[Haskell-cafe] Foldable for (,)

Jonathon Delgado voldermort at hotmail.com
Thu May 4 09:30:46 UTC 2017

Ay, there's the rub. Traversable over (,) can be expressed as a fold over a single element, but that doesn't mean it should be. All you want in this case is the effect.

From: Nickolay Kudasov <nickolay.kudasov at gmail.com>
Sent: 04 May 2017 08:16Subject: Re: [Haskell-cafe] Foldable for (,)
It turns out that by using Const applicative functor you can use traverse to perform foldMap! Thus every Traversable is trivially a Foldable (see foldMapDefault).
Similarly, every Traversable is trivially a Functor if you use Identity applicative functor (see fmapDefault).
This is why Traversable has those Functor and Foldable constraints, not because it relies on fmap or foldMap.

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