Which of the predefined Selective instances are rigid?

[kozross]

It would be really useful to know this. While it can be inferred from the implementations, having this information in the documentation for the instances would be great.

[snowleopard]

Most of the instances are rigid. Three notable exceptions are: Under and Validation, as well as Lift f that inherits the rigidity of the underlying selective functor f.

I agree that we should document this. Perhaps, marking the few non-rigid instances should be sufficient?

[kozross]

@snowleopard If as you say most instances are rigid, marking the exceptions alone, while adding something like "Unless stated otherwise, the instances of Selective defined in this package are rigid.` to the paragraph describing rigidity, would do.

Out of curiosity, how does rigidity interact with Compose? I assume it inherits the rigidity of the Selective 'component'? This kind of thing is also worth explaining on 'lifty' instances (that is, instances of types similar to Compose).

[snowleopard]

Hmm, interesting. It's not immediately clear to me whether Selective (Compose f g) inherits the rigidity of the inner instance Selective g.

For Product f g the answer seems simple: the result is rigid only if both f and g are rigid.