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