Chapter 04: Currying

    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.

    Here we’ve made a function 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. 

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

    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 if you like.)

    1. match(/r/g, 'hello world'); // [ 'r' ]
    3. const hasLetterR = match(/r/g); // x => x.match(/r/g)
    4. hasLetterR('hello world'); // [ 'r' ]
    5. hasLetterR('just j and s and t etc'); // null
    7. filter(hasLetterR, ['rock and roll', 'smooth jazz']); // ['rock and roll']
    9. const removeStringsWithoutRs = filter(hasLetterR); // xs => xs.filter(x => x.match(/r/g))
    10. removeStringsWithoutRs(['rock and roll', 'smooth jazz', 'drum circle']); // ['rock and roll', 'drum circle']
    11. const noVowels = replace(/[aeiou]/ig); // (r,x) => x.replace(/[aeiou]/ig, r)
    12. const censored = noVowels('*'); // x => x.replace(/[aeiou]/ig, '*')
    13. 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 https://github.com/MostlyAdequate/mostly-adequate-guide.git), 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.

    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:

    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.

    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 B and 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.

    Running Exercises on Your Machine (optional)

    • clone the repository (git clone /MostlyAdequate/mostly-adequate-guide.git)
    • 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!

    {% exercise %}
    Refactor to remove all arguments by partially applying the function.

    {% initial src=”./exercises/ch04/exercise_a.js#L3;” %} 

    1. const words = str => split(' ', str);

    {% solution src=”./exercises/ch04/solution_a.js” %}
    {% validation src=”./exercises/ch04/validation_a.js” %}
    {% context src=”./exercises/support.js” %}
    {% endexercise %}

    {% exercise %}
    Refactor to remove all arguments by partially applying the functions.

    {% initial src=”./exercises/ch04/exercise_b.js#L3;” %}

    {% solution src=”./exercises/ch04/solution_b.js” %}
    {% validation src=”./exercises/ch04/validation_b.js” %}
    {% context src=”./exercises/support.js” %}
    {% endexercise %}

    Considering the following function:

      {% exercise %}
      Refactor max to not reference any arguments using the helper function keepHighest.

      {% initial src=”./exercises/ch04/exercise_c.js#L7;” %}