<= =>
Exemplo foldl'
foldl'           :: (a -> b -> a) -> a -> [b] -> a
foldl' f a []     = a
foldl' f a (x:xs) = (foldl' f $! f a x) xs

exG3 = printO (observe "foldl'" foldl' (+) 0  [1..4::Int])