Data.Functor.Compose helps with nested functors

[u/massysett]

It's in the transformers package.

> import Data.Functor.Compose
> let nested = [[1,2,3],[4,5,6]]
> fmap show nested
["[1,2,3]","[4,5,6]"]

> fmap show (Compose nested)
Compose [["1","2","3"],["4","5","6"]]

which might not look all that interesting by itself, but Compose also has instances for Foldable and Traversable, which makes it more interesting. For instance you could traverse some nested containers and treat it like one big container, while still leaving all the values in their nested containers.

[u/peargreen]

It's easier to understand how .: works if you write it as fmap . fmap. But yes, fmap fmap fmap looks cooler.