The above examples demonstrate how the ability to pass functions as arguments significantly enhances the expressive power of our programming language. We can achieve even more expressive power by creating functions whose returned values are themselves functions.

We can illustrate this idea by looking again at the fixed-point example described at the end of section 1.3.3. We formulated a new version of the square-root function as a fixed-point search, starting with the observation that $\sqrt{x}$ is a fixed-point of the function $y\mapsto x/y$. Then we used average damping to make the approximations converge. Average damping is a useful general technique in itself. Namely, given a function $f$, we consider the function whose value at $x$ is equal to the average of $x$ and $f(x)$.

We can express the idea of average damping by means of the following function:

function average_damp(f) { return x => average(x, f(x)); }

The function
`average_damp`
is a
function
that takes as its argument a
function
`f` and returns as its value a
function
(produced by the function definition expression)
that, when applied to a number `x`, produces the
average of `x` and
`f(x)`.
For example, applying
`average_damp`
to the `square`
function
produces a
function
whose value at some number $x$ is the average of
$x$ and $x^2$.
Applying this resulting
function
to 10 returns the average of 10 and 100, or 55:[1]

average_damp(square)(10);

Using
`average_damp`,
we can reformulate the square-root
function
as follows:

function sqrt(x) { return fixed_point(average_damp(y => x / y), 1.0); }

Notice how this formulation makes explicit the three ideas in the method: fixed-point search, average damping, and the function $y\mapsto x/y$. It is instructive to compare this formulation of the square-root method with the original version given in section 1.1.7. Bear in mind that these functions express the same process, and notice how much clearer the idea becomes when we express the process in terms of these abstractions. In general, there are many ways to formulate a process as a function. Experienced programmers know how to choose process formulations that are particularly perspicuous, and where useful elements of the process are exposed as separate entities that can be reused in other applications. As a simple example of reuse, notice that the cube root of $x$ is a fixed point of the function $y\mapsto x/y^2$, so we can immediately generalize our square-root function to one that extracts cube roots:[2]

function cube_root(x) { return fixed_point(average_damp(y => x / square(y)), 1.0); }

When we first introduced the square-root
function,
in section 1.1.7, we mentioned that this was a
special case of *Newton's method*. If
$x\mapsto g(x)$ is a differentiable function,
then a solution of the equation $g(x)=0$ is a
fixed point of the function $x\mapsto f(x)$ where
\[
f(x) = x - \frac{g(x)}{Dg(x)}
\]
and $Dg(x)$ is the derivative of
$g$ evaluated at $x$.
Newton's method is the use of the fixed-point method we saw above to
approximate a solution of the equation by finding a fixed point of the
function $f$.[3]
For many functions $g$ and for sufficiently good
initial guesses for $x$, Newton's method
converges very rapidly to a solution of
$g(x)=0$.[4]

In order to implement Newton's method as a
function,
we must first express the idea of derivative. Note that
derivative,

like average damping, is something that
transforms a function into another function. For instance, the derivative
of the function $x\mapsto x^3$ is the function
$x \mapsto 3x^2$. In general, if
$g$ is a function and
$dx$ is a small number, then the derivative
$Dg$ of $g$ is the
function whose value at any number $x$ is given
(in the limit of small $dx$) by
\[Dg(x) = \frac {g(x+dx) - g(x)}{dx} \]
Thus, we can express the idea of derivative (taking
$dx$ to be, say, 0.00001) as the
function

function deriv(g) { return x => (g(x + dx) - g(x)) / dx; }

const dx = 0.00001;

Like
`average_damp`,
`deriv` is a
function
that takes a
function
as argument and returns a
function
as value. For example, to approximate the derivative of
$x \mapsto x^3$ at 5 (whose exact value is 75)
we can evaluate

function cube(x) { return x * x * x; } deriv(cube)(5);

With the aid of `deriv`, we can express
Newton's method as a fixed-point process:

function newton_transform(g) { return x => x - g(x) / deriv(g)(x); } function newtons_method(g, guess) { return fixed_point(newton_transform(g), guess); }

The
`newton_transform`
function
expresses the formula at the beginning of this section, and
`newtons_method`
is readily defined in terms of this. It takes as arguments a
function
that computes the function for which we want to find a zero, together with
an initial guess. For instance, to find the
square root of $x$, we can use Newton's
method to find a zero of the function
$y\mapsto y^2-x$ starting with an initial guess
of 1.[5]
This provides yet another form of the square-root
function:

function sqrt(x) { return newtons_method(y => square(y) - x, 1.0); }

We've seen two ways to express the square-root computation as an instance of a more general method, once as a fixed-point search and once using Newton's method. Since Newton's method was itself expressed as a fixed-point process, we actually saw two ways to compute square roots as fixed points. Each method begins with a function and finds a fixed point of some transformation of the function. We can express this general idea itself as a function:

function fixed_point_of_transform(g, transform, guess) { return fixed_point(transform(g), guess); }

This very general
function
takes as its arguments a
function
`g`
that computes some function, a
function
that transforms `g`, and an initial guess.
The returned result is a fixed point of the transformed function.

Using this abstraction, we can recast the first square-root computation from this section (where we look for a fixed point of the average-damped version of $y \mapsto x/y$) as an instance of this general method:

function sqrt(x) { return fixed_point_of_transform( y => x / y, average_damp, 1.0); }

Similarly, we can express the second square-root computation from this section (an instance of Newton's method that finds a fixed point of the Newton transform of $y\mapsto y^2-x$) as

function sqrt(x) { return fixed_point_of_transform( y => square(y) - x, newton_transform, 1.0); }

We began section 1.3 with the observation that compound functions are a crucial abstraction mechanism, because they permit us to express general methods of computing as explicit elements in our programming language. Now we've seen how higher-order functions permit us to manipulate these general methods to create further abstractions.

As programmers, we should be alert to opportunities to identify the underlying abstractions in our programs and to build upon them and generalize them to create more powerful abstractions. This is not to say that one should always write programs in the most abstract way possible; expert programmers know how to choose the level of abstraction appropriate to their task. But it is important to be able to think in terms of these abstractions, so that we can be ready to apply them in new contexts. The significance of higher-order functions is that they enable us to represent these abstractions explicitly as elements in our programming language, so that they can be handled just like other computational elements.

In general, programming languages impose restrictions on the ways in which
computational elements can be manipulated. Elements with the fewest
restrictions are said to have
*first-class* status. Some of the rights and
privileges

of first-class elements are:[6]

- They may be referred to using names.
- They may be passed as arguments to functions.
- They may be returned as the results of functions.
- They may be included in data structures.[7]

JavaScript, unlike other common programming languages, awards functions full first-class status. This poses challenges for efficient implementation, but the resulting gain in expressive power is enormous.[8]

newtons_method(cubic(a, b, c), 1);

function cubic(a, b, c) { return x => cube(x) + a * square(x) + b * x + c; }

double(double(double))(inc)(5); // ?

function double(f) { return x => f(f(x)); }

compose(square, inc)(6);

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

repeated(square, 2)(5);

function repeated(f, n) { return n === 0 ? x => x : compose(f, repeated(f, n - 1)); }

const dx = 0.00001; function smooth(f) { return x => (f(x - dx) + f(x) + f(x + dx)) / 3; } function n_fold_smooth(f, n) { return repeated(smooth, n)(f); }

function nth_root(n, x) { return fixed_point(repeated(average_damp, math_floor(math_log2(n))) (y => x / fast_expt(y, n - 1)), 1.0); }

function iterative_improve(good_enough, improve) { function iterate(guess) { return good_enough(guess) ? guess : iterate(improve(guess)); } return iterate; }

[1]
Observe that this
is a combination whose operator is itself
a combination. Exercise 1.4 already
demonstrated the ability to form such combinations, but that was only a toy
example. Here we begin to see the real need for such
combinations—when applying a
function
that is obtained as the value returned by a higher-order
function.

[3]
Elementary calculus books
usually describe Newton's method in terms of the sequence of
approximations $x_{n+1}=x_n-g(x_n)/Dg(x_n)$.
Having language for talking about processes and using the idea of fixed
points simplifies the description of the method.

[4]
Newton's method does not
always converge to an answer, but it can be shown that in favorable cases
each iteration doubles the number-of-digits accuracy of the approximation
to the solution. In such cases,
Newton's method will converge much more rapidly than the half-interval
method.

[5]
For finding square roots, Newton's method converges
rapidly to the correct solution from any starting point.

[6]
The notion of
first-class status of programming-language
elements is due to the British computer scientist Christopher
Strachey (1916–1975).

[7]
We'll see
examples of this after we introduce data structures in
chapter 2.

[8]
The major implementation cost of first-class
functions
is that allowing
functions
to be returned as values requires reserving storage for a
function's free names
even while the
function
is not executing.
In the JavaScript implementation we will study in
section 4.1, these names are stored in the
function's
environment.

1.3.4 Functions as Returned Values