「Real World Haskell」読書メモ 4.関数プログラミング foldl を foldr で理解する

Real World Haskell　4.関数プログラミング 4.6 ループをどのように考えるか

-- file: ch04/Fold.hs
myFoldl :: (a -> b -> a) -> a -> [b] -> a
 
myFoldl f z xs = foldr step id xs z
    where step x g a = g (f a x)

import Debug.Trace
 
myFoldl f z xs = trace("foldr step id "++show xs++" "++show z)
                      (foldr step id xs z)
    where step x g a = trace("g (f "++show a++" "++show x++")")
                             (g (f a x))

myFoldl (\x y->"["++x++" "++y++"]") "[start]" ["a","b","c"]

"foldr step id ["a","b","c"] "[start]"

g (f "[start]" "a")         -- f は(\x y->"["++x++" "++y++"]")
g (f "[[start] a]" "b")     -- g は id
g (f "[[[start] a] b]" "c")
[[[[start] a] b] c]"