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javascript/examples/1_4_functions-hi.js

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+"use strict";        4
+
+chapter("Hi-order functions");
+section("Minimum by absolute value");
+
+let minimumByAbs = function(...args) {
+    let result = Infinity;
+    for (const arg of args) {
+        if (Math.abs(result) > Math.abs(arg)) {
+            result = arg;
+        }
+    }
+    return result;
+};
+println("minimumByAbs =", minimumByAbs);
+example("minimumByAbs(1, -2, 3)");
+
+
+section("Unify minimum and minimumByAbs");
+
+subsection("High-order functions");
+function minimumBy(comparator, init = Infinity) {
+    return (...args) => {
+        let result = init;
+        for (const arg of args) {
+            if (comparator(result, arg) > 0) {
+                result = arg;
+            }
+        }
+        return result;
+    }
+}
+println(minimumBy);
+
+function comparing(f) {
+    return (a, b) => f(a) - f(b);
+}
+println(comparing);
+
+const identity = a => a;
+println("const identity =", identity);
+
+function maximumBy(comparator, init = -Infinity) {
+    return minimumBy((a, b) => -comparator(a, b), init);
+}
+println(maximumBy);
+
+
+subsection("Definitions");
+
+let minimumByV = minimumBy(comparing(identity));
+minimumByAbs = minimumBy(comparing(Math.abs));
+let maximumByLength = maximumBy(comparing(s => s.length), "");
+
+example("minimumByV");
+example("minimumByAbs");
+example("maximumByLength");
+example("minimumByV(1, -2, 3)");
+example("minimumByAbs(1, -2, 3)");
+example("maximumByLength('aa', 'bbb', 'c')");
+
+
+section("Unify minimumBy and sum");
+
+subsection("High-order functions");
+function foldLeft(f, zero) {
+    return (...args) => {
+        let result = zero;
+        for (const arg of args) {
+            result = f(result, arg);
+        }
+        return result;
+    }
+}
+println(foldLeft);
+
+function minBy(f) {
+    return (a, b) => f(a) < f(b) ? a : b;
+}
+println(minBy);
+
+subsection("Definitions");
+const sum2 = foldLeft((a, b) => a + b, 0);
+const product = foldLeft((a, b) => a * b, 1);
+minimumByAbs = foldLeft(minBy(comparing(Math.abs)), Infinity);
+example("sum2(1, -2, 3)");
+example("product(1, -2, 3)");
+example("minimumByAbs(1, -2, 3)");
+
+
+section("sumSquares and sumAbs");
+
+let square = x => x * x;
+let sumSquares = foldLeft((a, b) => a + square(b), 0);
+let sumAbs = foldLeft((a, b) => a + Math.abs(b), 0);
+example("sumSquares(1, -2, 3)");
+example("sumAbs(1, -2, 3)");
+
+
+subsection("High-order functions");
+function map(f) {
+    return (...args) => {
+        const result = [];
+        for (const arg of args) {
+            result.push(f(arg));
+        }
+        return result;
+    }
+}
+println(map);
+
+function compose(f, g) {
+    return (...args) => f(g(...args));
+}
+println(compose);
+
+function unspread(f) {
+    return args => f(...args);
+}
+println(unspread);
+
+subsection("Definitions");
+sumSquares = compose(unspread(sum2), map(square));
+sumAbs = compose(unspread(sum2), map(Math.abs));
+example("sumSquares(1, -2, 3)");
+example("sumAbs(1, -2, 3)");
+
+
+section("diff");
+
+let diff = dx => f => x => (f(x + dx) - f(x - dx)) / 2 / dx;
+
+let dsin = diff(1e-7)(Math.sin);
+for (let i = 0; i < 10; i++) {
+    println(i + " " + Math.cos(i) + " " + dsin(i) + " " + Math.abs(Math.cos(i) - dsin(i)));
+}
+
+
+section("Currying");
+subsection("curry");
+
+const curry = f => a => b => f(a, b);
+
+const addC = curry((a, b) => a + b);
+const add10 = addC(10);
+example("addC(10)(20)");
+example("add10(20)");
+
+
+subsection("uncurry");
+
+const uncurry = f => (a, b) => f(a)(b);
+const addU = uncurry(a => b => a + b);
+example("addU(10, 20)");
+
+
+subsection("mCurry");
+
+println("bind");
+let bind = (f, ...as) => (...args) => f(...[...as, ...args]);
+let add100 = bind((a, b) => a + b, 100);
+example("    add100(200)");
+
+println("mCurry");
+let mCurry = curry(bind);
+let sub = mCurry((a, b, c) => a - b - c);
+let sub10 = sub(10);
+example("    sub(10)(20, 30)");
+example("    sub10(20, 30)");