Try to observe evaluation process.

observe_evaluation.hs

import System.IO.Unsafe (unsafePerformIO)
import Prelude hiding (foldr, foldl, foldl')
import System.Mem(performGC)
import GHC.HeapView

instance Monoid Integer where
    mappend = (+)
    mempty = 0

foldr f z l = result
    where
    result = loop f z l
    loop f z []     = observe result `seq` z
    loop f z (x:xs) = observe result `seq` (x `f` loop f z xs)

foldl f z l = loop f z l
    where
    loop f z []     = observe l `seq` z
    loop f z (x:xs) = let z' = z `f` x
                      in observe l `seq` (loop f z' xs)

l1 :: Integer
l1 = foldr (+) 0 [1..10]
l2 :: Integer
l2 = foldl (+) 0 [1..10]

observe :: Monoid a => a -> a
observe a = unsafePerformIO $ do
    performGC
    s <- ppHeapGraph <$> buildHeapGraph 1000 a (asBox a)
    putStrLn s
    return a
{-# NOINLINE observe #-}

main = do
    print "======foldr=========="
    print l1
    print "======foldl=========="
    print l2