Created
September 30, 2017 03:57
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Notes from Allonge Six
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| const _ = require('lodash'); | |
| const map = _.map; | |
| const mapWith = (fn) => (list) => map(list, fn); | |
| // Bounding to a name | |
| ((diameter_fn) => | |
| console.log(diameter_fn(2)) //=> 6.2831853 | |
| )( | |
| ((PI) => | |
| (diameter) => diameter * PI | |
| )(3.14159265) | |
| ); | |
| // Combinators | |
| const identity = (x) => x; // I Combinator | |
| const kestrel = (x) => (y) => x; // K Combinator | |
| const composeComb = (a, b) => (c) => a(b(c)); // B Combinator | |
| // Decorators | |
| const not = (fn) => (x) => !fn(x); // decorator | |
| const something = (x) => x != null; | |
| // const nothing = (x) => !something(x); | |
| const nothing = not(something); | |
| // Partial Appliacation | |
| const callLeft = (fn, ...args) => | |
| (...remainingArgs) => | |
| fn(...args, ...remainingArgs); | |
| const callRight = (fn, ...args) => | |
| (...remainingArgs) => | |
| fn(...remainingArgs, ...args); | |
| const squareAll = callRight(map, (n) => n * n); | |
| console.log(squareAll([1, 2, 3])); //=> [1, 4, 9] | |
| // Turns function into a function taking exactly one argument. | |
| const unary = (fn) => | |
| fn.length === 1 | |
| ? fn | |
| : function (something) { | |
| return fn.call(this, something); | |
| }; | |
| ['1', '2', '3'].map(unary(parseInt)); //=> [1, 2, 3] | |
| // Does something with value for side-effects, but keeps the value around | |
| const tap = (value, fn) => { | |
| const curried = (fn) => ( | |
| typeof(fn) === 'function' && fn(value), | |
| value | |
| ); | |
| return fn === undefined | |
| ? curried | |
| : curried(fn); | |
| }; | |
| tap([1, 2, 3])((a) => map(a, (e) => e * 4)); //=> [1, 2, 3] | |
| // or | |
| tap([1, 2, 3], (a) => map(a, (e) => e * 4)); // same | |
| // Returns nothing if any of function arguments is null | |
| const maybe = (fn) => | |
| function (...args) { | |
| if (args.length === 0) return; | |
| for(let arg of args) { | |
| if(arg == null) return; | |
| } | |
| return fn.apply(this, args); | |
| }; | |
| maybe((a, b, c) => a + b + c)(1, 2, 3); //=> 6 | |
| maybe((a, b, c) => a + b + c)(1, null, 3); //=> undefined | |
| // Ensures that a function can only be called once | |
| const once = (fn) => { | |
| let done = false; | |
| return function () { | |
| return done ? void 0 : ((done = true), fn.apply(this, arguments)); | |
| }; | |
| } | |
| const askedOnBlindDate = once( | |
| () => "sure, why not?" | |
| ); | |
| askedOnBlindDate() | |
| //=> 'sure, why not?' | |
| askedOnBlindDate() | |
| //=> undefined | |
| askedOnBlindDate() | |
| //=> undefined | |
| // Variadic Compose | |
| const compose = (a, ...rest) => | |
| rest.length === 0 | |
| ? a | |
| : (c) => a(compose(...rest)(c)); | |
| // Flatten the given array | |
| const flatten = ([first, ...rest]) => { | |
| if (first === undefined) return []; | |
| if (!Array.isArray(first)) return [first, ...flatten(rest)]; | |
| return [...flatten(first), ...flatten(rest)]; | |
| } | |
| flatten(["foo", [3, 4, []]]) | |
| //=> ["foo",3,4] | |
| // Just reduce | |
| const foldWith = (fn, terminalValue, [first, ...rest]) => | |
| first === undefined | |
| ? terminalValue | |
| : fn(first, foldWith(fn, terminalValue, rest)); | |
| foldWith((number, rest) => number * number + rest, 0, [1, 2, 3, 4, 5]) | |
| //=> 55 |
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