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This repository contains an interpreter for the "Monkey" programming language, as described in Write an Interpreter in Go.

My changes

The interpreter in this repository has been significantly extended from the starting point:

  • Added single-line & multi-line comments.
  • Added postfix operators (i++, i--).
  • Allow accessing individual characters of a string via the index-operator.
  • Added a driver to read from STDIN, or a named file, rather than a REPL.
    • This allows executing the examples easily (for example "./monkey examples/hello.mon".)
  • Added a collection of standard-library functions.
    • Including file input/output, type-discovery, string, and math functions.
  • Added a new way to define functions, via function.
  • Added the general-purpose comparision functions <= & >=.
  • Allow string comparisons via ==, !=, <=, & >=.
  • Allow comparisions to be complex:
    • if ( a >= 'a' && a <= 'z' ) ..
    • if ( a || b ) ..
  • Allow assignments without let.
    • This also allows operators such as "+=", "-=", "*=", & "/=" to work.
  • Added command-line handling, so that scripts can read their own arguments.
  • Added global-constants available by default
    • For example PI, E, STDIN, STDOUT, & STDERR.
  • Most scripts will continue running in the face of errors.
    • To correct/detect "obvious" errors add pragma("strict"); to your script, which will cause the interpreter to show a suitable error-message and terminate.
  • Function arguments may have defaults. For example:
    • function greet( name = "World" ) { puts("Hello, " + name + "\n"); }
  • Moved parts of the standard-library to 100% pure monkey, rather than implementing it in go.
  • Added the eval function.
    • Which allows executing monkey-code from a string.
  • Improved error-reporting from the parser.
    • It will now show the line-number of failures (where possible).
  • Added support for regular expressions, both literally and via match
    • if ( name ~= /steve/i ) { puts( "Hello Steve\n"); }
  • Added support for ternary expressions.
  • Added support for creating arrays of consecutive integers via the range operator (1..10).
  • Added the ability to iterate over the contents of arrays, hashes, and strings via the foreach statement.
  • Added printf and sprintf primitives, which work as you would expect.
    • printf( "%d %s", 3, "Steve" );
  • Added support for switch statements, with block-based case expressions.
    • No bugs due to C-style "fall-through".
  • Add support for explicit null usage:
    • a = null; if ( a == null ) { .. }

See Also

If you enjoyed this repository you might find the related ones interesting:

Finally I put together a couple of "complex" compilers, which convert input into AMD64 assembly language:

1. Installation

Due to the embedded standard-library implementation, which is implemented in monkey, you'll need to compile this project with go version 1.16beta1 or higher.

You can install from source like so:

git clone
cd monkey
go install

Binary Releases

Alternatively you could install a binary-release, from the release page.

If you're an emacs user might also wish to install the monkey.el file, which provides syntax highlighting for monkey-scripts.

1.1 Usage

To execute a monkey-script simply pass the name to the interpreter:

 $ monkey ./example/hello.mon

Scripts can be made executable by adding a suitable shebang line:

 $ cat hello.mon
 #!/usr/bin/env monkey
 puts( "Hello, world!\n" );

Execution then works as you would expect:

 $ chmod 755 hello.mon
 $ ./hello.mon
 Hello, world!

If no script-name is passed to the interpreter it will read from STDIN and execute that instead, allowing simple tests to be made.

2 Syntax

NOTE: Example-programs can be found beneath examples/ which demonstrate these things, as well as parts of the standard-library.

2.1 Definitions

Variables are defined using the let keyword, with each line ending with ;.

  let a = 3;
  let b = 1.2;

Variables may be integers, floats, strings, or arrays/hashes (which are discussed later).

Some variables are defined by default, for example:

puts( PI ); // Outputs: 3.14159..
puts( E );  // Outputs: 2.71828..

Variables may be updated without the need for let, for example this works as you would expect:

let world = "Earth";
world = "world";
puts( "Hello, " + world + "!\n");

If you're not running with pragma("strict"); you can also declare and use variables without the need for let, but that should be avoided as typos will cause much confusion!

 name = "Steve";
 puts( "Hello, " + name + "\n");

2.2 Arithmetic operations

monkey supports all the basic arithmetic operation of int and float types.

The int type is represented by int64 and float type is represented by float64.

   let a = 3;
   let b = 1.2;

   puts( a + b  );  // Outputs: 4.2
   puts( a - b  );  // Outputs: 1.8
   puts( a * b  );  // Outputs: 3.6
   puts( a / b  );  // Outputs: 2.5
   puts( 2 ** 3 ) ; // Outputs: 8

Here ** is used to raise the first number to the power of the second. When operating with integers the modulus operator is available too, via %.

2.3 Builtin containers

monkey contains two builtin containers: array and hash.

2.3.1 Arrays

An array is a list which organizes items by linear sequence. Arrays can hold multiple types.

 let a = [1, 2.3, "array"];
 let b = [false, true, "Hello World", 3, 3.13];

Adding to an array is done via the push function:

 let a = push(a, "another");

You can iterate over the contents of an array like so:

 let i = 0;
 for( i < len(a) ) {
    puts( "Array index ", i, " contains ", a[i], "\n");

With the definition we included that produces this output:

 Array index 0 contains 1
 Array index 1 contains 2.3
 Array index 2 contains array
 Array index 3 contains another

As a helper you may define an array of consecutive integers via the range operator (..):

 let a = 1..10;

2.3.2 Hashes

A hash is a key/value container, but note that keys may only be of type boolean, int and string.

let a = {"name":"monkey",

puts(a); // Outputs: {name: monkey, true: 1, 7: seven}

puts(a["name"]); // Outputs: monkey

Updating a hash is done via the set function, but note that this returns an updated hash - rather than changing in-place:

let b = set(a, 8, "eight");
puts(b);  // Outputs: {name: monkey, true: 1, 7: seven, 8: eight}

You can iterate over the keys in a hash via the keys function, or delete keys via delete (again these functions returns an updated value rather than changing it in-place).

Hash functions are demonstrated in the examples/hash.mon sample.

2.4 Builtin functions

The core primitives are:

  • delete
    • Deletes a hash-key.
  • int
    • convert the given float/string to an integer.
  • keys
    • Return the keys of the specified array.
  • len
    • Yield the length of builtin containers.
  • match
    • Regular-expression matching.
  • pragma
    • Allow the run-time environment to be controlled.
    • We currently support only pragma("strict");.
  • push
    • push an elements into the array.
  • puts
    • Write literal value of objects to STDOUT.
  • printf
    • Write values to STDOUT, via a format-string.
  • set
    • insert key value pair into the map.
  • sprintf
    • Create strings, via a format-string.
  • string
    • convert the given item to a string.
  • type
    • returns the type of a variable.

The following functions are also part of our standard library, but are implemented in 100% pure monkey:

  • first
    • yield the first element of array.
  • last
    • yield the last element of array.
  • rest
    • yield an array which excludes the first element.

2.4.1 The Standard Library

In addition to the core built-in functions we also have a minimal-standard library. The library includes some string/file primitives, a regular-expression matcher, and some maths-helpers.

You can see the implementation of the go-based standard-library beneath evaluator/stdlib*, and several of these functions are documented in the various examples/.

NOTE: Parts of our standard-library are implemented in 100% pure monkey, and these are embedded in our compiled interpreter. The source of the functions can be viewed in data/stdlib.mon.

If you wish to make changes to the monkey-based standard-library you'll need to rebuild the interpreter after making your changes, to ensure they are bundled into the executable.

Nothing special is required, the following will suffice as you'd expect:

go build .

2.5 Functions

monkey uses fn to define a function which will be assigned to a variable for naming/invocation purposes:

let add = fn(a, b) { return a + b;};
puts(add(1,2));  // Outputs: 3

// functions can be used via their variables
let addTwo = fn(a,b, f) { return 2 + f(a, b);};
puts( addTwo(1,2, add) ); // outputs: 5.

It is also possible to define a function without the use of let, via the function keyword. This was added to make the language feel more natural to C-developers:

function hello() { puts "Hello, world\n" ; };
hello();   // Outputs: Hello, world" to the console.

You may specify a default value for arguments which are not provided, for example:

let foo = fn( name = "World!") {
  puts( "Hello, " + name + "\n" );

foo( "Steve" );

This will output what you expect:

Hello, World!
Hello, Steve

The same thing works for literal functions:

// Function with a default (string) argument
function meh( arg = "Steve" ) {
  puts( "Argument:", arg, " has type:", type(arg), "\n");

// Call it with no argument and the default will be used.

// But of course all the rest work just fine.
meh( 1 );
meh( 1/3.0 );
meh( "Steve" );
meh( [1,2,3,4] );
meh( {"Steve":"Kemp", true:1, false:0, 7:"seven"} );

2.6 If-else statements

monkey supports if-else statements.

let max = fn(a, b) {
  if (a > b) {
    return a;
  } else {
    return b;

puts( max(1, 2) );  // Outputs: 2

2.6.1 Ternary Expressions

monkey supports the use of ternary expressions, which work as you would expect with a C-background:

function max(a,b) {
  return( a > b ? a : b );

puts( "max(1,2) -> ", max(1, 2), "\n" );
puts( "max(-1,-2) -> ", max(-1, -2), "\n" );

Note that in the interests of clarity nested ternary-expressions are illegal!

2.7 Switch Statements

Monkey supports the switch and case expressions, as the following example demonstrates:

  name = "Steve";

  switch( name ) {
    case /^steve$/i {
       printf("Hello Steve - we matched you via a regexp\n");
    case "St" + "even" {
       printf("Hello SteveN, you were matched via an expression\n" );
    case 3 {
       printf("Hello number three, we matched you literally.\n");
    default {
       printf("Default case: %s\n", string(name) );

See also examples/switch.mon.

2.8 For-loop statements

monkey supports a golang-style for-loop statement.

 let sum = fn(x) {
    let i = 1;
    let sum = 0;

    for (i < x) {
       sum += i;
    return sum;

 puts(sum(100));  // Outputs: 4950

2.8.1 Foreach statements

In addition to iterating over items with the for statement, as shown above, it is also possible to iterate over various items via the foreach statement.

For example to iterate over an array:

 a = [ "My", "name", "is", "Steve" ]
 foreach item in a {
      puts( "\t",  item , "\n");

Here you see that we've iterated over the items of the array, we can also see their offsets like so:

 foreach offset, item in a {
      puts( offset, "\t",  item , "\n");

The same style of iteration works for Arrays, Hashes, and the characters which make up a string. You can see examples of this support in examples/iteration.mon.

When iterating over hashes you can receive either the keys, or the keys and value at each step in the iteration, otherwise you receive the value and an optional index.


monkey support two kinds of comments:

  • Single-line comments begin with // and last until the end of the line.
  • Multiline comments between /* and */.

2.10 Postfix Operators

The ++ and -- modifiers are permitted for integer-variables, for example the following works as you would expect showing the numbers from 0 to 5:

let i = 0;
for ( i <= 5 ) {
   puts( i, "\n" );

Another feature borrowed from C allows variables to be updated in-place via the operators +=, -=, *=, & /=.

Using += our previous example could be rewritten as:

let i = 0;
for ( i <= 5 ) {
   puts( i, "\n" );
   i += 1;

The update-operators work with integers and doubles by default, when it comes to strings the only operator supported is +=, allowing for a string-append:

let str = "Forename";
str += " Surname";
str += "\n";
puts( str );           // -> "Forename Surname\n"

2.11 Command Execution

As with many scripting languages commands may be executed via the backtick operator (```).

  let uptime = `/usr/bin/uptime`;

  if ( uptime ) {
      puts( "STDOUT: ", uptime["stdout"].trim() , "\n");
      puts( "STDERR: ", uptime["stderr"].trim() , "\n");
  } else {
      puts( "Failed to run command\n");

The output will be a hash with two keys stdout and stderr. NULL is returned if the execution fails. This can be seen in examples/exec.mon.

2.12 Regular Expressions

The match function allows matching a string against a regular-expression.

If a match fails NULL will be returned, otherwise a hash containing any capture groups in the match.

This is demonstrated in the examples/regexp.mon example.

You can also perform matching (complete with captures), with a literal regular expression object:

if ( Name ~= /steve/i ) { puts( "Hello Steve\n" ); }
if ( Name !~ /[aeiou]/i ) { puts( "You have no vowels.\n" ); }

// captures become $1, $2, $N, etc.
ip = "";
if ( ip ~= /([0-9]+)\.([0-9]+)\.([0-9]+)\.([0-9]+)/  ) {
    printf("Matched! %s.%s.%s.%s\n", $1, $2, $3, $4 );

2.13 File I/O

The open primitive is used to open files, and can be used to open files for either reading, or writing:

// Open a file for reading
fh = open( "/etc/passwd" );
fh = open( "/etc/passwd", "r" );

// Open a file for writing
fh = open( "/tmp/blah", "w" );

// Open a file for appending
fh = open( "/tmp/blah", "wa" );

Once you have a file-object you can invoke methods upon it:

  • read()
    • Read a line of input, returning that input as a string.
  • readlines()
    • Read the lines of the given file, and return them as an array.
  • write(data)
    • Write the data to the given file.

These are demonstrated in the following examples:

By default three filehandles will be made available, as constants:

    • Use for reading STDIN.
    • Used for writing messages.

2.14 File Operations

The primitive stat will return a hash of details about the given file, or directory entry.

You can change the permissions of a file via the chmod function, but note that the second argument is an octal string:

chmod( "/tmp/", "755")
chmod( "/tmp/normal", "644")

To remove a file, use unlink:

unlink( "/tmp/trash.txt" )

And finally to make a directory:

mkdir( "/tmp/blah" );

3. Object Methods

There is now support for "object-methods". Object methods are methods which are defined against a type. For example all of our primitive types allow a methods() method, which returns the methods which are available against them.

Similarly each of them implement a type() function which returns the type involved:

let i = 1;
puts( i.type() );

let s = "Steve";
puts( s.type() );

Or even:

puts( "Steve".type() );

Seeing methods available works as you would expect:

a = [ "Array", "Is", "Here" ];

let i = 0;
for ( i < len(a.methods() ) ) {
   puts( "Method " + a.methods()[i] + "\n" );

This shows:

Method find
Method len
Method methods
Method string

The string object has the most methods at the time of writing, but no doubt things will change over time.

3.1 Defininig New Object Methods

The object-methods mentioned above are implemented in Go, however it is also possible to define such methods in 100% monkey!

You can define a method via something like:

function string.steve() {
   puts( "Hello, I received '", self, "' as an argument\n" );

Note that the function has access to the object it was invoked upon via the implicit self name. Invocation would look as you expect:

let s = "Hello, world";
s.steve();   -> Hello, I received 'Hello, world' as an argument

You can see data/stdlib.mon implements some primitives in this fashion, for example the functional-programming methods, array.filter, string.toupper, etc, etc.

Github Setup

This repository is configured to run tests upon every commit, and when pull-requests are created/updated. The testing is carried out via .github/ which is used by the github-action-tester action.

Releases are automated in a similar fashion via .github/build, and the github-action-publish-binaries action.