[Haskell-beginners] Foldable of Foldable
tonymorris at gmail.com
Fri Jan 2 15:16:35 UTC 2015
A foldable of a foldable is itself foldable. That fact is expressed in
instance (Foldable g, Foldable f, Functor g) => Foldable (:. g f)
On 03/01/15 01:14, Julian Birch wrote:
> Apologies if this isn't clear, I suspect if I understood the
> terminology better I'd already have solved the problem. I've got a
> foldable of a foldable of a. (Specifically `[Set a]`) and I want to
> be able to express the concept that I can treat `(Foldable f1,
> Foldable f2) => f1 (f2 a)` as `f3 a` i.e. a foldable of a foldable of
> a can be newtyped to a foldable of a. At least, I think that's right.
> Sadly, my attempts at actually expressing this concept have met with
> incomprehension from GHC, could someone help me out, please?
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