Functional Programming: Alternatives to the IF #Functional #JavaScript #Monads

in #javascript3 years ago (edited)

couple of times I have been asked "How would you do X in functional programming?" I absolutely love these types of questions.

I do my best to answer every question, but I think there are many questions interesting enough to warrant their own articles.

So for this article I would like to demonstrate how I could recreate this imperative function in a more functional manner.

This function has an if statement without an else. So while a ternary operator could work, it is not ideal.

// A basic redux-thunk action that only dispatches when value exists
const someAction = value => dispatch => {
  const item = getItem(value)
  if (item != null) {
    dispatch({ type: 'ACTION', item })
  }
}

Here we need to run dispatch only when we have a value, otherwise we do nothing.

One option is to use a short circuit operator:

// short circuit
const someAction = value => dispatch => {
  const item = getItem(value)
  item && dispatch({ type: 'ACTION', item })
}

// ternary
const someAction = value => dispatch => {
  const item = getItem(value)
  item ? dispatch({ type: 'ACTION', item }) : null
}

Short circuit and ternary and both ways you can solve this problem, but let's take a look at a couple more.

Solution A: ifVal Helper Function

To do this in a more functional way, I will create a helper function. I'll write this helper old school first then boil it down piece by piece so everyone can understand it easily.

// 1: old school
function ifVal (x, f) {
  if (x == null) {
    return null
  } else {
    return f(x)
  }
}

// 2: convert to arrow function
const ifVal = (x, f) => {
  if (x == null) {
    return null
  } else {
    return f(x)
  }
}

// 3: convert if/else to a ternary operator
const ifVal = (x, f) => {
  return x == null ? null : f(x)
}

// 4: voilà!
const ifVal = (x, f) => x == null ? null : f(x)

Now we can modify our someAction function to use ifVal instead of the classic if block.

// functional alternative
const someAction = value => dispatch =>
  ifVal(getItem(value), item => dispatch({ type: 'ACTION', item }))

Here's the comparison for a quick and easy twitter screenshot:

/**
 * execute the function if the value is not null or undefined
 * @param {Object} val - the value to test
 * @param {Function} fn - the function to execute.
 * @returns {Object} - null or the value of the executed function.
 */
const ifVal = (val, fn) => val == null ? null : fn(val)

// imperative example
const someAction = value => dispatch => {
  const item = getItem(value)
  if (item!= null) {
    dispatch({ type: 'ACTION', item })
  }
}

// functional example
const someAction = value => dispatch =>
  ifVal(getItem(value), item => dispatch({ type: 'ACTION', item }))

Further reading

Take a look at Ramda's when, unless, and ifelse for other useful and similar functions.

Solution B: Functors

We could also use the Maybe Type. The Maybe Type consists of either a Just type which holds a value or a Nothing type which is exactly what is says.

For this example, I'll use the Maybe Type from the Sanctuary library.

It looks a little like this:

/* Examples of Sanctuary's Maybe */

toMaybe(null) //=> Nothing
toMaybe(undefined) //=> Nothing
toMaybe(0) //=> Just(0)
toMaybe(false) //=> Just(false)
toMaybe(123) //=> Just(123)
toMaybe({ name: 'joel' }) //=> Just({ name: 'joel' })

The Maybe Type is pretty simple and the reason why I want to use it is because map and Maybe work together so well, it was been given a special name, a Functor.

Some libraries like ramda-fantasy have a fluent syntax, but this article is using Sanctuary. Though, it is good to know that these two are equivalent.

const double = x => x * 2

// ramda-fantasy
Maybe.toMaybe(333).map(double) //=> Just(666)

// Sanctuary
map(double, toMaybe(333)) //=> Just(666)

Examples:

const double = x => x * 2

// map is ignored
map(double, toMaybe(null)) //=> Nothing

// no more null exceptions!
map(val => val.something(), toMaybe(null)) //=> Nothing

// map is mapped
map(double, toMaybe(333)) //=> Just(666)

All together doSomething with the Maybe will look like this:

import { toMaybe, map } from 'sanctuary'

const someAction = value => dispatch => {
  const item = getItem(value)
  const maybeValue = toMaybe(item)
  map(item => dispatch({ type: 'ACTION', item }))(maybeValue)
}

Here we have three functions, getItem, toMaybe and map that can be composed together into a new single function. We can pass value into that new function and it will flow through first into getItem, then toMaybe and finally that value will flow into our map.

const someAction = value => dispatch => pipe([
  getItem,
  toMaybe,
  map(value => dispatch({ type: 'ACTION', value })),
], value)

And for one last picture perfect Tweet:

import { toMaybe, map, pipe } from 'sanctuary'

// imperative example
const someAction = value => dispatch => {
  const item = getItem(value)
  if (item != null) {
    dispatch({ type: 'ACTION', item })
  }
}

// maybe functor example
const someAction = value => dispatch => pipe([
  getItem,
  toMaybe,
  map(value => dispatch({ type: 'ACTION', value })),
], value)

someAction(666)(dispatch) // Action is dispatched!
someAction(null)(dispatch) // Nothing happens

End

I hope you enjoyed this small functional journey. Let me know in the comments if you did, if you got lost, if you have any questions, or if you want more articles like this one. If you really want to motivate me, follow me here or Twitter and if you loved this article, pass it around!

Cheers!

This article was originally posted on dev.to.

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