handlers and combined effects

[dcurrie]

It surprised me that nesting two effect handlers let one of the effects leak... (I have no certainty that my terminology here or interpretation of what is really happening is correct, hence in Q&A section!)...

My goal is to write a unit test that has a mutable vector and uses random numbers. Each of those two things work separately for me. When I combine them, I get

(1, 0): error: there are unhandled effects for the main expression
  inferred effect : <pure,console,rstest/istate<int>>
  unhandled effect: rstest/istate<int>
  hint            : wrap the main function in a handler

and I have no idea how to wrap the main function in a handler. I found a workaround, but I don't understand it.

This fails:

fun istate-test(): <exn,div,random,istate<int>> string {
    istate(100, -9, show-a-couple-prng)
}
public fun main() {
  println("Random numbers " + pseudo-random(7, istate-test))
}

This fails:

public fun main() {
  fun istate-test(): <exn,div,random,istate<int>> string {
    istate(100, -9, show-a-couple-prng)
  }
  println("Random numbers " + pseudo-random(7, istate-test))
}

This works!?:

public fun main() {
  println("Random numbers " + pseudo-random(7, fn() { istate(100, -9, show-a-couple-prng) }))
}

Full test code is here

[anfelor]

I think your type signature for istate-test is wrong:
fun istate-test(): <exn,div,random,istate<int>> string
still has the istate effect. I think if you remove that effect from the type signature you should be fine.
In particular, if you add an effect to the type signature that is not needed in the function, Koka will subsequently assume that the function needed the effect.

[dcurrie]

Thanks, Anton! That fixes it. I was confused by the error message.