Ah, I realise it's ok so long as the original definition of foldl/foldl' gets optimised to the right thing. If that's the case just ignore me. Cheers, Simon On 17/02/2014 10:22, Simon Marlow wrote:

This worries me a bit. If foldl isn't inlined, I get a less efficient version, so it has to be inlined everywhere. So -O0 code gets worse, and binary sizes for -O1+ get bigger - foldl, sum, and product are now INLINE.

What I'm arguing is that we should have more flexibility to *not* inline things (INLINABLE is much better than INLINE), and when not inlining things we should be calling an efficient version of the function. This is why map is not defined in terms of foldr, for instance.

Cheers, Simon

On 10/02/2014 13:52, git@git.haskell.org wrote:

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Repository : ssh://git@git.haskell.org/base

On branch : master Link : http://ghc.haskell.org/trac/ghc/changeset/b63facef165b957183b65604ef99b2b857...

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commit b63facef165b957183b65604ef99b2b8574747a5 Author: Joachim Breitner Date: Tue Jan 28 14:31:05 2014 +0100

Implement foldl with foldr

together with the call arity analysis and the following patch (about inlining maximum), we get nice benefits from fusing foldl and foldl' with good producers:

Min -0.1% -74.5% -6.8% -8.3% -50.0% Max +0.2% 0.0% +38.5% +38.5% 0.0% Geometric Mean -0.0% -4.1% +7.7% +7.7% -0.8%

Because this depends on a compiler optimisation, we have to watch out for cases where this is not an improvements, and whether they occur in the wild.

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b63facef165b957183b65604ef99b2b8574747a5 Data/List.hs | 34 +++++++++------------------------- GHC/List.lhs | 13 +++++++------ 2 files changed, 16 insertions(+), 31 deletions(-)

diff --git a/Data/List.hs b/Data/List.hs index 130ceb2..4796055 100644 --- a/Data/List.hs +++ b/Data/List.hs @@ -1,5 +1,5 @@ {-# LANGUAGE Trustworthy #-} -{-# LANGUAGE CPP, NoImplicitPrelude, MagicHash #-} +{-# LANGUAGE CPP, NoImplicitPrelude, ScopedTypeVariables, MagicHash #-}

-----------------------------------------------------------------------------

-- | @@ -989,10 +989,11 @@ unfoldr f b = -- -----------------------------------------------------------------------------

-- | A strict version of 'foldl'. -foldl' :: (b -> a -> b) -> b -> [a] -> b -foldl' f z0 xs0 = lgo z0 xs0 - where lgo z [] = z - lgo z (x:xs) = let z' = f z x in z' `seq` lgo z' xs +foldl' :: forall a b . (b -> a -> b) -> b -> [a] -> b +foldl' k z0 xs = foldr (\(v::a) (fn::b->b) (z::b) -> z `seq` fn (k z v)) (id :: b -> b) xs z0 +-- Implementing foldl' via foldr is only a good idea if the compiler can optimize +-- the resulting code (eta-expand the recursive "go"), so this needs -fcall-arity! +-- Also see #7994

-- | 'foldl1' is a variant of 'foldl' that has no starting value argument, -- and thus must be applied to non-empty lists. @@ -1008,32 +1009,15 @@ foldl1' _ [] = errorEmptyList "foldl1'" -- -----------------------------------------------------------------------------

-- List sum and product

-{-# SPECIALISE sum :: [Int] -> Int #-} -{-# SPECIALISE sum :: [Integer] -> Integer #-} -{-# INLINABLE sum #-} -{-# SPECIALISE product :: [Int] -> Int #-} -{-# SPECIALISE product :: [Integer] -> Integer #-} -{-# INLINABLE product #-} --- We make 'sum' and 'product' inlinable so that we get specialisations --- at other types. See, for example, Trac #7507. - -- | The 'sum' function computes the sum of a finite list of numbers. sum :: (Num a) => [a] -> a -- | The 'product' function computes the product of a finite list of numbers. product :: (Num a) => [a] -> a -#ifdef USE_REPORT_PRELUDE + +{-# INLINE sum #-} sum = foldl (+) 0 +{-# INLINE product #-} product = foldl (*) 1 -#else -sum l = sum' l 0 - where - sum' [] a = a - sum' (x:xs) a = sum' xs (a+x) -product l = prod l 1 - where - prod [] a = a - prod (x:xs) a = prod xs (a*x) -#endif

-- -----------------------------------------------------------------------------

-- Functions on strings diff --git a/GHC/List.lhs b/GHC/List.lhs index b7b78c7..e004ded 100644 --- a/GHC/List.lhs +++ b/GHC/List.lhs @@ -1,6 +1,6 @@ \begin{code} {-# LANGUAGE Trustworthy #-} -{-# LANGUAGE CPP, NoImplicitPrelude, MagicHash #-} +{-# LANGUAGE CPP, NoImplicitPrelude, ScopedTypeVariables, MagicHash #-} {-# OPTIONS_HADDOCK hide #-}

-----------------------------------------------------------------------------

@@ -178,11 +178,12 @@ filterFB c p x r | p x = x `c` r -- can be inlined, and then (often) strictness-analysed, -- and hence the classic space leak on foldl (+) 0 xs

-foldl :: (b -> a -> b) -> b -> [a] -> b -foldl f z0 xs0 = lgo z0 xs0 - where - lgo z [] = z - lgo z (x:xs) = lgo (f z x) xs +foldl :: forall a b. (b -> a -> b) -> b -> [a] -> b +{-# INLINE foldl #-} +foldl k z0 xs = foldr (\(v::a) (fn::b->b) (z::b) -> fn (k z v)) (id :: b -> b) xs z0 +-- Implementing foldl via foldr is only a good idea if the compiler can optimize +-- the resulting code (eta-expand the recursive "go"), so this needs -fcall-arity! +-- Also see #7994

-- | 'scanl' is similar to 'foldl', but returns a list of successive -- reduced values from the left:

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