Re: [Haskell-cafe] A systematic method for deriving a defintion of foldl using foldr?

On Wed, 11 Mar 2009, R J wrote:

foldl and foldr are defined as follows:

  foldr                :: (a -> b -> b) -> b -> [a] -> b   foldr f e []         =  e   foldr f e (x : xs)   =  f x (foldr f e xs)

  foldl                :: (b -> a -> b) -> b -> [a] -> b   foldl f e []         =  e   foldl f e (x : xs)   =  foldl f (f e x) xs

1.  I understand how these definitions work, and yet I'm unable to implement foldl in terms of foldr.  What's a systematic approach to identifying such an implementation, and what is the implementation?

You are lucky, I recently wrote http://haskell.org/haskellwiki/Foldl_as_foldr