golox: Lox Implementation in Go

This is a complete interpreter for the Lox programming language as defined in Crafting Interpreters.

This implementation closely follows the described Java implementation.

Notable differences:

  • The Java implementation utilizes Java Exceptions. Go does not have true exceptions but Go panic/recover works well enough to model the same exception use cases.
  • Java inheritance is also often used in the Java version when walking the AST with the visitor pattern. In this implementation I use a Go type switch to delegate to the proper struct type. The consequence is that after running the AST generator, any new AST node type must be added to all type switches. If you forget to do so, when the new node is encountered, the switch default case will trigger and typically stop with an ‘unsupported ast node type‘ panic.


  • Start REPL: go run cmd/golox/golox.go
  • Run file.lox: go run cmd/golox/golox.go file.lox
  • Build golox binary: go build cmd/golox/golox.go

Adding and Updating the AST types in pkg/ast/gen

  • To Generate AST node types: go run cmd/tool/gen/ast/ast-gen.go pkg/ast/gen

ast-gen.go has the defineAst method that determines what types to generate.

After generation you should probably update the parser and interpreter to utilize the new AST node types. Otherwise you may get an unsupported *new node type* panic.


go run cmd/golox/golox.go

Use ctrl+d to exit.

Running Samples

The samples directory has many samples from Crafting Interpreters.

You can run them like any other lox file: go run cmd/golox/golox.go samples/hello.lox

CPU Profile

You can run the interpreter with cpu profiling enabled.


# run `14-fib-bench.lox` file and output cpu profile to `cpuprofile`:
go run cmd/golox/golox.go samples/14-fib-bench.lox cpuprofile

# use pprof the examine `cpuprofile`
go tool pprof cpuprofile


As expected the tree-walk interpreter is very slow.

On a Macbook Air M1, fib(35) runs in:

  • golox: 17 seconds
  • go: 43 milliseconds


go run cmd/golox/golox.go samples/14-fib-bench.lox

go run cmd/other/go-fib/main.go

For comparison jlox in Chunks of Bytecode runs fib(40) in 72 seconds. golox runs fib(40) in 197 seconds and plain Go runs in 351ms on a Macbook Air M1. Since fib(40) takes too long to complete I left the samples at fib(35).


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