Folding over [Word8] and ByteString with same function?

Question

It turns out that there is no instance of Foldable available for ByteString. I'd like to write a function that uses foldl' on either [Word8] or ByteString but I can't. Since a ByteString is, data-wise, the same as [Word8], it seems that I should be able to.

Is there a package available that integrates the two or must I roll my own with a type class?

Answer 1

Have a look at the MonoFoldable class defined in mono-traversable package.

It has instances for both ByteString and [a].

Answer 2

ErikR's answer is great. I just want to insert a sidenote. If you have lens you have the bytes traversal:

λ> import Data.ByteString.Lens
λ> import Control.Lens
λ> :t foldrOf bytes
foldrOf bytes
  :: IsByteString s => (GHC.Word.Word8 -> r -> r) -> r -> s -> r

In a sense the question you are asking is the motivation for the lens package: can we extend the functions in Data.Foldable and Data.Traversable to consume not only regular instances of Foldable and Traversable but also objects that behave and compose like foldables and traversables?