Record.md

Records

Examples

Initialization

const a = Record({ x: 1, y: 2 });

Initialization Sugar

const a = #{ x: 1, y: 2 };

Extension

const b = Record(Object.assign({}, a, { z: 3 }));

Extension Sugar

const b = #{ ...a, z: 3 };

Type

typeof #{}; // 'record'

Syntax

RecordLiteral:

• # { (... AssignmentExpression)? }
• # { PropertyDataAssignment (, PropertyDataAssignment)* (, ... AssignmentExpression)? }

PropertyDataAssignment:

• PropertyName : AssignmentExpression

Comparison Semantics

== and === test value equality.

<, >= etc. test value order/equality in enumeration order.

Prototype

Value types are not objects and have no prototype. Member expressions normally access a prototype but the prototype of records is null.

This makes it easier to verify that a record has a particular signature without hasOwnProperty checks.

Notes

Enumeration always proceeds in lexicographic order of property names.

Polyfill in Terms of Typed Objects

let Record = (function() {
  let RecordSymbol = new Symbol('record');
  let RecordConstructor = function(object) {
    const keys = Object.keys(object).sort();
    let fields = {};
    for (let i = 0; i < keys.length; i++) {
      fields[keys[i]] = any;
    }
    // Note: Enumeration order of fields is not guaranteed which
    // makes ValueType a problematic API for this use case.
    let type = ValueType(RecordSymbol, fields);
    // Note: We can't replace the ValueType's prototype object
    // so we have to do the next best thing.
    Object.setPrototypeOf(type.prototype, null);
    return new type(object);
  };
  RecordConstructor.prototype = null;
  return RecordConstructor;
})();