Deriving Contravariant and Profunctor

[Edward Kmett]

Actually it is trickier than you'd think.

With "Functor" you can pretend that contravariance doesn't exist.

With both profunctor and contravariant it is necessarily part of the puzzle.

data Compose f g a = Compose (f (g a))

* are both f and g contravariant leading to a functor?
* is f contravariant and g covariant leading to a contravariant functor?
* is f covariant and g contravariant leading to a contravariant functor?

data Wat p f a b = Wat (p (f a) b)

is p a Profunctor or a Bifunctor? is f Contravariant or a Functor?

We investigated adding TH code-generation for the contravariant package,
and ultimately rejected it on these grounds.

https://github.com/ekmett/contravariant/issues/17

-Edward



On Fri, Sep 11, 2015 at 12:49 PM, David Feuer <david.feuer at gmail.com> wrote:

> Would it be possible to add mechanisms to derive Contravariant and
> Profunctor instances? As with Functor, each algebraic datatype can
> only have one sensible instance of each of these.
>
> David Feuer
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