[Haskell-cafe] Foldable for (,)

[Benno Fünfstück]

The relation between Foldable and Traversable is similar to the one between
Applicative and Monad: Traversable does not require Foldable, but every
Traversable has a Foldable instance (given by foldMalDefault). That's why
there is the superclass constraint.

Jonathon Delgado <voldermort at hotmail.com> schrieb am Do., 4. Mai 2017,
09:49:

> It seems that Traversable is doing two things:
>
> 1) Changing the shape of a data structure.
> 2) Folding over the contents of a data structure.
>
> Traversable requires Foldable to enable 2, but Traversable is also applied
> to types such as (,) where only 1 is relevant.
>
> If this is correct, follow-up questions would be:
>
> 1) For educational purposes, could these concerns be split without
> substantial drawback?
> 2) For practical purposes, could this be done without breaking a lot of
> existing code?
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