Chapter 04: Currying

Can't Live If Livin' Is without You

My Dad once explained how there are certain things one can live without until one acquires them. A microwave is one such thing. Smart phones, another. The older folks among us will remember a fulfilling life sans internet. For me, currying is on this list.

The concept is simple: You can call a function with fewer arguments than it expects. It returns a function that takes the remaining arguments.

You can choose to call it all at once or simply feed in each argument piecemeal.

const add = x => y => x + y;
const increment = add(1);
const addTen = add(10);

increment(2); // 3
addTen(2); // 12

Here we've made a function add that takes one argument and returns a function. By calling it, the returned function remembers the first argument from then on via the closure. Calling it with both arguments all at once is a bit of a pain, however, so we can use a special helper function called curry to make defining and calling functions like this easier.

Let's set up a few curried functions for our enjoyment. From now on, we'll summon our curry function defined in the Appendix A - Essential Function Support.

const match = curry((what, s) => s.match(what));
const replace = curry((what, replacement, s) => s.replace(what, replacement));
const filter = curry((f, xs) => xs.filter(f));
const map = curry((f, xs) =>;

The pattern I've followed is a simple, but important one. I've strategically positioned the data we're operating on (String, Array) as the last argument. It will become clear as to why upon use.

(The syntax /r/g is a regular expression that means match every letter 'r'. Read more about regular expressions if you like.)

match(/r/g, 'hello world'); // [ 'r' ]

const hasLetterR = match(/r/g); // x => x.match(/r/g)
hasLetterR('hello world'); // [ 'r' ]
hasLetterR('just j and s and t etc'); // null

filter(hasLetterR, ['rock and roll', 'smooth jazz']); // ['rock and roll']

const removeStringsWithoutRs = filter(hasLetterR); // xs => xs.filter(x => x.match(/r/g))
removeStringsWithoutRs(['rock and roll', 'smooth jazz', 'drum circle']); // ['rock and roll', 'drum circle']

const noVowels = replace(/[aeiou]/ig); // (r,x) => x.replace(/[aeiou]/ig, r)
const censored = noVowels('*'); // x => x.replace(/[aeiou]/ig, '*')
censored('Chocolate Rain'); // 'Ch*c*l*t* R**n'

What's demonstrated here is the ability to "pre-load" a function with an argument or two in order to receive a new function that remembers those arguments.

I encourage you to clone the Mostly Adequate repository (git clone, copy the code above and have a go at it in the REPL. The curry function (and actually anything defined in the appendixes) has been made available from the exercises/support.js module.

More Than a Pun / Special Sauce

Currying is useful for many things. We can make new functions just by giving our base functions some arguments as seen in hasLetterR, removeStringsWithoutRs, and censored.

We also have the ability to transform any function that works on single elements into a function that works on arrays simply by wrapping it with map:

const getChildren = x => x.childNodes;
const allTheChildren = map(getChildren);

Giving a function fewer arguments than it expects is typically called partial application. Partially applying a function can remove a lot of boiler plate code. Consider what the above allTheChildren function would be with the uncurried map from lodash (note the arguments are in a different order):

const allTheChildren = elements => map(elements, getChildren);

We typically don't define functions that work on arrays, because we can just call map(getChildren) inline. Same with sort, filter, and other higher order functions (a higher order function is a function that takes or returns a function).

When we spoke about pure functions, we said they take 1 input to 1 output. Currying does exactly this: each single argument returns a new function expecting the remaining arguments. That, old sport, is 1 input to 1 output.

No matter if the output is another function - it qualifies as pure. We do allow more than one argument at a time, but this is seen as merely removing the extra ()'s for convenience.

In Summary

Currying is handy and I very much enjoy working with curried functions on a daily basis. It is a tool for the belt that makes functional programming less verbose and tedious.

We can make new, useful functions on the fly simply by passing in a few arguments and as a bonus, we've retained the mathematical function definition despite multiple arguments.

Let's acquire another essential tool called compose.

Chapter 05: Coding by Composing


Note about Exercises

Throughout the book, you might encounter an 'Exercises' section like this one. Exercises can be done directly in-browser provided you're reading from gitbook (recommended).

Note that, for all exercises of the book, you always have a handful of helper functions available in the global scope. Hence, anything that is defined in Appendix A, Appendix B and Appendix C is available for you! And, as if it wasn't enough, some exercises will also define functions specific to the problem they present; as a matter of fact, consider them available as well.

Hint: you can submit your solution by doing Ctrl + Enter in the embedded editor!

Running Exercises on Your Machine (optional)

Should you prefer to do exercises directly in files using your own editor:

  • clone the repository (git clone
  • go in the exercises section (cd mostly-adequate-guide/exercises)
  • install the necessary plumbing using npm (npm install)
  • complete answers by modifying the files named exercises_* in the corresponding chapter's folder
  • run the correction with npm (e.g. npm run ch04)

Unit tests will run against your answers and provide hints in case of mistake. By the by, the answers to the exercises are available in files named answers_*.

Let's Practice!

Refactor to remove all arguments by partially applying the function.
// words :: String -> [String] const words = str => split(' ', str);

Refactor to remove all arguments by partially applying the functions.
// filterQs :: [String] -> [String] const filterQs = xs => filter(x => x.match(/q/i), xs);

Considering the following function:

const keepHighest = (x, y) => (x >= y ? x : y);

Refactor `max` to not reference any arguments using the helper function `keepHighest`.
// max :: [Number] -> Number const max = xs => reduce((acc, x) => (x >= acc ? x : acc), -Infinity, xs);

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