Currying~ Popular concepts ~

Another popular functional programming paradigm is currying. It is kind of similar to the partial application in the sense that it allows us to split the function call. However, this time we apply one argument at a time. While with a partial application, we may do multiple arguments for each call. Look at the update function below. It has three arguments:

const user = { age: 33, job: "developer" };

function update(user, prop, value) {
  user[prop] = value;
}
update(user, "age", 36);

Ideally, we don't want to pass the user object repeatedly. A good step forward is to create a function that has it already applied and only accepts prop and value. We can do that by using a partial application.

const user = { age: 33, job: "developer" };
function update(user, prop, value) {
  user[prop] = value;
}
const updateUser = partial(update, user);
updateUser("age", 36);

function partial(func, ...args1) {
  return (...args2) => {
    return func(...args1, ...args2);
  }
}

Now, let's transform this example to currying:

const user = { age: 33, job: "developer" };
function update(user, prop, value) {
  user[prop] = value;
}
const curriedUpdate = curry(update);
const updateUser = curriedUpdate(user);
const updateAge = updateUser('age');
updateAge(36);

function curry(f) {
  return function curried(...params) {
    if (params.length >= f.length) {
      return f.apply(this, params);
    } else {
      return function(...params2) {
        return curried.apply(this, params.concat(params2));
      }
    }
  };
}

Instead of partial we now have curry. This is a helper that returns a curried version of our functions. Our original function with N arguments is transformed into N functions with one argument.