Unhelpful error message when effect keyword is misused as an interface

[jiribenes]

The original Effekt paper uses the effect keyword for both an "interface" and a singleton operation:

effect Lexer {
  def peek(): Option[Token]
  def next(): Token
}

effect Emit {
  def text(content: String): Unit
  def newline(): Unit
}

effect LayoutChoice {
  def fail[A](): A
  def choice(): Direction
}

effect Next(): String
effect Choice(): Boolean

However, when you read the paper and just want to try things out on the website, you get a bizarre parser error:

I think it would be nice to special case this exact syntax in the parser to provide a much nicer error message: something that underlines the effect keyword, says "Did you mean interface? ...", and ideally provides a quick-fix that works both in VSCode and on the website.

---

Relevant code

effekt/effekt/shared/src/main/scala/effekt/RecursiveDescent.scala

Lines 469 to 486 in 6e71bcd

	// effect <NAME>[...](...): ...
	def operationDef(): Def =
	nonterminal:
	`effect` ~> operation() match {
	case op @ Operation(id, tps, vps, bps, ret) =>
	// TODO is the `true` flag used at all anymore???
	InterfaceDef(IdDef(id.name), tps, List(Operation(id, Nil, vps, bps, ret) withPositionOf op))
	}
	def operation(): Operation =
	nonterminal:
	idDef() ~ params() ~ returnAnnotation() match {
	case id ~ (tps, vps, bps) ~ ret => Operation(id, tps, vps, bps, ret)
	}
	def interfaceDef(): InterfaceDef =
	nonterminal:
	InterfaceDef(`interface` ~> idDef(), maybeTypeParams(), `{` ~> manyWhile(`def` ~> operation(), `def`) <~ `}`)