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This essay has been incorporated into Why Y? Deriving the Y Combinator in JavaScript
In To Grok a Mockingbird, we explored the mockingbird, a recursive combinator that decouples recursive functions from themselves. Decoupling recursive functions from themselves allows us to compose them more flexibly, such as with decorators.^{1}
This also separates the concern of the recursive algorithm to be performed from the mechanism of implementing recursion. This allowed us to implement the Jackson’s Widowbird, a variation of the mockingbird that uses trampolining to execute tailrecursive functions in constant space.
In this essay, we’re going to look at the Why Bird, known most famously as the Y Combinator. The why bird provides all the benefits of the mockingbird, but allows us to write more idiomatic JavaScript.
revisiting the mockingbird
To review what we saw in To Grok a Mockingbird, a typical recursive function calls itself by name, like this:^{2}
function exponent (x, n) {
if (n === 0) {
return 1;
} else if (n % 2 === 1) {
return x * exponent(x * x, Math.floor(n / 2));
} else {
return exponent(x * x, n / 2);
}
}
exponent(2, 7)
//=> 128
Because it calls itself by name, it is tightly coupled to itself. This means that if we want to decorate it–such as by memoizing its return values, or if we want to change its implementation strategy–like employing trampolining–we have to rewrite the function.
We saw that we can decouple a recursive function from itself. Instead of calling itself by name, we arrange to pass the recursive function to itself as a parameter. We begin by rewriting our function to take itself as a parameter, and also to pass itself as a parameter.
We call that writing a recursive function in mockingbird form. It looks like this:
(myself, x, n) => {
if (n === 0) {
return 1;
} else if (n % 2 === 1) {
return x * myself(myself, x * x, Math.floor(n / 2));
} else {
return myself(myself, x * x, n / 2);
}
};
Given a function written in mockingbird form, we use a JavaScript implementation of the mockingbird to turn it into a recursive function:
const mockingbird =
fn =>
(...args) =>
fn(fn, ...args);
const exponent =
mockingbird(
(myself, x, n) => {
if (n === 0) {
return 1;
} else if (n % 2 === 1) {
return x * myself(myself, x * x, Math.floor(n / 2));
} else {
return myself(myself, x * x, n / 2);
}
}
);
exponent(3, 3)
//=> 27
Because the recursive function has been decoupled from itself, we can do things like memoize it:
const memoized = (fn, keymaker = JSON.stringify) => {
const lookupTable = Object.create(null);
return function (...args) {
const key = keymaker.call(this, args);
return lookupTable[key]  (lookupTable[key] = fn.apply(this, args));
}
};
const ignoreFirst = ([_, ...values]) => JSON.stringify(values);
const exponent =
mockingbird(
memoized(
(myself, x, n) => {
if (n === 0) {
return 1;
} else if (n % 2 === 1) {
return x * myself(myself, x * x, Math.floor(n / 2));
} else {
return myself(myself, x * x, n / 2);
}
},
ignoreFirst
)
);
Memoizing our recursive function does not require any changes to its code. We can easily reuse it elsewhere if we wish.
why the mockingbird needs improvement
The mockingbird is a useful tool, but it has a drawback: In addition to rewriting our functions to take themselves as a parameter, we also have to rewrite them to pass themselves along. So in addition to this:
(myself, x, n) => ...
We must also write this:
myself(myself, x * x, n / 2)
The former is the point of decoupling. The latter is nonsense!
The idea behind the way we use the mockingbird (as opposed to a literal interpretation of the M combinator) is to write idiomatic JavaScript. But there’s nothing “idiomatic” about a function invoking itself with myself(myself, ...)
.
What we want is a function like the mockingbird, but it must support functions calling themselves idiomatically, e.g. myself(x * x, n / 2)
.
Let’s visualize exactly what we want. With the mockingbird, we write:
const exponent =
mockingbird(
(myself, x, n) => {
if (n === 0) {
return 1;
} else if (n % 2 === 1) {
return x * myself(myself, x * x, Math.floor(n / 2));
} else {
return myself(myself, x * x, n / 2);
}
}
);
We want a better recursive combinator, one that lets us write:
const exponent =
_____(
(myself, x, n) => {
if (n === 0) {
return 1;
} else if (n % 2 === 1) {
return x * myself(x * x, Math.floor(n / 2));
} else {
return myself(x * x, n / 2);
}
}
);
Let’s build that!
a few ground rules
Before we begin, there are some rules we have to follow if we are to take the mockingbird and derive another combinator from it. Every combinator has the following properties:
 It is a function.
 It can only use its parameters or previously defined combinators that have these same properties. This means it cannot refer to noncombinators, like constants, ordinary functions, or object from the global namespace.
 It cannot be a named function declaration or named function expression.
 It cannot create a named function expression.
 It cannot declare any bindings other than via parameters.
 It can invoke a function in its implementation.
In combinatory logic, all combinators take exactly one parameter, as do all of the functions that combinators create. Combinatory logic also eschews all gathering and spreading of parameters. When creating an idiomatic JavaScript combinator (like mockingbird
), we eschew these limitations. Idiomatic JavaScript combinators can:
 Gather parameters, and;
 Spread parameters.
deriving the why bird in seven easy pieces
Step Zero: We begin with the mockingbird:
const mockingbird =
fn =>
(...args) =>
fn(fn, ...args);
Step One, we name our combinator. Honouring Raymond Smullyan’s choice, we shall call it the why bird:
const why =
fn =>
(...args) =>
fn(fn, ...args);
Step Two, we identify the key change we have to make:
const why =
fn =>
(...args) =>
fn(?, ...args);
We’ve replaced that one fn
with a placeholder. Why? Well, our fn
is a function that looks like this: (myself, arg0, arg1, ..., argn) => ...
. But whatever we pass in for myself
will look like this: (arg0, arg1, ..., argn) => ...
. So it can’t be fn
.
But what will it be?
Well, the approach we are going to take is to think about the mockingbird. What does it do? It takes a function like (myself, arg0, arg1, ..., argn) => ...
, and returns a function that looks like (arg0, arg1, ..., argn) => ...
.
The mockingbird isn’t what we want, but let’s airily assume that there is such a function. We’ll call it maker
, because it makes the function we want.
Step Three, we replace ?
with maker(??)
. We know it will make the function we want, but we don’t yet know what we must pass to it:
const why =
fn =>
(...args) =>
fn(maker(??), ...args);
This leaves us two things to figure out:
 Where do we get
maker
, and;  What parameter(s) do we pass to it.
For 1
, we could define maker
as an anonymous function expression. Another option arises. In the days before ES6, if we wanted to define variables within a scope smaller than a function, we created an immediately invoked function expression.
Step Four, we could, after some experimentation, consider this format that binds a function expression to maker
with an immediately invoked function expression;
const why =
fn =>
(
maker =>
(...args) =>
fn(maker(??), ...args)
)(???);
This still leaves us two things to work out: ??
is what we pass to maker
, and ???
is maker’s expression. Here’s the “Eureka!” moment:
maker
is a function that takes one or more parameters and returns a function that looks like (...args) => fn(maker(??), ...args)
. That’s the function we want to pass to fn
as myself
. The mockingbird isn’t such a function, but we can see one right in front of us:
maker => (...args) => fn(maker(??), ...args)
is a function that takes one parameter(maker
) and returns a function that looks like (...args) => fn(maker(??), ...args)
!
Step Five, let’s fill that in for ???
:
const why =
fn =>
(
maker =>
(...args) =>
fn(maker(??), ...args)
)(
maker =>
(...args) =>
fn(maker(??), ...args)
);
Now what about ??
? Well, we have just decided two things:
maker
takes one or more parameters and returns a function that looks like(...args) => fn(maker(??), ...args)
, and;maker => (...args) => fn(maker(??), ...args)
is a function that takes one parameter(maker
) and returns a function that looks like(...args) => fn(maker(??), ...args)
.
Conclusion: maker
takes one parameter, maker
, and returns a function that looks like (...args) => fn(maker(??), ...args)
. Therefore, the expression we want is maker(maker)
, and ??
is nothing more than maker
!
Step Six:
const why =
fn =>
(
maker =>
(...args) =>
fn(maker(maker), ...args)
)(
maker =>
(...args) =>
fn(maker(maker), ...args)
);
Let’s test it:
const exponent =
why(
(myself, x, n) => {
if (n === 0) {
return 1;
} else if (n % 2 === 1) {
return x * myself(x * x, Math.floor(n / 2));
} else {
return myself(x * x, n / 2);
}
}
);
exponent(2, 9)
//=> 512
Voila! A working why bird!!
from why bird to y combinator
Our why bird is written in–and for–idiomatic JavaScript, especially with respect to employing functions that take more than one parameter. A direct implementation of a formal combinator only takes one parameter and only works with functions that take one parameter.
We can translate our why bird to its formal combinator, the y combinator. To aid us, let’s first imagine a recursive function:
const isEven =
n =>
(n === 0)  !isEven(n  1);
In why bird form, it becomes:
const _isEven =
(myself, n) =>
(n === 0)  !myself(n  1);
Alas, it now takes two parameters. We fix this by currying it:
const __isEven =
myself =>
n =>
(n === 0)  !myself(n  1);
Instead of taking two parameters (myself
and n
), it is now a function taking one parameter, myself
, and returning a function that takes another parameter, n
.
To accommodate functions in this form, we take our why bird and perfom some similar modifications. We’ll start as above by renaming it:
const Y =
fn =>
(
maker =>
(...args) =>
fn(maker(maker), ...args)
)(
maker =>
(...args) =>
fn(maker(maker), ...args)
);
Next, we observe that (...args) => fn(maker(maker), ...args)
is not allowed, we do not gather and spread parameters. First, we change ...args
into just arg
, since only one parameter is allowed:
const Y =
fn =>
(
maker =>
arg => fn(maker(maker), arg)
)(
maker =>
arg => fn(maker(maker), arg)
);
fn(maker(maker), arg)
is also not allowed, we do not pass two parameters to any function. Instead, we pass one parameter, get a function back, and pass the second parameter to that function. Like this:
const Y =
fn =>
(
maker =>
arg => fn(maker(maker))(arg)
)(
maker =>
arg => fn(maker(maker))(arg)
);
Let’s try it:
Y(
myself =>
n =>
(n === 0)  !myself(n  1)
)(1962)
It works too, and now we have derived one of the most important results in theoretical computer science. The Y Combinator matters deeply, because in the kind of formal computation models that are simple enough to prove results (like the Lambda Calculus and Combinatory Logic), we do not have any iterative constructs, and must use recursion for nearly everything nontrivial.
The Y Combinator makes recursion possible without requiring variable declarations. As we showed above, we can even make an anonymous function recursive, which is necessary in systems where functions do not have names.^{3}
if a forest contains a mockingbird, it also contains a why bird
Looking this expression of the Y combinator, we can see why it was named after the letter “Y,” the code literally looks like a forking branch:
const Y =
fn =>
(m => a => fn(m(m))(a))(
m => a => fn(m(m))(a)
);
Since we’re talking direct implementations of formal combinators, let’s have a look at the M combinator:
const M =
fn => fn(fn);
We can combine M
and Y
to create a more compact expression of the Y combinator:
const Y =
fn =>
M(m => a => fn(m(m))(a));
The compact expression of the Y combinator is usually expressed with the M combinator “reduced” to (x => x(x))
:
const Y =
fn =>
(x => x(x))(m => a => fn(m(m))(a));
We can use the reduced M combinator make a compact why bird, too:
const why =
fn =>
(x => x(x))(
maker =>
(...args) =>
fn(maker(maker), ...args)
);
And with that, we have derived compact implementations of both the Y combinator and its idiomatic JavaScript equivalent, the why bird, from our mockingbird implementation.
summary
The mockingbird that we explored in To Grok a Mockingbird is easy to understand, and decouples recursive functions from themselves. This provides us with more ways to compose recursive functions with other functions like decorators.
However, it requires us to pass myself
along when making recursive calls. This is decidedly not idiomatic, so we derived the why bird, an idiomatic JavaScript recursive combinator that enables recursive functions to call themselves without any additional parameters.
We then derived a JavaScript implementation of the Y combinator from the why bird, and finished by using a reduced version of the M combinator to produce “compact” implementations of both the why bird and the Y combinator.
the end
The essays in this series on recursive combinators are: To Grok a Mockingbord, Deriving the Y Combinator and Why Bird from the Mockingbird, and A practical (albeit infrequently needed) use for the Y Combinator. Enjoy them all!
Notes

The mockingbird is more formally known as the M Combinator. Our naming convention is that when discussing formal combinators from combinatory logic, or direct implementations in JavaScript, we will use the formal name. But when using variations designed to work more idiomatically in JavaScript–such as versions that work with functions taking more than one argument), we will use Raymond Smullyan’s ornithological nicknames.
For a formalist, the M Combinator’s direct translation isconst M = fn => fn(fn)
. This is only useful iffn
is implemented in “curried” form, e.g.const isEven = myself => n => n === 0  !myself(n  1)
. If we wish to use a function written in idiomatic JavaScript form, such asconst isEven = (myself, n) => n === 0  !myself(n  1)
, we use the mockingbird, which is given later asconst mockingbird = fn => (...args) => fn(fn, ...args)
. This is far more practical for programming purposes. ↩ 
The paradox of instructional explorations is that if we wish to illustrate a mechanism like recursive combinators, choosing trivial functions like exponentiation makes it easier to focus on the thing we’re exploring, the combinators. The tradeoff is that with such simple functions, it will always feel overcomplicated to use recursive combinators. Whereas, if we work with functions with realworld implications, the mechanism we’re exploring gets lost in the complexity of the functions it operates upon. ↩

As alluded to, there is an enormous significance to the Y combinator beyond writing recursive JavaScript functions that are decoupled from themselves. Deriving the Y combinator is interesting in its own right, and highlighting the relationship between the M combinator and the Y combinator is something that is rarely mentioned in casual blogs.
If the subject piques your interest, be sure to look into pointfree programming, fixed point functions, recursion theory, … and most especially, read Raymond Smullyan;s To Mock a Mockingbird. ↩