monkey — Monkey Language Interpreter in Zig

A lexer, parser, and REPL for the Monkey programming language, implemented in Zig following Thorsten Ball's "Writing An Interpreter In Go".

Status

Component	Milestone
Tokenizer / Lexer	Full Monkey token set, keyword lookup, two-char operators (==, !=)
Parser	let statements, structured error reporting (no heap allocs per error)
AST	Tagged unions (Statement / Expression) — no vtables, no interface casting
REPL	Reads from stdin, tokenizes and prints tokens (zig build repl)

Quick start

# Run all tests
zig test src/Interpreter/Parser.zig
zig test src/Interpreter/Lexer.zig
zig test src/Interpreter/Ast.zig
zig test src/Interpreter/Token.zig

# Or use the build system
zig build test

# Run the REPL
zig build repl

# Check compilation (all packages)
zig build check

Project structure

src/Interpreter/
  Token.zig     — Token type, Type enum, keyword lookup (StaticStringMap)
  Lexer.zig     — Character-by-character tokenizer
  Ast.zig       — AST types: Program, Statement (tagged union), Identifier, LetStatement
  Parser.zig    — Recursive-descent parser, structured error collection
  Repl.zig      — Interactive read-eval-print loop
src/main.zig    — Stub entry point (not wired to interpreter)
build.zig       — Data-driven package registration, one loop for test + check steps

Design notes

• Tagged unions for AST — Statement / Expression are union(enum), not Go-style interfaces or Zig vtables. The fixed set of node types means no runtime dispatch.
• Allocation-free parse errors — parser errors store Token.Type values directly, no allocPrint per error. The ErrorFmt struct uses the {f} format specifier for custom display.
• Borrowed literals — Token.literal slices into the original source input. No string copies during lexing.
• Build loop — build.zig uses std.meta.fields over a Packages named struct to register every module, its test artifact, and its check executable in a single comptime loop.
• Zig 0.16 conventions — ArrayList is unmanaged by default (allocator passed per-operation), toOwnedSlice returns right-sized allocations, pub fn format with two args responds to the {f} specifier.

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The above was AI generated