E4224

Compiler diagnostic name: lexscan_catchall_missing.

The regex cases in a lexmatch or lexscan expression are not exhaustive, and the expression has no catch-all case.

The compiler checks whether the regex cases cover every possible input, including empty input and, for streaming scans, end of input. An exhaustive expression may omit the catch-all. Otherwise, add a final catch-all branch to define the result when no regex case matches.

Erroneous example

///|
pub fn classify(text : String) -> String {
  lexmatch text with longest {
    re"^[a-z]+$" => "word"
  }
}

Suggestion

Make the regex cases exhaustive, or add a final catch-all case. A lexmatch catch-all may bind the unmatched input; a lexscan catch-all must be written as _, including when its target is a StringScanner. If a catch-all exists but is not last, see E4171.

///|
pub fn classify(text : String) -> String {
  lexmatch text with longest {
    re"^.*$" => "text"
  }
}