Patterns read the shape of a value. They are most visible in match branches.

let Port := data {
  | Configured(port : Int, secure : Bool)
  | Default
};

let configured : Port := .Configured(secure := .True, port := 8080);
match configured (
| .Configured(port, secure := _) => port
| .Default => 3000
);

Match constructors

let Port := data {
  | Configured(Int)
  | Default
};

let configured : Port := .Configured(8080);
match configured (
| .Configured(port) => port
| .Default => 3000
);

A constructor pattern starts with a dot. Names inside the pattern bind the carried values.

Match named fields

Named variant fields can be read by name. Use _ when the field exists but the branch does not need it.

let Port := data {
  | Configured(port : Int, secure : Bool)
  | Default
};

let configured : Port := .Configured(secure := .True, port := 8080);
match configured (
| .Configured(port, secure := _) => port
| .Default => 3000
);

Record-shaped data uses the normal record value shape. Use field access when you want one field after construction.

let Box[T] := data {
  value : T;
};

let boxedPort : Box[Int] := { value := 8080 };
boxedPort.value;

Ignore values

Use _ when a value is present but not needed.

match configured (
| .Configured(_) => 8080
| .Default => 3000
);

Do not bind a name you will not use. _ tells the reader that the value is intentionally ignored.