Data.Foldable causes missed foldr/build opportunities

Edward Kmett ekmett at
Thu Jul 25 20:22:15 CEST 2013

In your case here mapM_ f would turn into Foldable.foldr ((>>) . f) (return
()) if it were to inline (which it isn't set up to do) rather than into
Control.Monad.mapM_. Once it became Foldable.foldr it'd get stuck on the
way to becoming GHC.List.foldr and then to fusion by the fact that we don't
inline the members of the Foldable [] instance.

As an aside, it is interesting that in
traverse is INLINE'd but not the mapM, though it is irrelevant to your
issue here, it is another thing that would likely be impacted.

So there appears to be at least two obstacles in the way of this fusing
away properly, and yet another in the way of normal mapM fusing.

Some of this could be mitigated by rephrasing the foldr/build rule directly
in terms of Foldable.foldr where it'd only typecheck if applied to a []
anyways, but it does look like a serious look will have to be made at what
gets inlined as we proceed to investigate how to get this to work right.


On Thu, Jul 25, 2013 at 1:55 PM, Gabriel Gonzalez <gabriel439 at>wrote:

> On 07/25/2013 10:41 AM, Edward Kmett wrote:
> One of the open concerns about it is definitely ensuring that we get the
> fusion opportunities we can.
>  If you put an INLINE pragma on your Foldable version of each do the
> fusion rules fire after it gets inlined into a call site that uses it as a
> list?
> I tried both INLINE and INLINABLE and neither causes the fusion rules to
> fire.  I also tried:
> * adding an orphan SPECIALIZE rule for `Data.Foldable.mapM_` in the module
> where I defined `each`
> * Specializing the type of `each` to consume lists, but still using the
> `Foldable` `mapM_`
> * Defining a new copy of `each` (using the `Foldable` version) in the same
> module as the code that uses it, specializing the type signature to lists,
> and trying out INLINE/INLINABLE or no pragma.
> None of those causes the rule to fire, either.
>  -Edward
> On Thu, Jul 25, 2013 at 1:22 PM, Gabriel Gonzalez <gabriel439 at>wrote:
>> I'm now in favor of the `Data.Foldable` proposal, but I just wanted to
>> mention that the proposal needs to include some extra pragma work to ensure
>> that build/foldr optimizations fire.  I was just experimenting with the
>> following combinator for `pipes` trying out the following two versions:
>>     each :: (Monad m) => [a] -> Producer a m ()
>>     each = mapM yield
>>     each :: (Monad m, Foldable f) => f a -> Producer a m ()
>>     each = Data.Foldable.mapM yield
>> When I do a pure `pipes`-based fold over both `Producers`s, the version
>> specialized to lists triggers a firing of the build/foldr fusion rule and
>> runs about 20% faster.  The true improvement for `mapM` by itself is
>> probably even greater than that because I haven't optimized the folding
>> code yet.  The latter version does not trigger the rule firing.  Either way
>> I'm going to include the latter `Foldable` version but I just wanted to
>> mention this because I remember people were asking if this would impact
>> fusion or not.
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