FWIW, i personally use `natural-transformation` library [1], which
provides this exact type alias, and also newtype around it (which is occasionally useful).

I’d rather promote the usage of the library (which has only `base` as dependency!).

- [1]: https://hackage.haskell.org/package/natural-transformation-0.3.1/docs/Control-Natural.html

On 14 Nov 2016, at 20:11, Baldur Blöndal <baldurpet@gmail.com> wrote:

What the subject says, add

    type f ~> g = forall a. f a -> g a

to ‘base’. Further motivation (and arguments against) in GHC Trac ticket 12772 [1].
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