[Functional Programming Monad] Combine Stateful Computations Using A State Monad

本文探讨了State ADT在组合离散、有状态交易时展现出的真正威力。通过使用'of'构造助手将任意类型的值提升到结果中,以及使用'chain'实例方法将简单交易组合成更复杂的流程,我们展示了如何利用'get'和'modify'构造助手创建易于阅读且功能强大的有状态交易集合。

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The true power of the State ADT really shows when we start combining our discrete, stateful transactions. We start looking at another construction helper named of, used to lift any given value of any type into the resultant. We also explore another instance method called chain that is used for combining our simple, discrete transactions into more complex flows that can be extended to meet our needs as they arise. To create a set of stateful transactions to be combined, we see how the get and modify construction helpers can be used to make simple, easy to read code.

 

const { constant, Pair, Unit, curry, objOf, compose, State, mapProps, prop, option } = require("crocks");

const { put, get, modify } = State;


const add = x => y => x+y;
const inc = add(1);

// State s a -> State(x => Pair(a, x))

// 'get' return result apply to variable a
const addState = n => 
    get(add(n))

const incState = n => 
    modify(inc) // modify return Unit() in variable position, Pair( (), 3 )
    .map(constant(n)) // to keep the vairable a, we can use constant to wrap a value into function, Pair( 12, 3 )

const compute = n => 
    State.of(n)
        .chain(addState)
        .chain(incState)


 console.log(
     compute(10)
        .runWith(2)
 )       

 

转载于:https://www.cnblogs.com/Answer1215/p/10338745.html

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