Challenge - Functional chain

Build a text-processing chain with pure functions, composition and currying.

Functional chain

In this challenge you will build a chain of pure functions. Pure functions always return the same output for the same input and have no side effects.

To combine two functions you write compose(f, g); the returned function computes f(g(x)). This is called function composition.

To make a multi-argument function callable step by step you use currying: curryAdd(a) returns a function that takes b and returns a + b.

Exercise Write compose and curryAdd

Write compose(f, g) that returns a function taking x and returning f(g(x)). Then write curryAdd(a) that takes a and returns a function taking b and returning a + b.

Show example solution

Start with function compose(f, g) { return function(x) { return f(g(x)); }; }. Then write function curryAdd(a) { return function(b) { return a + b; }; }.

Example solution:
function compose(f, g) {
  return function(x) {
    return f(g(x));
  };
}

function curryAdd(a) {
  return function(b) {
    return a + b;
  };
}

Test yourself

What does compose(f, g)(x) return?
```
function compose(f, g) {
  return function(x) {
    return f(g(x));
  };
}
```
- g(f(x))
- f(x) then g(x)
- f(g(x))
- x(f, g)
Show answer

f(g(x)) — Correct! compose(f, g)(x) applies g first, then passes the result to f.

Complete the compose function

function compose(f, g) {
  return function(x) {
    ____
  };
}

Show answer

function compose(f, g) {
  return function(x) {
    return f(g(x));
  };
}

Match the functional concepts

compose(f, g)

computes f(g(x))
curry(f)

step-by-step args
pure function

no side effects
higher-order

returns a function

Next lesson: Challenge - Custom iterable

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