Add OptionT.fromF

[lambdista]

I noticed that the 0.3.0 version includes a useful OptionT.fromOption function. It lets you build an OptionT from an Option.

Wouldn't it be the case to add an OptionT.fromF that lets you build an OptionT from an Applicative F? As in:

scala> OptionT.fromF(List(8))
res1: cats.data.OptionT[List,Int] = OptionT(List(Some(8)))

Of course I'd accept suggestions for a better name than fromF.

[ceedubs]

Thanks for the suggestion @lambdista. Just to make sure I understand clearly, you're talking about a function that will take an F[A] and lift it into an OptionT[F, A] where we essentially map on the F[A] and wrap every A in Some, right?

If so, here are a couple thoughts:

• I think we could get by with just a Functor and not require an Applicative.
• I think this would essentially be liftM from MonadTrans, which we have talked about adding, but I don't think there's a GitHub issue for yet. I think it would be useful to have an OptionT-specific method like you suggest even if we add MonadTrans, because OptionT could get by with a Functor while liftM has a more constrained Monad requirement. The OptionT instance of MonadTrans could simply delegate through to this method.

[lambdista]

Thanks for your answer @ceedubs. Yes, that's exactly what I mean. About your thoughts:

• You're totally right, I think we could get by with just a Functor too.
• Yes, it's essentially liftM from MonadTrans but, as you correctly wrote, having an OptionT-specific method would do no harm and require a less constrained Functor requirement compared to the Monad of liftM.

Let me know if you want me to add it and send a PR. Thank you.

[ceedubs]

I'd like this 👍. Anyone else?

I'd propose liftF as a name to parallel liftM but with Functor instead of Monad.

[non]

👍

[lambdista]

That's great! liftF is perfect. 👍

[lambdista]

Implemented, added test and updated related docs. Let me know if you want to me to change/add anything. Thank you.

[ceedubs]

Completed by #667.