Eff

Effects of type R, returning a value of type A

It is implemented as a "Free-er" monad with extensible effects:

• the "pure" case is a pure value of type A

• the "impure" case is:

  • a disjoint union of possible effects
  • a continuation of type X => Eff[R, A] indicating what to do if the current effect is of type M[X] this type is represented by the Arrs type

• the "impure applicative" case is:

  • list of disjoint unions of possible effects
  • a function to apply to the values resulting from those effects

The monad implementation for this type is really simple:

• point is Pure
• bind simply appends the binding function to the Arrs continuation

Important:

The list of continuations is NOT implemented as a type sequence but simply as a

  Vector[Any => Eff[R, Any]]

This means that various .asInstanceOf are present in the implementation and could lead to burns and severe harm. Use with caution!

Similarly the list of effects in the applicative case is untyped and interpreters for those effects are supposed to create a list of values to feed the mapping function. If an interpreter doesn't create a list of values of the right size and with the right types, there will be a runtime exception.

The Pure, Impure and ImpureAp cases also incorporate a "last" action returning no value but just used for side-effects (shutting down an execution context for example). This action is meant to be executed at the end of all computations, regardless of the number of flatMaps added on the Eff value.

Since this last action will be executed, its value never collected so if it throws an exception it is possible to print it by defining the eff.debuglast system property (-Deff.debuglast=true)