michael@0: /* This Source Code Form is subject to the terms of the Mozilla Public
michael@0:  * License, v. 2.0. If a copy of the MPL was not distributed with this
michael@0:  * file, You can obtain one at http://mozilla.org/MPL/2.0/.
michael@0:  */
michael@0: "use strict";
michael@0: 
michael@0: module.metadata = {
michael@0:   "stability": "experimental"
michael@0: };
michael@0: 
michael@0: const method = require("method/core");
michael@0: 
michael@0: // Utility function that is just an enhancement over `method` to
michael@0: // allow predicate based dispatch in addition to polymorphic
michael@0: // dispatch. Unfortunately polymorphic dispatch does not quite
michael@0: // cuts it in the world of XPCOM where no types / classes exist
michael@0: // and all the XUL nodes share same type / prototype.
michael@0: // Probably this is more generic and belongs some place else, but
michael@0: // we can move it later once this will be relevant.
michael@0: let dispatcher = hint => {
michael@0:   const base = method(hint);
michael@0:   // Make a map for storing predicate, implementation mappings.
michael@0:   let implementations = new Map();
michael@0: 
michael@0:   // Dispatcher function goes through `predicate, implementation`
michael@0:   // pairs to find predicate that matches first argument and
michael@0:   // returns application of arguments on the associated
michael@0:   // `implementation`. If no matching predicate is found delegates
michael@0:   // to a `base` polymorphic function.
michael@0:   let dispatch = (value, ...rest) => {
michael@0:     for (let [predicate, implementation] of implementations) {
michael@0:       if (predicate(value))
michael@0:         return implementation(value, ...rest);
michael@0:     }
michael@0: 
michael@0:     return base(value, ...rest);
michael@0:   };
michael@0: 
michael@0:   // Expose base API.
michael@0:   dispatch.define = base.define;
michael@0:   dispatch.implement = base.implement;
michael@0:   dispatch.toString = base.toString;
michael@0: 
michael@0:   // Add a `when` function to allow extending function via
michael@0:   // predicates.
michael@0:   dispatch.when = (predicate, implementation) => {
michael@0:     if (implementations.has(predicate))
michael@0:       throw TypeError("Already implemented for the given predicate");
michael@0:     implementations.set(predicate, implementation);
michael@0:   };
michael@0: 
michael@0:   return dispatch;
michael@0: };
michael@0: 
michael@0: exports.dispatcher = dispatcher;