Is it possible to change the kind of a Monad? Let’...

# arrow

[georg]

Is it possible to change the kind of a Monad?
Let’s say I have Kind<F, A> and I want a function that turns it into Kind<G, A> . Or even to Kind<G, B>

[PhBastiani]

Yes... In category theory, this kind of transformation is called 'natural transformation'. Not really promoted in Arrow. Consider also the use of MTL typeclasses (Arrow monad transformers are in incubation)

[Jannis]

A natural transformation in arrow is implemented as FunctionK iirc. Outside of free monads or other special uses its usually not what one needs. If your use case is something special or free monads by all means go for it but otherwise there are likely easier methods to do specific transformations

[raulraja]

https://arrow-kt.io/docs/0.10/apidocs/arrow-core-data/arrow.core/-function-k/index.html#functionk

[raulraja]

you need to know how to go from F to G to get an exhaustive transformation

[raulraja]

this essentially let’s you compose ADTs

[raulraja]

is A -> B at the kind level

[georg]

Thanks everyone. I’ll have a look at FunctionK .