[Functional Programming] Arrow contramap vs map and promap

In previous post, Arrow Functor with contramap, we have seen how to opreating on params before we invoke the function by using Arrow + contramap. It happens before function get inovked, before we get result. We can say this opreation happens on the left hand side of function (which is the params).

In this post, we will see how to opreate on the right handside of function, which is our result.

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 welcome = str => `Welcome. ${str}!`;
const nameUpper = arrUpper
    .contramap(getName)
    .map(welcome)

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

So after 'contramap', we chian 'map(welcome)'. this will wrap the result from 'arrUpper' into a new String.

Notice that it is equivalent that:

const nameUpper = arrUpper
    .map(welcome)
    .contramap(getName)

const nameUpper = arrUpper
    .contramap(getName)
    .map(welcome)

We can also shorter the syntax by using 'promap', it is the same as 'contramap + map':

const nameUpper = arrUpper
    .promap(getName, welcome)