Member implicits

Type inference with the Eff monad can be a bit tricky to get right if we want to avoid type annotations. Here are some tips to help you.

Running effects with several type parameters

Some effects use 2 type variables, like Reader or Writer. If you want to use those effects in an effect stack you need to make sure you have the following scalac option:

scalacOptions += "-Ypartial-unification"

Use context bounds and type aliases

When creating effects you can always “require” a stack containing the right effects with the MemberIn typeclass:

import org.atnos.eff._
import org.atnos.eff.all._

type StateInt[A] = State[Int, A]
type WriterString[A] = Writer[String, A]

// for creating state effects
def putAndTell[R](i: Int)(implicit s: StateInt |= R, w: WriterString |= R): Eff[R, Int] =
  for {
    // no type annotations needed!
    _ <- put(i)
    _ <- tell("stored " + i)
  } yield i

You can even use context bounds to make the declaration of putAndTell more concise:

import org.atnos.eff.all._

type _stateInt[R] = State[Int, ?] |= R
type _writerString[R] = Writer[String, ?] |= R

def putAndTell[R :_stateInt :_writerString](i: Int): Eff[R, Int] =
  for {
    _ <- put(i)
    _ <- tell("stored " + i)
  } yield i

Know your Member typeclasses

There are 3 different ways to declare that an effect is part of an effect stack with 3 typeclasses:

Typeclass Alias Meaning When to use it
MemberIn[M, R] M |= R “M is part of R” to create M effects in R
MemberInOut[M, R] M /= R “M is part of R and can be extracted from it” to intercept the effect M (see Interpreter.scala) and transform it while staying in the same stack. For example to handleError for an Error effect
Member[M, R] M <= R “M is part of R, can be extracted from it, and the resulting stack is m.Out” to interpret the effect in terms of special values or other effects and remove the effect from the stack

“Packing” member instances

Some function signatures can be repetitive when they always require the same list of effects:

def foo1R :_foo :_bar :_baz: Eff[R, Int] def foo2R :_foo :_bar :_baz: Eff[R, Int] def foo3R :_foo :_bar :_baz: Eff[R, Int]

It is possible to “pack” them with the following _effects type definition:

import org.atnos.eff.Members.{&:, &&:}

trait Foo[A]
trait Bar[A]
trait Baz[A]
trait Boo[A]

type _foo[R] = Foo |= R
type _bar[R] = Bar |= R
type _baz[R] = Baz |= R

// Note the last &&:, you can write instead: _foo[R] &: _bar[R] &: _baz[R] &: NoMember
type _effects[R] = _foo[R] &: _bar[R] &&: _baz[R]

def getFoo[R :_foo :_bar]: Eff[R, Int] = Eff.pure(1)
def getBar[R :_bar]: Eff[R, Int] = Eff.pure(1)
def getBaz[R :_baz]: Eff[R, Int] = Eff.pure(1)

object t {

  import org.atnos.eff.Members.extractMember

  def foo[R :_effects](i: Int): Eff[R, Int] =
    getFoo[R] >>
    getBar[R] >>
    getBaz[R]

}

// Then call foo with a concrete stack
type S = Fx.fx3[Foo, Bar, Baz]
type U = Fx.fx4[Foo, Bar, Baz, Boo]

t.foo[S](1)
t.foo[U](1)

The implicit definition extractMember in the Members object will take care of “unpacking” the member instances where required. Note that this implicit must not be in scope when calling the methods requiring the “packed” implicits otherwise it will trigger a divergent implicit search.