comment added

Here's another exampe, transferred from #1094 (Azmo):

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).