Monkey

A tree-walking interpreter for the Monkey programming language, written in Go with no dependencies. I built it while working through Thorsten Ball's Writing an Interpreter in Go, and it covers the full pipeline from source text to evaluated result.

$ go run .
Hello you! This is the Monkey programming language!
>> let add = fn(a, b) { a + b };
>> add(2, 3);
5
>> let m = {"name": "monkey"};
>> m["name"];
monkey
>> len([1, 2, 3]);
3

The language

• Integers, booleans, strings, arrays, and hash maps

• let bindings, if / else, and return

• First-class functions and closures:

  let newAdder = fn(x) { fn(y) { x + y } };
  let addTwo = newAdder(2);
  addTwo(3); // 5

• Built-in functions: len, first, last, rest, push, puts

How it works

source ──► lexer ──► tokens ──► parser ──► AST ──► evaluator ──► object
                                  (Pratt)          (tree-walking, with environments)

Package	Role
token/	Token types and keyword lookup
lexer/	Turns source text into tokens
parser/	Pratt parser (top-down operator precedence) that builds the AST, with optional tracing to debug precedence
ast/	AST node types
evaluator/	Recursively evaluates AST nodes, plus the built-in functions
object/	The runtime object system, and Environment for scopes and closures
repl/	Read-eval-print loop

Interesting parts:

• Pratt parsing. Each token type registers prefix and infix parse functions with a precedence. That handles operator precedence and associativity without a grammar generator.
• Closures. A function object captures the environment it was defined in. Calling it creates an enclosed environment that extends the captured one.
• Error values. Runtime errors are first-class objects that short-circuit evaluation, instead of Go panics.

Run it

Requires Go 1.19+.

git clone https://github.com/learnercys/monkey && cd monkey
go run .        # start the REPL
go test ./...   # lexer, parser, AST, object, and evaluator tests

What's next

• Compile to bytecode and run on a stack-based VM (the follow-up book, Writing a Compiler in Go)
• Macros