[Functional Programming] Arrow Functor with contramap

最新推荐文章于 2020-06-16 15:00:27 发布

转载最新推荐文章于 2020-06-16 15:00:27 发布 · 76 阅读

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原文链接：http://www.cnblogs.com/Answer1215/p/10569099.html

本文介绍了Arrow Functor，它能提升特定类型函数并实现懒执行，可将执行拆分为不同路径。还通过示例展示了如何结合使用Arrow和contramap，当输入数据类型不匹配时，用contramap处理输入参数，避免修改原函数。

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What is Arrow Functor?

Arrow is a Profunctor that lifts a function of type a -> b and allows for lazy execution of the function. Arrow can be considered a Strong Profunctorif the underlying data running through the Arrow is a Pair, typically in the form of Arrow (Pair a c) (Pair b d).

This will allow you to split execution into two distinct paths, applying Arrow to a specific path. The parameters of Arrow represent the function that it wraps, with the input being on the left, and the output on the right. When anArrow wraps an endomorphism, the signature typically represents both the input and output.

In short, Arrow is

Take a function
Function will be lazy
We can apply params later

const Arrow = require('crocks/Arrow');

const arrUpper = Arrow(
    str => str.toUpperCase()
)

log(
    arrUpper.runWith('zhentian')
) // ZHENTIAN

Why this can be useful?

In this post, we are going to see an exmple about how to use 'Arrow' with 'contramap':

Currently 'arrUpper' take a string as input, sometimes the data we receive might be is an Object:

// {name: 'zhentian'}

In this case, we cannot directly using 'arrUpper', and we don't want to modify our 'arrUpper' function to be:

const arrUpper = Arrow(o => o.name && o.name.toUpperCase())

What we can do is using 'contramap' to apply an function on the input param of 'arrUpper', to change the Object type to String type, we can keep the 'arrUpper' untouched:

const Arrow = require('crocks/Arrow');
const chain = require('crocks/pointfree/chain');
const option = require('crocks/pointfree/option');
const prop = require('crocks/Maybe/prop');
const safe = require('crocks/Maybe/safe');

const getName = compose(
    option('no name'),
    chain(safe(isString)),
    prop('name')
)
const arrUpper = Arrow(
    str => str.toUpperCase()
)
const nameUpper = arrUpper
    .contramap(getName)

log(
    nameUpper.runWith({name: 'zhentian'})
) // ZHENTIAN

What 'contramap' does is apply the given function (getName) to the target function (arrUpper)'s params ({name: 'zhentian'}), before the target function get invoked.

So in our example, we transform the {name: 'zhentian'} to just 'zhentian' or 'no name' before passing to arrUpper.

转载于:https://www.cnblogs.com/Answer1215/p/10569099.html