CODEWITHSUNDEEP
scalascala-cats
user355252
user355252

Reputation:

How to go from an functor over an effect to an effect over the functor?

I have two different types of users; let's call them BasicUser and EnrichedUser.

I get a value of the Foo user type inside a container C that has a cats.Functor instance:

val basicUserF: C[BasicUser] = getBasicUser(…)

Now I'd like to map over this value (using the functor instance) to turn the contained BasicUser into an EnrichedUser user, staying in the container:

import cats.syntax.all._

val enrichedUserF: C[EnrichedUser] = basicUserF.map(user => enrichUser(user))

However, enrichUser doesn't return EnrichedUser, but rather OptionT[Future, EnrichedUser], so I end up with the following type:

val enrichedUserThing: C[OptionT[Future, EnrichedUser]] = …

How can I get from this type to OptionT[Future, C[EnrichedUser]] (which I'll ultimately turn into a JSON-serialized Akka HTTP response)?

Upvotes: 2

Views: 95

Answers (1)

Jasper-M
Jasper-M

Reputation: 15086

If you have a Traverse[C] instance you can use sequence.

scala> implicit def CTraverse: Traverse[C] = ???
CTraverse: cats.Traverse[C]

scala> def sequenced = enrichedUserThing.sequence[OptionT[Future,?], EnrichedUser]
sequenced: cats.data.OptionT[scala.concurrent.Future,C[EnrichedUser]]

Extra: If you have -Ypartial-unification enabled you don't need the explicit type arguments.

Upvotes: 1

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