E0041

E0041#

Warning name: missing_rest_mark

Closed map pattern.

MoonBit's map pattern syntax does not support closed pattern (i.e. asserting that the map does not contain any other unmatched elements). So map patterns must always contain a .., otherwise this warning will be reported.

Erroneous example#

///|
fn f(map : Map[String, Int]) -> Bool {
  match map {
    // the pattern still matches 
    // if `map` contain elements other than "x" and "y"
    { "x": x, "y": y } => true
    _ => false
  }
}

///|
test {
  inspect(f({ "x": 1, "y": 2, "z": 3 }), content="true")
}

Suggestion#

  • If the intended semantic is open matching (i.e. allow existence of unmatched elements), just add .. to the map pattern.

  • If the intended semantic is closed matching, use pattern guard instead:

    ///|
fn f(map : Map[String, Int]) -> Bool {
  match map {
    { "x": x, "y": y, .. } if map.length() == 2 => true
    _ => false
  }
}

///|
test {
  inspect(f({ "x": 1, "y": 2, "z": 3 }), content="false")
}
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