id summary reporter owner description type status priority milestone component version resolution keywords cc os architecture failure difficulty testcase blockedby blocking related
1094 loop optimization by removing constants 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)." feature request closed normal 6.8.1 Compiler 6.6 duplicate loop constants optimization Unknown/Multiple Unknown/Multiple Unknown