loop optimization by removing constants

the current version of Data.List.foldl' is:

foldl'           :: (a -> b -> a) -> a -> [b] -> a
foldl' f a []     = a
foldl' f a (x:xs) = let a' = f a x in a' `seq` foldl' f a' xs

which for some reason is faster if removing the first parameter, e.g:

foldl2'           :: (a -> b -> a) -> a -> [b] -> a
foldl2' f = loop
  where loop a []     = a
        loop a (x:xs) = let a' = f a x in a' `seq` loop a' xs

---

Testing speed difference:

module Main (main) where

import System (getArgs)

sumFoldl' :: Int -> Int
sumFoldl' n = foldl' (+) 0 [1..n]

sumFoldl2' :: Int -> Int
sumFoldl2' n = foldl2' (+) 0 [1..n]

-- copy of Data.List.foldl'
foldl'           :: (a -> b -> a) -> a -> [b] -> a
foldl' f a []     = a
foldl' f a (x:xs) = let a' = f a x in a' `seq` foldl' f a' xs

-- altered version of foldl'
foldl2'           :: (a -> b -> a) -> a -> [b] -> a
foldl2' f = loop
  where loop a []     = a
        loop a (x:xs) = let a' = f a x in a' `seq` loop a' xs

main = do
  [v,n] <- getArgs
  case v of
    "1" -> print (sumFoldl' (read n))
    "2" -> print (sumFoldl2' (read n))

---

I have no idea how the optimization of loops is done, but the request is that this kind of loop rewriting should be done by the compiler, not by the user (to get a faster loop). It seems to just be a matter of finding and removing constants (identifiers just passed along).

Trac metadata

Trac field	Value
Version	6.6
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
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Operating system	Multiple
Architecture	Multiple