zoukankan      html  css  js  c++  java
  • [Functional Programming] Read and Transform Values from a State ADT’s State (get)

    Many times we need to access and transform state, either in part or in full, to be used when calculating other state transitions. We will look at how we can leverage the get function on the State ADT to read and modify portions of our application state, putting the result in the Resultant portion. We will create two transactions: one to select a card from a list of cards and another to access the hint portion of our state.

    The code below is just trying to get 'cards' and 'hint' props from an object with two fns.

    const {prop, State, option,chain, find, propEq, isNumber, compose, safe} = require('crocks');
    const  {get, modify} = State; 
    
    const state = {
        cards: [
            {id: 'green-square', color: 'green', shape: 'square'},
            {id: 'orange-square', color: 'orange', shape: 'square'},
            {id: 'blue-square', color: 'blue', shape: 'triangle'}
        ],
        hint: {
            color: 'green',
            shape: 'square'
        }
    }
    
    // getState :: String ->  State Object (Maybe a)
    const getState = key => get(prop(key))
    
    const findById = id => find(propEq('id', id));
    
    // getCard :: String -> State Object Card
    const getCard = id => get(prop('cards'))
        .map(chain(findById(id)))
        .map(option({id: 'urk', color: '', shape: ''}))
    
    const getCard2 = (id) => compose(option({}), chain(findById(id)), prop('cards'))    
    
    const getHint = () => getState('hint').map(option({
        color: 'yay',
        shape: 'wow'
    }));
    
    const res = getCard('green-square') 
        .evalWith(state)  // { id: 'green-square', color: 'green', shape: 'square' }
    
    const res1 = get(getCard2('green-square'))  //{ id: 'green-square', color: 'green', shape: 'square' } 
        .evalWith(state)
    
    const res2 = getHint()
        .evalWith(state); // { color: 'green', shape: 'square' }
    console.log(res)
    console.log(res1)
    console.log(res2)

    The important thing to understand is the difference between  'getCard' & 'getCard2' functions. They have the same functionalities.

    const getCard = id => get(prop('cards'))
        .map(chain(findById(id)))
        .map(option({id: 'urk', color: '', shape: ''}))

    Once we use 'get', it creates a State instance, therefore we have to use instance method 'map' to access its value.

    const getCard2 = (id) => compose(option({}), chain(findById(id)), prop('cards'))    

    By the time we define 'getCard2', we haven't lift it into State with 'get' function. We can do normal fp. The reason using 'chian' in both functions is because prop('cards') return a Maybe, findById return a Maybe, would be a nested Maybe type, so using 'chain' to flatten it.

  • 相关阅读:
    Beyond Compare 4 使用30天后过期续用方法
    cookie和Session是啥?
    如何添加博客园访问次数统计
    获取DataFrame列名的3种方法
    GET与POST方法和用curl命令执行
    Series拼接回DataFrame
    pandas.to_json&to_dict&from_json&from_dict解读
    Mysql判断是否某个字符串在某字符串字段的4种方法
    alias 命令
    python中的pyc和pyo文件和__pyc__文件夹
  • 原文地址:https://www.cnblogs.com/Answer1215/p/10246974.html
Copyright © 2011-2022 走看看