11_language.txt

:chap_num: 11
:prev_link: 10_modules
:next_link: 12_browser
:load_files: ["code/chapter/11_language.js"]
:zip: node/html

= Project: A Programming Language =

[chapterquote="true"]
[quote, Hal Abelson and Gerald Sussman, Structure and Interpretation of Computer Programs]
____
The evaluator, which determines the meaning of expressions in a
programming language, is just another program.
____

ifdef::interactive_target[]

[chapterquote="true"]
[quote, Master Yuan-Ma, The Book of Programming]
____
When a student asked the master about the nature of the cycle of Data
and Control, Yuan-Ma replied ‘Think of a compiler, compiling itself.’
____

endif::interactive_target[]

(((Abelson+++,+++ Hal)))(((Sussman+++,+++
Gerald)))(((SICP)))(((project chapter)))Building your own
((programming language)) is surprisingly easy (as long as you do not
aim too high) and very enlightening.

The main thing I want to show in this chapter is that there is no
((magic)) involved in building your own language. I've often felt that
some human inventions were so immensely clever and complicated that
I'd never be able to understand them. But with a little reading and
tinkering, such things often turn out to be quite mundane.

(((Egg language)))We will build a programming language called Egg. It
will be a tiny, simple language but one that is powerful enough to
express any computation you can think of. It will also allow simple
((abstraction)) based on ((function))s.

[[parsing]]
== Parsing ==

(((parsing)))(((validation)))The most immediately visible part of a
programming language is its _((syntax))_, or notation. A _parser_ is a
program that reads a piece of text and produces a data structure that
reflects the structure of the program contained in that text. If the
text does not form a valid program, the parser should complain and
point out the error.

(((special form)))Our language will have a simple and uniform
syntax. Everything in Egg is an ((expression)). An expression can be a
variable, a number, a string, or an _application_. Applications are
used for function calls but also for constructs such as `if` or `while`.

(((double-quote character)))(((parsing)))(((escaping,in strings)))To
keep the parser simple, strings in Egg do not support anything like
backslash escapes. A string is simply a sequence of characters that
are not double quotes, wrapped in double quotes. A number is a
sequence of digits. Variable names can consist of any character that
is not ((whitespace)) and does not have a special meaning in the
syntax.

(((comma character)))Applications are written the way they are in
JavaScript, by putting ((parentheses)) after an expression and having
any number of ((argument))s between those parentheses, separated by
commas.

----
do(define(x, 10),
   if(>(x, 5),
      print("large"),
      print("small")))
----

(((block)))The ((uniformity)) of the ((Egg language)) means that
things that are ((operator))s in JavaScript (such as `>`) are normal
variables in this language, applied just like other ((function))s. And
since the ((syntax)) has no concept of a block, we need a `do`
construct to represent doing multiple things in sequence.

(((type property)))(((parsing)))The ((data structure)) that the parser will
use to describe a program will consist of ((expression)) objects, each
of which has a `type` property indicating the kind of expression it is
and other properties to describe its content.

(((identifier)))Expressions of type `"value"` represent literal strings
or numbers. Their `value` property contains the string or number value
that they represent. Expressions of type `"word"` are used for
identifiers (names). Such objects have a `name` property that holds
the identifier's name as a string. Finally, `"apply"` expressions
represent applications. They have an `operator` property that refers
to the expression that is being applied, and they have an `args` property that
refers to an array of argument expressions.

The `>(x, 5)` part of the previous program would be represented like this:

[source,application/json]
----
{
  type: "apply",
  operator: {type: "word", name: ">"},
  args: [
    {type: "word", name: "x"},
    {type: "value", value: 5}
  ]
}
----

indexsee:[abstract syntax tree,syntax tree]
Such a ((data structure)) is called a _((syntax tree))_. If you
imagine the objects as dots and the links between them as lines
between those dots, it has a ((tree))like shape. The fact that
expressions contain other expressions, which in turn might contain
more expressions, is similar to the way branches split and split again.

image::img/syntax_tree.svg[alt="The structure of a syntax tree",width="5cm"]

(((parsing)))Contrast this to the parser we wrote for the
configuration file format in link:09_regexp.html#ini[Chapter 9], which
had a simple structure: it split the input into lines and
handled those lines one at a time. There were only a few simple forms
that a line was allowed to have.

(((recursion)))(((nesting,of expressions)))Here we must find a
different approach. Expressions are not separated into lines, and they
have a recursive structure. Application expressions _contain_ other
expressions.

(((elegance)))Fortunately, this problem can be solved elegantly by
writing a parser function that is recursive in a way that reflects the
recursive nature of the language.

(((parseExpression function)))(((syntax tree)))We define a function
`parseExpression`, which takes a string as input and returns an
object containing the data structure for the expression at the start
of the string, along with the part of the string left after parsing
this expression. When parsing subexpressions (the argument to an
application, for example), this function can be called again, yielding
the argument expression as well as the text that remains. This text
may in turn contain more arguments or may be the closing parenthesis
that ends the list of arguments.

This is the first part of the parser:

// include_code

[source,javascript]
----
function parseExpression(program) {
  program = skipSpace(program);
  var match, expr;
  if (match = /^"([^"]*)"/.exec(program))
    expr = {type: "value", value: match[1]};
  else if (match = /^\d+\b/.exec(program))
    expr = {type: "value", value: Number(match[0])};
  else if (match = /^[^\s(),"]+/.exec(program))
    expr = {type: "word", name: match[0]};
  else
    throw new SyntaxError("Unexpected syntax: " + program);

  return parseApply(expr, program.slice(match[0].length));
}

function skipSpace(string) {
  var first = string.search(/\S/);
  if (first == -1) return "";
  return string.slice(first);
}
----

(((skipSpace function)))Because Egg allows any amount of
((whitespace)) between its elements, we have to repeatedly cut the
whitespace off the start of the program string. This is what the
`skipSpace` function helps with.

(((literal expression)))(((SyntaxError type)))After skipping any
leading space, `parseExpression` uses three ((regular expression))s to
spot the three simple (atomic) elements that Egg supports: strings,
numbers, and words. The parser constructs a different kind of data
structure depending on which one matches. If the input does not match
one of these three forms, it is
not a valid expression, and the parser throws an error. `SyntaxError` is a
standard error object type, which is raised when an attempt is made to
run an invalid JavaScript program.

(((parseApply function)))We can then cut off the part that we matched
from the program string and pass that, along with the object for the
expression, to `parseApply`, which checks whether the expression is an
application. If so, it parses a parenthesized list of arguments.

// include_code

[source,javascript]
----
function parseApply(expr, program) {
  program = skipSpace(program);
  if (program[0] != "(")
    return {expr: expr, rest: program};

  program = skipSpace(program.slice(1));
  expr = {type: "apply", operator: expr, args: []};
  while (program[0] != ")") {
    var arg = parseExpression(program);
    expr.args.push(arg.expr);
    program = skipSpace(arg.rest);
    if (program[0] == ",")
      program = skipSpace(program.slice(1));
    else if (program[0] != ")")
      throw new SyntaxError("Expected ',' or ')'");
  }
  return parseApply(expr, program.slice(1));
}
----

(((parsing)))If the next character in the program is not an opening
parenthesis, this is not an application, and `parseApply` simply
returns the expression it was given.

(((recursion)))Otherwise, it skips the opening parenthesis and
creates the ((syntax tree)) object for this application expression. It
then recursively calls `parseExpression` to parse each argument until a
closing parenthesis is found. The recursion is indirect, through
`parseApply` and `parseExpression` calling each other.

Because an application expression can itself be applied (such as in
`multiplier(2)(1)`), `parseApply` must, after it has parsed an
application, call itself again to check whether another pair of
parentheses follows.

(((syntax tree)))(((Egg language)))(((parse function)))This is all we
need to parse Egg. We wrap it in a convenient `parse` function that
verifies that it has reached the end of the input string after parsing
the expression (an Egg program is a single expression), and that
gives us the program's data structure.

// include_code strip_log
// test: join

[source,javascript]
----
function parse(program) {
  var result = parseExpression(program);
  if (skipSpace(result.rest).length > 0)
    throw new SyntaxError("Unexpected text after program");
  return result.expr;
}

console.log(parse("+(a, 10)"));
// → {type: "apply",
//    operator: {type: "word", name: "+"},
//    args: [{type: "word", name: "a"},
//           {type: "value", value: 10}]}
----

(((error message)))It works! It doesn't give us very helpful
information when it fails and doesn't store the line and column on
which each expression starts, which might be helpful when reporting
errors later, but it's good enough for our purposes.

== The evaluator ==

(((evaluate function)))(((evaluation)))(((interpretation)))(((syntax
tree)))(((Egg language)))What can we do with the syntax tree for a
program? Run it, of course! And that is what the evaluator does. You
give it a syntax tree and an environment object that associates names
with values, and it will evaluate the expression that the tree
represents and return the value that this produces.

// include_code

[source,javascript]
----
function evaluate(expr, env) {
  switch(expr.type) {
    case "value":
      return expr.value;

    case "word":
      if (expr.name in env)
        return env[expr.name];
      else
        throw new ReferenceError("Undefined variable: " +
                                 expr.name);
    case "apply":
      if (expr.operator.type == "word" &&
          expr.operator.name in specialForms)
        return specialForms[expr.operator.name](expr.args,
                                                env);
      var op = evaluate(expr.operator, env);
      if (typeof op != "function")
        throw new TypeError("Applying a non-function.");
      return op.apply(null, expr.args.map(function(arg) {
        return evaluate(arg, env);
      }));
  }
}

var specialForms = Object.create(null);
----

(((literal expression)))(((environment)))The evaluator has code for
each of the ((expression)) types. A literal value expression simply
produces its value. (For example, the expression `100` just evaluates
to the number 100.) For a variable, we must check whether it is
actually defined in the environment and, if it is, fetch the
variable's value.

(((function,application)))Applications are more involved. If they are
a ((special form)), like `if`, we do not evaluate anything and simply
pass the argument expressions, along with the environment, to the
function that handles this form. If it is a normal call, we evaluate
the operator, verify that it is a function, and call it with the
result of evaluating the arguments.

We will use plain JavaScript function values to represent Egg's
function values. We will come back to this
link:11_language.html#egg_fun[later], when the special form called
`fun` is defined.

(((readability)))(((evaluate
function)))(((recursion)))(((parsing)))The recursive structure of
`evaluate` resembles the similar structure of the parser. Both mirror
the structure of the language itself. It would also be possible to
integrate the parser with the evaluator and evaluate during parsing,
but splitting them up this way makes the program more readable.

(((Egg language)))(((interpretation)))This is really all that is
needed to interpret Egg. It is that simple. But without defining a few
special forms and adding some useful values to the ((environment)),
you can't do anything with this language yet.

== Special forms ==

(((special form)))(((specialForms object)))The `specialForms` object
is used to define special syntax in Egg. It associates words with
functions that evaluate such special forms. It is currently empty.
Let's add some forms.

// include_code

[source,javascript]
----
specialForms["if"] = function(args, env) {
  if (args.length != 3)
    throw new SyntaxError("Bad number of args to if");

  if (evaluate(args[0], env) !== false)
    return evaluate(args[1], env);
  else
    return evaluate(args[2], env);
};
----

(((conditional execution)))Egg's `if` construct expects exactly three
arguments. It will evaluate the first, and if the result isn't the
value `false`, it will evaluate the second. Otherwise, the third gets
evaluated. This `if` form is more similar to JavaScript's ternary `?:`
operator than to JavaScript's `if`. It is an expression, not a statement,
and it produces a value, namely, the result of the second or third
argument.

(((Boolean)))Egg differs from JavaScript in how it handles the
condition value to `if`. It will not treat things like zero or the
empty string as false, but only the precise value `false`.

(((short-circuit evaluation)))The reason we need to represent `if` as
a special form, rather than a regular function, is that all arguments
to functions are evaluated before the function is called, whereas
`if` should evaluate only _either_ its second or its third argument,
depending on the value of the first.

The `while` form is similar.

// include_code

[source,javascript]
----
specialForms["while"] = function(args, env) {
  if (args.length != 2)
    throw new SyntaxError("Bad number of args to while");

  while (evaluate(args[0], env) !== false)
    evaluate(args[1], env);

  // Since undefined does not exist in Egg, we return false,
  // for lack of a meaningful result.
  return false;
};
----

Another basic building block is `do`, which executes all its arguments
from top to bottom. Its value is the value produced by the last
argument.

// include_code

[source,javascript]
----
specialForms["do"] = function(args, env) {
  var value = false;
  args.forEach(function(arg) {
    value = evaluate(arg, env);
  });
  return value;
};
----

(((= operator)))To be able to create ((variable))s and give them new
values, we also create a form called `define`. It expects a word as
its first argument and an expression producing the value to assign to
that word as its second argument. Since `define`, like everything, is
an expression, it must return a value. We'll make it return the value
that was assigned (just like JavaScript's `=` operator).

// include_code

[source,javascript]
----
specialForms["define"] = function(args, env) {
  if (args.length != 2 || args[0].type != "word")
    throw new SyntaxError("Bad use of define");
  var value = evaluate(args[1], env);
  env[args[0].name] = value;
  return value;
};
----

== The environment ==

(((Egg language)))(((evaluate function)))The ((environment)) accepted
by `evaluate` is an object with properties whose names correspond to
variable names and whose values correspond to the values those
((variable))s are bound to. Let's define an environment object to
represent the ((global scope)).

To be able to use the `if` construct we just defined, we must
have access to ((Boolean)) values. Since there are only two
Boolean values, we do not need special syntax for them. We simply bind
two variables to the values `true` and `false` and use those.

// include_code

[source,javascript]
----
var topEnv = Object.create(null);

topEnv["true"] = true;
topEnv["false"] = false;
----

We can now evaluate a simple expression that negates a Boolean value.

[source,javascript]
----
var prog = parse("if(true, false, true)");
console.log(evaluate(prog, topEnv));
// → false
----

(((arithmetic)))(((Function constructor)))To supply basic
((arithmetic)) and ((comparison)) ((operator))s, we will also add some
function values to the ((environment)). In the interest of keeping the
code short, we'll use `new Function` to synthesize a bunch of operator
functions in a loop, rather than defining them all individually.

// include_code

[source,javascript]
----
["+", "-", "*", "/", "==", "<", ">"].forEach(function(op) {
  topEnv[op] = new Function("a, b", "return a " + op + " b;");
});
----

A way to ((output)) values is also very useful, so we'll wrap
`console.log` in a function and call it `print`.

// include_code

[source,javascript]
----
topEnv["print"] = function(value) {
  console.log(value);
  return value;
};
----

(((parsing)))(((run function)))That gives us enough elementary tools
to write simple programs. The following `run` function provides a
convenient way to write and run them. It creates a fresh environment
and parses and evaluates the strings we give it as a single program.

// include_code

[source,javascript]
----
function run() {
  var env = Object.create(topEnv);
  var program = Array.prototype.slice
    .call(arguments, 0).join("\n");
  return evaluate(parse(program), env);
}
----

(((join method)))(((call method)))The use of
`Array.prototype.slice.call` is a trick to turn an ((array-like
object)), such as `arguments`, into a real array so that we can call
`join` on it. It takes all the arguments given to `run` and treats
them as the lines of a program.

[source,javascript]
----
run("do(define(total, 0),",
    "   define(count, 1),",
    "   while(<(count, 11),",
    "         do(define(total, +(total, count)),",
    "            define(count, +(count, 1)))),",
    "   print(total))");
// → 55
----

(((summing example)))(((Egg language)))This is the program we've seen
several times before, which computes the sum of the numbers 1 to 10,
expressed in Egg. It is clearly uglier than the equivalent JavaScript
program but not bad for a language implemented in less than 150
((lines of code)).

[[egg_fun]]
== Functions ==

(((function)))(((Egg language)))A programming language without
functions is a poor programming language indeed.

Fortunately, it is not hard to add a `fun` construct, which treats its
last argument as the function's body and treats all the arguments before that as
the names of the function's arguments.

// include_code

[source,javascript]
----
specialForms["fun"] = function(args, env) {
  if (!args.length)
    throw new SyntaxError("Functions need a body");
  function name(expr) {
    if (expr.type != "word")
      throw new SyntaxError("Arg names must be words");
    return expr.name;
  }
  var argNames = args.slice(0, args.length - 1).map(name);
  var body = args[args.length - 1];

  return function() {
    if (arguments.length != argNames.length)
      throw new TypeError("Wrong number of arguments");
    var localEnv = Object.create(env);
    for (var i = 0; i < arguments.length; i++)
      localEnv[argNames[i]] = arguments[i];
    return evaluate(body, localEnv);
  };
};
----

(((local scope)))(((Object.create function)))(((prototype)))Functions
in Egg have their own local environment, just like in JavaScript. We
use `Object.create` to make a new object that has access to the
variables in the outer environment (its prototype) but that can also
contain new variables without modifying that outer scope.

(((power example)))(((evaluation)))(((interpretation)))The function
created by the `fun` form creates this local environment and adds the
argument variables to it. It then evaluates the function body in this
environment and returns the result.

// start_code

[source,javascript]
----
run("do(define(plusOne, fun(a, +(a, 1))),",
    "   print(plusOne(10)))");
// → 11

run("do(define(pow, fun(base, exp,",
    "     if(==(exp, 0),",
    "        1,",
    "        *(base, pow(base, -(exp, 1)))))),",
    "   print(pow(2, 10)))");
// → 1024
----

== Compilation ==

(((interpretation)))(((compilation)))What we have built is an
interpreter. During evaluation, it acts directly on the representation
of the program produced by the parser.

(((efficiency)))(((performance)))_Compilation_ is the process of
adding another step between the parsing and the running of a program,
which transforms the program into something that can be evaluated more
efficiently by doing as much work as possible in advance. For example,
in well-designed languages it is obvious, for each use of a
((variable)), which variable is being referred to, without actually
running the program. This can be used to avoid looking up the variable
by name every time it is accessed and to directly fetch it from some
predetermined ((memory)) location.

Traditionally, ((compilation)) involves converting the program to
((machine code)), the raw format that a computer's processor can
execute. But any process that converts a program to a different
representation can be thought of as compilation.

(((simplicity)))(((Function constructor)))(((transpilation)))It would
be possible to write an alternative ((evaluation)) strategy for Egg,
one that first converts the program to a JavaScript program, uses `new
Function` to invoke the JavaScript compiler on it, and then runs the
result. When done right, this would make Egg run very fast while
still being quite simple to implement.

If you are interested in this topic and willing to spend some time on
it, I encourage you to try to implement such a compiler as an
exercise.

== Cheating ==

(((Egg language)))When we defined `if` and `while`, you probably
noticed that they were more or less trivial wrappers around
JavaScript's own `if` and `while`. Similarly, the values in Egg are
just regular old JavaScript values.

If you compare the implementation of Egg, built on top of JavaScript,
with the amount of work and complexity required to build a programming
language directly on the raw functionality provided by a machine, the
difference is huge. Regardless, this example hopefully gave you an
impression of the way ((programming language))s work.

And when it comes to getting something done, cheating is more
effective than doing everything yourself. Though the toy language in
this chapter doesn't do anything that couldn't be done better in
JavaScript, there _are_ situations where writing small languages helps
get real work done.

Such a language does not have to resemble a typical programming
language. If JavaScript didn't come equipped with regular expressions,
you could write your own parser and evaluator for such a sublanguage.

(((artificial intelligence)))Or imagine you are building a giant
robotic ((dinosaur)) and need to program its ((behavior)). JavaScript
might not be the most effective way to do this. You might instead opt
for a language that looks like this:

----
behavior walk
  perform when
    destination ahead
  actions
    move left-foot
    move right-foot

behavior attack
  perform when
    Godzilla in-view
  actions
    fire laser-eyes
    launch arm-rockets
----

(((expressivity)))This is what is usually called a _((domain-specific
language))_, a language tailored to express a narrow domain of
knowledge. Such a language can be more expressive than a
general-purpose language because it is designed to express exactly the
things that need expressing in its domain and nothing else.

== Exercises ==

=== Arrays ===

(((Egg language)))Add support for ((array))s to Egg by adding the
following three functions to the top scope: `array(...)` to
construct an array containing the argument values, `length(array)` to
get an array's length, and `element(array, n)` to fetch the n^th^
element from an array.

ifdef::interactive_target[]

// test: no

[source,javascript]
----
// Modify these definitions...

topEnv["array"] = "...";

topEnv["length"] = "...";

topEnv["element"] = "...";

run("do(define(sum, fun(array,",
    "     do(define(i, 0),",
    "        define(sum, 0),",
    "        while(<(i, length(array)),",
    "          do(define(sum, +(sum, element(array, i))),",
    "             define(i, +(i, 1)))),",
    "        sum))),",
    "   print(sum(array(1, 2, 3))))");
// → 6
----
endif::interactive_target[]

!!hint!!

The easiest way to do this is to represent Egg arrays
with JavaScript arrays.

(((slice method)))The values added to the top environment must be
functions. `Array.prototype.slice` can be used to convert an
`arguments` array-like object into a regular array.

!!hint!!

=== Closure ===

(((closure)))(((function,scope)))The way we have defined `fun` allows
functions in Egg to “close over” the surrounding environment, allowing
the function's body to use local values that were visible at the time
the function was defined, just like JavaScript functions do.

The following program illustrates this: function `f` returns a function
that adds its argument to `f`'s argument, meaning that it needs access
to the local ((scope)) inside `f` to be able to use variable `a`.

[source,javascript]
----
run("do(define(f, fun(a, fun(b, +(a, b)))),",
    "   print(f(4)(5)))");
// → 9
----

Go back to the definition of the `fun` form and explain which
mechanism causes this to work.

!!hint!!

(((closure)))Again, we are riding along on a JavaScript mechanism to
get the equivalent feature in Egg. Special forms are passed the local
environment in which they are evaluated so that they can evaluate
their subforms in that environment. The function returned by `fun`
closes over the `env` argument given to its enclosing function and
uses that to create the function's local ((environment)) when it is
called.

(((compilation)))This means that the ((prototype)) of the local
environment will be the environment in which the function was created,
which makes it possible to access variables in that environment from
the function. This is all there is to implementing closure (though to
compile it in a way that is actually efficient, you'd need to do some
more work).

!!hint!!

=== Comments ===

(((hash character)))(((Egg language)))It would be nice if we could
write ((comment))s in Egg. For example, whenever we find a hash sign
(`#`), we could treat the rest of the line as a comment and ignore it,
similar to `//` in JavaScript.

(((skipSpace function)))We do not have to make any big changes to the
parser to support this. We can simply change `skipSpace` to skip
comments like they are ((whitespace)) so that all the points where
`skipSpace` is called will now also skip comments. Make this change.

ifdef::interactive_target[]

// test: no

[source,javascript]
----
// This is the old skipSpace. Modify it...
function skipSpace(string) {
  var first = string.search(/\S/);
  if (first == -1) return "";
  return string.slice(first);
}

console.log(parse("# hello\nx"));
// → {type: "word", name: "x"}

console.log(parse("a # one\n   # two\n()"));
// → {type: "apply",
//    operator: {type: "word", name: "a"},
//    args: []}
----
endif::interactive_target[]

!!hint!!

(((comment)))Make sure your solution handles multiple comments in a
row, with potentially ((whitespace)) between or after them.

A ((regular expression)) is probably the easiest way to solve this.
Write something that matches “whitespace or a comment, zero or more
times”. Use the `exec` or `match` method and look at the length of
the first element in the returned array (the whole match) to find out
how many characters to slice off.

!!hint!!

=== Fixing scope ===

(((variable,definition)))(((assignment)))Currently, the only way to
assign a ((variable)) a value is `define`. This construct acts as
a way both to define new variables and to give existing ones a new value.

(((local variable)))This ((ambiguity)) causes a problem. When you try
to give a nonlocal variable a new value, you will end up defining a
local one with the same name instead. (Some languages work like this
by design, but I've always found it a silly way to handle ((scope)).)

(((ReferenceError type)))Add a special form `set`, similar to
`define`, which gives a variable a new value, updating the variable in
an outer scope if it doesn't already exist in the inner scope. If the
variable is not defined at all, throw a `ReferenceError` (which is
another standard error type).

(((hasOwnProperty method)))(((prototype)))(((getPrototypeOf
function)))The technique of representing scopes as simple objects,
which has made things convenient so far, will get in your way a
little at this point. You might want to use the
`Object.getPrototypeOf` function, which returns the prototype of an
object. Also remember that scopes do not derive from
`Object.prototype`, so if you want to call `hasOwnProperty` on them,
you have to use this clumsy expression:

// test: no

[source,javascript]
----
Object.prototype.hasOwnProperty.call(scope, name);
----

This fetches the `hasOwnProperty` method from the `Object` prototype
and then calls it on a scope object.

ifdef::interactive_target[]

// test: no

[source,javascript]
----
specialForms["set"] = function(args, env) {
  // Your code here.
};

run("do(define(x, 4),",
    "   define(setx, fun(val, set(x, val))),",
    "   setx(50),",
    "   print(x))");
// → 50
run("set(quux, true)");
// → Some kind of ReferenceError
----
endif::interactive_target[]

!!hint!!

(((variable,definition)))(((assignment)))(((getPrototypeOf
function)))(((hasOwnProperty method)))You will have to loop through
one ((scope)) at a time, using `Object.getPrototypeOf` to go the next
outer scope. For each scope, use `hasOwnProperty` to find out whether the
variable, indicated by the `name` property of the first argument to
`set`, exists in that scope. If it does, set it to the result of
evaluating the second argument to `set` and then return that value.

(((global scope)))(((run-time error)))If the outermost scope is
reached (`Object.getPrototypeOf` returns null) and we haven't found
the variable yet, it doesn't exist, and an error should be thrown.

!!hint!!