[Haskell-cafe] how to Enumerate without lists?

[Tom Ellis]

On Wed, Sep 05, 2018 at 11:37:07AM -0400, Ben Doyle wrote:
> {-#LANGUAGE GADTs#-}
> 
> data Enumerator a b where
>   Enumerator :: a -> a -> Enumerator a a
> 
> instance Enum a => Foldable (Enumerator a) where
>   foldMap f (Enumerator x y)
>     | fromEnum x > fromEnum y = mempty
>     | otherwise                           = f x <> foldMap f (Enumerator
> (succ x) y)
> 
> Here we're using a GADT to express that our two-parameter Enumerator type
> in practice always has a == b (at the type level).
> Which lets us constrain the values inside our new Foldable structure, while
> still having a type of kind (* -> *) like the the
> typeclass requires.

This is ingenious!