :@ (x :@ y) optimize (S :@ (K :@ x) :@ I) = x optimize x = x
(Here using Uniplate, mostly because it is the freshest in my mind of all of them).
Um... shouldn't that read
abstract v (a :@ b) = S :@ (transform (abstract v) a) :@: (transform (abstract v) b)
No, because the whole point of transform is that it handles recursion for you. (However, there is a bug! abstracting an unrecognized form (that is, a primitive combinator) should add a K.) Stefan