List

The List effect is used for computations which may return several values. A simple example using this effect would be

import org.atnos.eff._, all._, syntax.all._

type S = Fx.fx1[List]

// create all the possible pairs for a given list
// where the sum is greater than a value
def pairsBiggerThan[R :_list](list: List[Int], n: Int): Eff[R, (Int, Int)] = for {
  a <- values(list:_*)
  b <- values(list:_*)
  found <- if (a + b > n) singleton((a, b))
  else           empty
} yield found

pairsBiggerThan[S](List(1, 2, 3, 4), 5).runList.run

> List((2,4), (3,3), (3,4), (4,2), (4,3), (4,4))