I was looking through PrelBase.lhs and I can not make head or tails out
of the point in all the run around with build, mapFB, etc. is.
As I see it the simplification of "(map f. map g) l" would go as
follows:
(map f . map g) xs
map f (map g xs)
build$ \c n -> foldr (mapFB c f) n (map g xs)
build$ \c n -> foldr (mapFB c f) n
(build$ \c n -> foldr (mapFB c g) n xs)
--with rule: foldr k z (build g) = g k z
build$ \c n ->
(\c n -> foldr (mapFB c g) n xs) (mapFB c f) n
build$ \c n -> foldr (mapFB (mapFB c f) g) n xs
-- with rule: mapFB (mapFB c f) g = mapFB c (f.g)
build$ \c n -> foldr (mapFB c (f.g)) n xs
(\c n -> foldr (mapFB c (f.g)) n xs) (:) []
foldr (mapFB (:) (f.g)) [] xs
-- with rule: foldr (mapFB (:) f) [] = mapList f
mapList (f.g) xs
which seams like a hell of a lot of work when the rule "map f (map g xs)
= map (f.g) xs" should do the trick.
I am sure all this run-around is here for a good reason, but what is it?
I am also sure that understanding what all this run-around does is the
key to understanding why the rule "replaceMany (map f l) a =
replaceManyMap f l" is not behaving as expected.
Thanks in advance for any help with this. I can see how the rule
rewriting mechanism can be very powerful; however, it is not very useful
if I can't figure out how it works...
--
Kevin Atkinson
[EMAIL PROTECTED]
http://metalab.unc.edu/kevina/
