[commit: packages/base] master: Implement foldl with foldr (b63face)

[Simon Marlow]

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 at git.haskell.org wrote:
> Repository : ssh://git@git.haskell.org/base
>
> On branch  : master
> Link       : http://ghc.haskell.org/trac/ghc/changeset/b63facef165b957183b65604ef99b2b8574747a5/base
>
>> ---------------------------------------------------------------
>
> commit b63facef165b957183b65604ef99b2b8574747a5
> Author: Joachim Breitner <mail at joachim-breitner.de>
> 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.
>
>
>> ---------------------------------------------------------------
>
> 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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