Data.Foldable causes missed foldr/build opportunities

[Gabriel Gonzalez]

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.