Pattern matching

match is a statement. The scrutinee is evaluated once. Arms are tried in order. case _ is the wildcard.

match moved:
    case Message.Move(x, y):
        assert x == 1
    case Message.Quit if n > 0:
        pass
    case _:
        assert False

What binds

Enum payloads bind names in the arm. Message.Move(x, y) brings x and y into that suite with the payload field types. Unit variants bind nothing.

case pat if expr is a guard. The pattern must match and the guard must be true, or the next arm is tried.

What is is not

tone is Color.Green is an expression that compares variant identity. It is not a match. Use is in conditions; use match when you need to unpack.

Exhaustiveness

Write a case _ when you do not want a fall-through hole. The compiler will not silently invent a default. An unmatched value that falls out of a match without a wildcard is a runtime / control-flow problem — treat _ as part of the contract.

Flags

On a @flags enum, Flag.A in mask is a bitwise test, not a match. You can still match the whole mask if you need structure; in is the cheap membership check.