@fpjs/overture/src/data/identity.js

A Javascript prelude

// This Source Code Form is subject to the terms of the Mozilla Public
// License, v. 2.0. If a copy of the MPL was not distributed with this
// file, You can obtain one at http://mozilla.org/MPL/2.0/.

/**
 * The identity functor and monad.
 */

import {type} from "@fpjs/overture/base";
import {map} from "@fpjs/overture/algebras/functor";
import {equals} from "@fpjs/overture/algebras/setoid";
import {lte} from "@fpjs/overture/algebras/ord";

///:: a -> Identity a
export const Identity = (x) => ({
    '@@type': 'Identity',
    'constructor': Identity,
    'runIdentity': x,
    'fantasy-land/map': (f) => Identity (f (x)),
    'fantasy-land/ap': map ((f) => f(x)),
    'fantasy-land/chain': (f) => f (x),
    'fantasy-land/reduce': (f, z) => f (z, x),
    'fantasy-land/equals': (y) => equals (x) (y.runIdentity),
    'fantasy-land/lte': (y) => lte (x) (y.runIdentity),
    'fantasy-land/traverse': (F, f) => map (Identity) (f (x)),
});
Identity['fantasy-land/of'] = Identity;

/** Extract the value from an Identity functor. */
///:: Identity a -> a
export const runIdentity = (o) => {
    if (! ('runIdentity' in o)) {
        throw TypeError(`${type(o)} is not Identity`);
    }
    return o['runIdentity'];
};