Unhelpful error message on missing braces in an interface implementation

[jiribenes]

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 }
}

[b-studios]

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
  }
}

[b-studios]

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

[b-studios]

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

https://github.com/effekt-lang/effekt/blob/49f2166f85c228b6169d1e7d0c1f508081c5551a/effekt/shared/src/main/scala/effekt/Parser.scala#L426-L437

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

[jiribenes]

After #495, the error looks something like this:

which means I can somewhat improve it (or at least underline the relevant part) by consuming tokens after ; until I find } or def or EOF: