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A Unified Theory of JavaScript Style, Part I JavaScript Combinators

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we'll talk about Combinators and decorators

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but think about Flexibility and decluttering

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composition We compose entities to create new entities

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interfaces Not all entities "fit together"

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Homogeneous interfaces create dense spaces

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Heterogeneous interfaces create sparse spaces

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Dense is more flexible than sparse

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Sparse can be quicker to grasp

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enough with the math!

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pluck: "A convenient version of what is perhaps the most common use-case for map: extracting a list of property values."

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"pluckWith" is the flipped form of "pluck"

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function pluck (mappable, key) { return mappable.map(function (obj) { return obj[key]; }); }; function pluckWith (key, mappable) { return pluck(mappable, key); }; var stooges = [ {name: 'moe', age: 40}, {name: 'larry', age: 50}, {name: 'curly', age: 60}]; pluckWith('name', stooges); //=> ["moe", "larry", "curly"]

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Let's make "pluckWith" out of combinators

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A unary combinator function flip (fn) { return function flipped (a, b) { return fn.call(this, b, a); } } function arrow (a, b) { return "" + a + " -> " + b; } flip(arrow)("x", "y") //=> 'y -> x'

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curry Another unary combinator

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function curry (fn) { return function curried (a, optionalB) { if (arguments.length > 1) { return fn.call(this, a, optionalB); } else return function partiallyApplied (b) { return fn.call(this, a, b); } } }

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Currying: var curriedArrow = curry(arrow); //=> [function] curriedArrow('finger')('moon') //=> 'finger -> moon'

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Partial Application: var taoism = curry(arrow)('finger'); //=> [function] taoism('moon') //=> 'finger -> moon'

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nota bene Partial application transforms binary operations into unary operations

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function get (object, property) { return object[property]; } get({foo: 1}, 'foo') //=> 1

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var getWith = curry(flip(get)); getWith('foo')({foo: 1}) //=> 1

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function map (mappable, fn) { return mappable.map(fn, this); } function double (n) { return n * 2; } map([1, 2, 3], double) //=> [2, 4, 6]

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var mapWith = curry(flip(map)); mapWith(double, [1, 2, 3]); //=> [2, 4, 6] var doubleAll = mapWith(double); doubleAll([1, 2, 3]) //=> [2, 4, 6]

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“almost there...”

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function pluckWith (attr) { return mapWith(getWith(attr)); }

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Compose

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function compose (a, b) { return function composed (c) { return a(b(c)); } }

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quod erat demonstrandum The combinator implementation of "pluckWith"

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var pluckWith = compose(mapWith, getWith);

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Let's compare both implementations of "pluckWith"

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var pluckWith = compose(mapWith, getWith); //// versus //// function pluck (mappable, key) { return mappable.map(function (obj) { return obj[key]; }); }; function pluckWith (key, mappable) { return pluck(mappable, key); };

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lesson Composing functions with combinators increases code flexibility...

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lesson Composing functions with combinators demands increased mental flexibility

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using combinators to make Decorators

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function Cake () {} extend(Cake.prototype, { mix: function () { // mix ingredients together return this; }, rise: function (duration) { // let the ingredients rise return this; }, bake: function () { // do some baking return this; } });

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fluent function fluent (methodBody) { return function fluentized () { methodBody.apply(this, arguments); return this; } }

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function Cake () {} extend(Cake.prototype, { mix: fluent( function () { // mix ingredients together }), rise: fluent( function (duration) { // let the ingredients rise }), bake: fluent(function () { // do some baking }) });

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new requirements Mix before rising or baking

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extend(Cake.prototype, { mix: fluent( function () { // mix ingredients together }), rise: fluent( function (duration) { this.mix(); // let the ingredients rise }), bake: fluent(function () { this.mix(); // do some baking }) });

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before a combinator that transforms decorations into decorators var before = curry( function decorate (decoration, method) { return function decoratedWithBefore () { decoration.apply(this, arguments); return method.apply(this, arguments); }; } ); var mixFirst = before(function () { this.mix() });

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the final version extend(Cake.prototype, { // Other methods... mix: fluent( function () { // mix ingredients together }), rise: fluent( mixFirst( function (duration) { // let the ingredients rise })), bake: fluent( mixFirst( function () { // do some baking })) });

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lesson Decorators declutter secondary concerns

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after var after = curry( function decorate (decoration, method) { return function decoratedWithAfter () { var returnValue = method.apply(this, arguments); decoration.apply(this, arguments); return returnValue; }; } );

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around var around = curry( function decorate (decoration, method) { return function decoratedWithAround () { var methodPrepended = [method].concat( [].slice.call(arguments, 0) ); return decoration.apply(this, methodPrepended); }; } );

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call me maybe var maybe = around(function (fn, value) { if (value != null) { return fn.call(this, value);; } }); maybe(double)(2) //=> 4 maybe(double)(null) //=> undefined

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generalized guards function provided (guard) { return around(function () { var fn = arguments[0], values = [].slice.call(arguments, 1); if (guard.apply(this, values)) { return fn.apply(this, values); } }); } var maybe = provided( function (value) { return value != null; });

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inversions function not (fn) { return function notted () { return !fn.apply(this, arguments) } } var except = compose(provided, not); var maybe = except( function (value) { return value == null; });

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lessons

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lesson one Combinators increase code flexibility and require increased mental flexibility

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lesson two Decorators declutter secondary concerns

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lesson three Do not follow in the footsteps of the sages. Seek what they sought

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Reginald Braithwaite GitHub, Inc. raganwald.com @raganwald NDC Conference, Oslo, Norway, June 5, 2014