Records

Named records via the type keyword — fixed sets of named, typed fields.

A type declares a named record — a fixed set of named, typed fields. Records describe the shape of values, function parameters, and the backing type of every block.

§ 1Declaring a record type

type Dog {
  name: utf8
  age:  u32
}

let rex = Dog { name: "Rex", age: 4u32 }

§ 2extends

A record may extends another, inheriting all its fields. The child type is a structural superset; it satisfies any context that accepts the parent.

type Dog {
  name: utf8
  age:  u32
}

type Pet extends Dog {
  breed: utf8
}

§ 3Working with records

Four builtins operate on record values: keys and values list a record's field names and values (sorted by name); merge combines two records, with the second winning on a clash; map_values transforms every value while keeping the keys.

let rex      = Dog { name: "Rex", age: 4u32 }
let names    = keys(rex)                       // ["age", "name"]
let defaults = { host: "localhost", port: 80 }
let cfg      = merge(defaults, { port: 8080 }) // { host: "localhost", port: 8080 }
let doubled  = map_values({ low: 1, high: 9 }, fn(x: i64) -> i64 x * 2)

§ 4Examples

§ 4.1Declaring and constructing a record

A type declares a named record; a record value fills its fields.

type Dog {
  name: utf8
  age:  u32
}

let rex = Dog { name: "Rex", age: 4u32 }

Expected: rex is a Dog record with name = "Rex" and age = 4.