Unhelpful error message on missing braces in an interface implementation

[Jiří Beneš]

When trying to write an instance of an interface:

interface Eff {
  def op(): Int
}

def e: Eff = new Eff {
  def op() = println("42"); 42
}

shows an error: "Operation Eff is not part of interface Eff."

Here's a screenshot:

One way to work around this is to add braces into the method body:

interface Eff {
  def op(): Int
}

def e: Eff = new Eff {
  def op() = { println("42"); 42 }
}

[Jonathan Immanuel Brachthäuser]

TBH it took me quite a while to understand this error...

The parser parses it as

def e: Eff = new Eff { () =>
  def op() = println("42")
  42
}

which is our syntax for singleton operations and thus shorthand for:

def e: Eff = new Eff { 
  def Eff() = {
    def op() = println("42")
    42
  }
}

[Jonathan Immanuel Brachthäuser]

@jiribenes what would you expect as an error message? We could also forbid parsing it as single operation in case it starts with def

[Jonathan Immanuel Brachthäuser]

I wouldn't know an elegant way to prevent it though.

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

Lines 426 to 437 in 49f2166

 lazy val implementation: P[Implementation] = 
   ( interfaceType ~ (`{` ~> many(defClause) <~ `}`) ^^ { 
     case effect ~ clauses => 
       Implementation(effect, clauses) 
     } 
   | idRef ~ maybeTypeParams ~ implicitResume ~ functionArg ^^ { 
     case id ~ tparams ~ resume ~ BlockLiteral(_, vparams, _, body) => 
       val synthesizedId = IdRef(id.name) 
       val interface = BlockTypeRef(id, Nil) withPositionOf id 
       Implementation(interface, List(OpClause(synthesizedId, tparams, vparams, None, body, resume) withPositionOf id)) 
     } 
   ) 

The parser for implementation reuses functionArg which admits the program you wrote.