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.