[Functional Programming] Arrow Functor with contramap

Posted 2021-02-14 answer1215

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.