[Haskell-cafe] Ambiguous types

Brent Yorgey byorgey at gmail.com
Wed Aug 7 13:36:42 UTC 2019

On Wed, Aug 7, 2019, 8:22 AM Juan Casanova <juan.casanova at ed.ac.uk> wrote:

> List is special as a foldable because it is general. In
> other words, the function:
> tolist :: Foldable t => t a -> [a]
> tolist = foldr (:) []
> is such that for any x :: t a where Foldable t, it is true that
> foldr f v x = foldr f v (tolist x)
> which is to say, any structure a foldable may have is preserved when
> transforming into list shape, as a foldable.

This is true.  In fact "Listable" might have been a better name than
Foldable.  (In particular Foldable has nothing to do with generic folds aka


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