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Tests for basic composition properties in JS (with Jest)
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| function compose (...fns) { | |
| function _compose(fnA, fnB) { | |
| if (fnA === undefined) { | |
| throw new Error('compose needs at least one argument') | |
| } | |
| return (...args) => { | |
| return fnA(fnB(...args)); | |
| } | |
| } | |
| return fns.reduce((acc, cur) => { | |
| return _compose(acc, cur) | |
| }); | |
| } | |
| function id(x) { | |
| return x; | |
| }; | |
| function square(n) { | |
| return n * n; | |
| } | |
| // f :: A -> B | |
| // g :: B -> C | |
| // h :: C -> D | |
| // h . (g . f) = (h . g) . f = h . g . f | |
| test('β β (π β π ) = (β β π) β π = β β π β π', () => { | |
| const c = compose; | |
| const h = (x) => `h(${x})`; | |
| const g = (x) => `g(${x})`; | |
| const f = (x) => `f(${x})`; | |
| const r = c(h, c(g, f)) | |
| const s = c(c(h, g), f) | |
| const t = c(h, g, f); | |
| const EXPECTED = 'h(g(f(7)))'; | |
| expect(r(7)).toBe(EXPECTED); | |
| expect(s(7)).toBe(EXPECTED); | |
| expect(t(7)).toBe(EXPECTED); | |
| }); | |
| // id . f = f | |
| test('idπ΅ β π = π', () => { | |
| const r = compose(id, square); | |
| expect(r(4)).toBe(16); | |
| }); | |
| // f . id = f | |
| test('π β idπ΄ = π', () => { | |
| const r = compose(square, id); | |
| expect(r(9)).toBe(81); | |
| }); |
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