借助函数完成可组合的数据类型（软件编写）（第十部分）

const t = value = >{
    const fn = () = >value;
    fn.toString = () = >`t($ {
        value
    })`;
    return fn;
};
 
const someValue = t(2);
console.log(someValue.toString() // "t(2)"
);

const assert = {
  same: (actual, expected, msg) => {
    if (actual.toString() !== expected.toString()) {
      throw new Error(`NOT OK: ${ msg }
        Expected: ${ expected }
        Actual:   ${ actual }
      `);
    }
    console.log(`OK: ${ msg }`);
  }
};
 
{
  const msg = 'a value t(x) composed with t(0) ==== t(x)';
  const x = 20;
  const a = t(x)(t(0));
  const b = t(x);
  assert.same(a, b, msg);
}
{
  const msg = 'a value t(x) composed with t(1) ==== t(x + 1)';
  const x = 20;
  const a = t(x)(t(1));
  const b = t(x + 1);
  assert.same(a, b, msg);
}

NOT OK: a value t(x) composed with t(0) ==== t(x)
        Expected: t(20)
        Actual:   20

const t = value => {
  const add = n => t(value + n);
  return Object.assign(add, {
    toString: () => `t(${ value })`,
    valueOf: () => value
  });
};

"OK: a value t(x) composed with t(0) ==== t(x)"
"OK: a value t(x) composed with t(1) ==== t(x + 1)"

// 自顶向下的函数组合：
const pipe = (...fns) => x => fns.reduce((y, f) => f(y), x);
// 求和函数为 pipeline 传入需要的初始值
// curry 化的 pipeline 复用度更好，我们可以延迟传入任意的初始值
const sumT = (...fns) => pipe(...fns)(t(0));
sumT(
  t(2),
  t(4),
  t(-1)
).valueOf(); // 5

const result = compose(
  value1,
  value2,
  value3
);

.1 + .2 === .3 // false

npm install--save moneysafe

import { $ } from 'moneysafe';
$(.1) + $(.2) === $(.3).cents; // true

import { $ } from 'moneysafe';
import { $$, subtractPercent, addPercent } from 'moneysafe/ledger';
$$(
  $(40),
  $(60),
  // 减去折扣
  subtractPercent(20),
  // 上税
  addPercent(10)
).$; // 88

git clone git@github.com: ericelliott / moneysafe.git

npm install

npm run watch

rm source / moneysafe.js && touch source / moneysafe.js