On Wed, 5 Nov 2003 07:17:23 -0800 (PST)
Hal Daume III
Keith is entirely correct.
You can see this from the definition of foldr:
foldr :: (a -> b -> b) -> b -> [a] -> b foldr f z [] = z foldr f z (x:xs) = f x (foldr f z xs)
where clearly every [] is replaced by z and every : by f.
I had heard this before when I was first beginning and didn't really find it "clear" :). I think if you write foldr with f in infix notation it's a bit more clear:
foldr f z [] = z foldr f z (x:xs) = x `f` foldr f z xs
or even write the second line as
foldr f z (x:xs) = x `f` xs' where xs' = foldr f z xs
I think in this case it's a bit more clear how "f" is replacing the ":".
- Hal
Simply choose better names, data Tree a = Empty | Leaf a | Branch (Tree a) (Tree a) foldTree empty leaf branch Empty = empty foldTree empty leaf branch (Leaf a) = leaf a foldTree empty leaf branch (Branch l r) = branch (foldTree empty leaf branch l) (foldTree empty leaf branch r) data List a = Nil | Cons a (List a) foldList nil cons Nil = nil foldList nil cons (Cons a as) = cons a (foldList nil cons as)