Labelled Effects

Ability to label effects, is a very rarely found feature in effect systems. That’s a shame, because without it, there is no true modularity.

Related reading:

In Idris and Helium, effect labelling is optional.

In Turbolift, effects are always uniquely labeled, thanks to Scala’s singleton types:

import turbolift.effects.Error

// Unique value:
case object MyError extends Error[String]

// Unique type:
type MyError = MyError.type

There is nothing stopping us from instantiating given effect more than once:

import turbolift.effects.Error

case object MyError1 extends Error[String]
case object MyError2 extends Error[String]

type MyError1 = MyError1.type
type MyError2 = MyError2.type

Each instance is a fully independent effect:

  • They may be instantiated with different type parameters (e.g. State[Int] and State[String]).
  • They may be used together in a computation. The type of the computation will reflect this, showing 2 distinct effects (e.g. MyError1 & MyError2)
  • They may be handled at different points in program, and with different handlers.

Example: Using 2 State effects at the same time

import turbolift.!!
import turbolift.effects.State

case object Foo extends State[Int]
case object Bar extends State[Int]

val program =
    x <- Foo.get
    _ <- Bar.put(-x)
  yield ()
// program: Computation[Unit, Foo & Bar] = turbolift.Computation@712ea364

val result = program
// result: Tuple2[Tuple2[Unit, Int], Int] = (((), 42), -42)


Performance penalty for using multiple effects in Turbolift, is small. Here is comparison with old version of Turbolift, that was based on Monad Transformers: