java.lang.ClassCastException when handling an effect alias of an interface

[marvinborner]

Effekt crashes with the following error if you incorrectly try to use an effect that's aliased to an interface:

Exception in thread "main" java.lang.ClassCastException: class effekt.symbols.EffectAlias cannot be cast to class effekt.symbols.BlockTypeConstructor$Interface (effekt.symbols.EffectAlias and effekt.symbols.BlockTypeConstructor$Interface are in unnamed module of loader 'app')

On the web, it crashes with TypeError: can't access property "a", a is undefined.

Reproduction:

interface Term {
  def num(n: Int): Unit
}

effect Readback = Term

def main() =
  try go()
  with Readback {
    def num(n) = resume(println(n))
  }

[marvinborner]

Also, is there a way to create an alias of an interface?

[jiribenes]

Nice find! The part you've shown is working fine, it's the implementation part of the handler that's wrong — we likely don't allow try { ... } with <effect alias> { ... } for obvious reasons.
see playground

[marvinborner]

Interesting, I adapted the example. What are the "obvious reasons" for not allowing this?

[jiribenes]

Ah sorry, was on the bus.
In the general case, effect aliases take whole sets of effects such as Scan in the stdlib, so writing with <alias of mulitple effects> might be hard as they can even share methods...

effekt/libraries/common/scanner.effekt

Line 10 in 117f720

effect Scan[A] = {read[A], peek[A]}

EDIT: But you're right, we could technically allow this when the aliased effect set is a singleton.

[marvinborner]

Ah sure, I see the problem with sets of interfaces. I am only interested in the singleton-case.

I initially expected interface Readback = Term to work, maybe we should instead support this.

[jiribenes]

I initially expected interface Readback = Term to work, maybe we should instead support this.

I'm not certain we ought to support this, but it should be easy enough to add a similarly special cased error message to the parser for when people use effect to mean an interface:

effekt/effekt/shared/src/main/scala/effekt/Parser.scala

Lines 620 to 639 in 117f720

	// On the top-level both
	// effect Foo = {}
	// and
	// effect Foo(): Int
	// are allowed. Here we simply backtrack, since effect definitions shouldn't be
	// very long and cannot be nested.
	def effectOrOperationDef(info: Info): Def = {
	// We used to use `effect Foo { def a(); def b(); ... }` for multi-operation interfaces,
	// but now we use `interface Foo { ... }` instead.
	// If we can't parse `effectDef` or `operationDef`, we should try parsing an interface with the wrong keyword
	// and report an error to the user if the malformed interface would be valid.
	def interfaceDefUsingEffect(): Maybe[InterfaceDef] =
	backtrack(restoreRecoverable = false):
	softFailWith("Unexpected 'effect', did you mean to declare an interface of multiple operations using the 'interface' keyword?"):
	interfaceDef(info, `effect`)
	backtrack { effectDef(info) }
	.orElse { interfaceDefUsingEffect() }
	.getOrElse { operationDef(info) } // The `operationDef` should be last as to not cause spurious errors later.
	}