add minimal support for resolve function returning IO

[yanns]

Add experimental support for cats effect IO in a new experimental module.
The base classes are depending on Async, so it should be possible to support any effect library compatible with it.

The support is very minimal as the Async will be converted to a standard scala Future for the moment.

How to use it:

first import the library:

libraryDependencies += "org.sangria-graphql" %% "sangria-cats-effect-experimental" % "<latest version>"

to use IO in resolve functions:

import sangria.catseffect.schema.AsyncValue._
private val QueryType: ObjectType[Unit, Unit] = ObjectType(
  "Query",
  () =>
    fields[Unit, Unit](
      Field("parent", StringType, resolve = _ => IO("hello"))
    ))

to get an IO when executing a query:

import sangria.catseffect.execution.IOExecutionScheme._
val query =
  gql"""
    query q1 {
      parent
    }
  """
val res: IO[Json] = Executor.execute(schema, query)

FAQ

What happens if I don't use the same effect type in resolve and in the execution scheme?

This will fail miserably.
Making this type safe will probably never be implemented. I've tried, and the code is impacted at too many places.

What's the point of this change if we are still relying on Future under the hood?

• This allows to check how the API could look like.
• It opens the possibility to implement a Resolver without Future in a later phase.
• And for some use-cases, having one conversion is likely not being an issue.

When will we have a Resolver totally implemented with Async?

I don't know. If you want to join the party, you're welcome.

Does this change also provide better support for fs2 streams?

Not yet. I first focus on IO.

Why is the code in the sangria main library and not in a separated code base?

I have to change the structure of the code of sangria to enable this integration:

• Very minimal integration with cats effect IO #964
• sangria.execution.Resolver is now private #977
• prepare for custom resolver #986

Those changes are easier if the implementations live in the same code base for the moment. Once this is stable, we can extract it into another library.

Is sangria depending on cats-effect now?

No, sangria does not have any dependency on cats-effect. Only the new library has this dependency.