zoukankan      html  css  js  c++  java
  • [Functional Programming] Function modelling -- 3. Reader Monad

    Reader Monad

    Continue with previous post, here we have some helper functions:

    const toUpper = s => s.toUpperCase();
    const exclaim = s => `${s}!`;

    And here is current code we have:

    const Fn = run => ({
      run,
      map: f => Fn(x => f(run(x))),
      chain: f =>
        Fn(x => console.log("run(x)", run(x), f(run(x))) || f(run(x)).run(x)),
      concat(otherFn) {
        return Fn(x => run(x).concat(otherFn.run(x)));
      }
    });

    Monad

    Monad is all about nesting, for example, we have 'Fn' instead a 'Fn':

    Fn(x => Fn(y => ...))

    Monad job is to flatten it.

    To implement a Monad interface, we need to implement 'chain' function:

    const Fn = run => ({
       run,
       concat(otherFn) {
          return Fn(x => run(x).concat(otherFn.run(x)))
       },
       map(f) { 
          return Fn(f => f(run(x)))
       },
       chain(otherM) {
          
       }
    })
    

    'chain' function take other Monad as arguement. 

    The implementation of 'chain' is pretty similar to 'map', the difference in meaning between 'map' & 'chain' is that we want to apply two 'Fn(method)' on the same value.

    const Fn = run => ({
      run,
      map: f => Fn(
        x => f(run(x))
      ),
      chain: f => Fn(
        x => f(run(x))
              .run(x)
      ),
      concat(otherFn) {
        return Fn(x => run(x).concat(otherFn.run(x)));
      }
    });

    Taking an example:

    Fn(toUpper)
      .chain(upper => Fn(y => exclaim(upper)))
      .run("hi"); // HI!

    'run(x)' return value, in the example is the result of 'toUpper' which is 'HI'.

    Then 'f(run(x))' return 'Fn' Monad:

    Fn (
    { 
      run: [Function],
      map: [Function: map],
      chain: [Function: chain],
      concat: [Function: concat] 
    })

    Last when we call 'f(run(x)).run(x)', we will get 'HI!', which is running 'exclaim' function over 'HI'.

    Also .run function unwrap Fn, everytimes we call .run(), we will get its value, instead of Fn().

    For 'chain' method:

      chain(f) {
        // f() return another Fn
        // when you call .run(), it unwrapper Fn()
        return Fn((x) => f(run(x)).run(x));
      },

    it call two times 'run()' but only add one 'Fn', which means it unwrap one nested 'Fn'.


    Fn.of()

    We have seen the example: 'Fn(upper)', we don't get used to that... we somehow lift a function 'upper' into a Functor, then data come last when we run ".run('hi')".

    We can change that, by passing the data first into 'Fn.of(data)', then we can doing mapping, chainning.

    But there is another interesting thing, in the previous example, data: 'hi' is the only input. If now we lift data into Fn.of('hi'), then we run ".run(another_data)", now we have two data input.

    Let's see 'Fn.of()' function first:

    const Fn = run => ({
      run,
      map: f => Fn(x => f(run(x))),
      chain: f =>
        Fn(x => console.log("run(x)", run(x), f(run(x))) || f(run(x)).run(x)),
      // run two functions and concat the result
      concat(otherM) {
        return Fn(x => run(x).concat(otherM.run(x)));
      }
    });
    Fn.of = x => Fn(() => x);
    

    Example:

    Fn.of("hi")
      .map(toUpper)
      .chain(upper => Fn(x => [upper, exclaim(x)]))
      .run("hello"); // [ 'HI', 'hello!' ]
    

    As you can see, now we have two values. We have applied 'toUpper' transform on 'hi', and apply 'exclaim' function on "hello".

    Example 2:

    Fn.of("hi")
      .map(toUpper)
      .chain(upper => Fn(x => [upper, exclaim(x)])) // ["HI", 'hello!']
      .chain(ary => Fn(x => [exclaim(ary[0]), x])) // ["HI!", "hello"]
      .run("hello");

    We can keep chaining based on previous chain's result. But this time, we keep 'x' as it is. So now, this is interesting thing, the data we pass with '.run('hello')', always keep the same value every time we try to read it.

    You can use it can enviormenet configuration object, anytime you need to read env variable you can get it from 'chain'. This partten is called: "Read Monda".

    Think about Angular, for example, it is similar to dependency injection. For React, you can use it as Provider / Context.


    Fn.ask()

    Let's define a convenient function call 'ask'. So instead of using "Fn()" constructor

    .chain(upper => Fn(x => [upper, exclaim(x)])

    We want to do:

    const Fn = run => ({
      run,
      map: f => Fn(x => f(run(x))),
      chain: f =>
        Fn(x => console.log("run(x)", run(x), f(run(x))) || f(run(x)).run(x)),
      // run two functions and concat the result
      concat(otherFn) {
        return Fn(x => run(x).concat(otherFn.run(x)));
      }
    });
    Fn.of = x => Fn(() => x);
    Fn.ask = Fn(x => x);
    

      

    Then we can refactor the previous example:

    Fn.of("hi")
      .map(toUpper)
      .chain(upper => Fn.ask.map(x => [upper, exclaim(x)])) // ["HI", 'hello!']
      .chain(ary => Fn.ask.map(x => [exclaim(ary[0]), x])) // ["HI!", "hello"]
      .run("hello");
    

      

  • 相关阅读:
    论如何O(1)快速乘
    luogu3605晋升者计数
    分数规划小结
    洛谷 P1589 泥泞路 & 2019青岛市竞赛(贪心)
    洛谷 P3368 【模板】树状数组 2(区间加,单点查询)
    前缀和序列 & 差分序列
    洛谷 P3374 【模板】树状数组 1(单点加,区间和)
    2019青岛市程序设计竞赛游记
    0x3f3f3f3f 0xbfbfbfbf 等的原理及应用
    memset 初始化数组 & 实现原理
  • 原文地址:https://www.cnblogs.com/Answer1215/p/12484174.html
Copyright © 2011-2022 走看看