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📜 Specification for type identifiers
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A type is a set of values. Boolean, for example, is the type comprising true and false. A value may be a member of multiple types (42 is a member of Number, PositiveNumber, Integer, and many other types).

In certain situations it is useful to divide JavaScript values into non-overlapping types. The language provides two constructs for this purpose: the typeof operator and Object.prototype.toString. Each has pros and cons, but neither supports user-defined types.

sanctuary-type-identifiers comprises:

  • an npm and browser -compatible package for deriving the type identifier of a JavaScript value; and
  • a specification which authors may follow to specify type identifiers for their types.


For a type to be compatible with the algorithm:

  • every member of the type MUST have a constructor property pointing to an object known as the type representative;

  • the type representative MUST have a @@type property (the type identifier); and

  • the type identifier MUST be a string primitive and SHOULD have format '<namespace>/<name>[@<version>]', where:

    • <namespace> MUST consist of one or more characters, and SHOULD equal the name of the npm package which defines the type (including scope where appropriate);

    • <name> MUST consist of one or more characters, and SHOULD be the unique name of the type; and

    • <version> MUST consist of one or more digits, and SHOULD represent the version of the type.

If the type identifier does not conform to the format specified above, it is assumed that the entire string represents the name of the type; namespace will be null and version will be 0.

If the version is not given, it is assumed to be 0.

For example:

//  Identity :: a -> Identity a
function Identity(x) {
  if (!(this instanceof Identity)) return new Identity(x);
  this.value = x;

Identity['@@type'] = 'my-package/Identity';

Note that by using a constructor function the constructor property is set implicitly for each value created. Constructor functions are convenient for this reason, but are not required. This definition is also valid:

//  IdentityTypeRep :: TypeRep Identity
var IdentityTypeRep = {
  '@@type': 'my-package/Identity'

//  Identity :: a -> Identity a
function Identity(x) {
  return {constructor: IdentityTypeRep, value: x};


const type = require('sanctuary-type-identifiers');
> function Identity(x) {
.   if (!(this instanceof Identity)) return new Identity(x);
.   this.value = x;
. }
. Identity['@@type'] = 'my-package/Identity@1';

> type.parse(type(Identity(0)))
{namespace: 'my-package', name: 'Identity', version: 1}


type :: Any -> String

Takes any value and returns a string which identifies its type. If the value conforms to the specification, the custom type identifier is returned.

> type(null)

> type(true)

> type(Identity(0))

type.parse :: String -> { namespace :: Nullable String, name :: String, version :: Number }

Takes any string and parses it according to the specification, returning an object with namespace, name, and version fields.

> type.parse('my-package/List@2')
{namespace: 'my-package', name: 'List', version: 2}

> type.parse('nonsense!')
{namespace: null, name: 'nonsense!', version: 0}

> type.parse(Identity['@@type'])
{namespace: 'my-package', name: 'Identity', version: 1}
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