Add IdentityT

[JordanMartinez]

For context, this is included in the Haskell 'transformers' library

[hdgarrood]

I'd prefer to wait until we have a concrete use case for it, personally.

[JordanMartinez]

I am trying to port hedgehog to PureScript and they use it. That's why I opened this issue. However, I can keep that code in the port until we have more use cases than this.

[hdgarrood]

Ah okay, I reckon that probably counts. Can you describe how it's useful for your hedgehog port?

[JordanMartinez]

It is used to create 3 different type class instances and I'm guessing that TreeT is somehow affected by it:

• TreeT definition
• Distributive instance
• MonadGen instance
• MonadTest instance

[hdgarrood]

Instance definitions don't really justify the existence of IdentityT though, I think the part that's more relevant here is places where those instances would be used. I think perhaps similarly to how identity is useful for functions which take functions as arguments, IdentityT could be useful for types which take monad transformers as arguments. Does hedgehog have any of those?

[JordanMartinez]

IdentityT could be useful for types which take monad transformers as arguments.

Yeah, that would make sense. Similar to how Identity acts like a placeholder monad argument.

After looking around a bit more, seems like distributeT from MonadTransDistributive has a MFunctor constraint that takes a monad transformer as an argument. However, the purescript port of mmorph's MFunctor doesn't have an instance for IdentityT. I imagine it's because IdentityT isn't included in this library. Does that concrete example suffice?

[hdgarrood]

Yep, that works for me.

[JordanMartinez]

I can submit a PR for this. However, what should the file's module name be? Control.Monad.Identity.Trans?

[hdgarrood]

Yep, that conforms to the convention we're using in this library.