Re: Proposal: Add ‘type f ~> g = forall a. f a -> g a’

[Edward Kmett]

I'm -1 on the proposal of adding this type to base.

One one hand, natural-transformation already provides this definition in a
"centralized" locale for folks who really do want to share it with a
ridiculously stripped down set of dependencies, which provides a place for
those who really want to get it from a central location as well. Not as
nice as no dependency at all in a world where we could all agree this is
the best thing to add to base, but darn close.

On the other:

As a type alias there is no real compatibility gain to be had. If both of
us define (~>) as above, then types written with types against my (~>) and
types written against your (~>) are *freely interchangeable*, even if you
might have to play with your import lists more carefully.

We can't hang any instances off of it, so almost all of the standard
arguments for moving it "closer to base" fail to take effect.

On the other, other hand, something like

newtype f ~> g = Nat { runNat :: forall a. f a -> g a }

is slightly easier to make a case for, because you can hang instances off
of it and share more than one line of code, but I'd be -1 on that as well,
because there are a host of issues with that definition as well not being
"one size fits all" either.

There are several viable definitions for a natural transformation depending
on how general you want to get -- and this one (like the one in
natural-transformation) conflates parametricity with naturality, so rapidly
stops being good enough as you drift into PolyKinds and then start wanting
a more accurate Category and Functor notions, which then force you to
parameterize over the underlying `->` in the definition.

Both of these definitions occupy awkward suboptimal points in the design
space, so I'd rather not see us locked into their use by enshrining either
one in base.

-Edward


On Tue, Nov 15, 2016 at 4:01 AM, Oleg Grenrus <oleg.grenrus at iki.fi> wrote:

> 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 at 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].
>
> [1] https://ghc.haskell.org/trac/ghc/ticket/12772
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