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Interactive Go interpreter and debugger with REPL, Eval, generics and Lisp-like macros


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gomacro - interactive Go interpreter and debugger with generics and macros

gomacro is an almost complete Go interpreter, implemented in pure Go. It offers both an interactive REPL and a scripting mode, and does not require a Go toolchain at runtime (except in one very specific case: import of a 3rd party package at runtime).

It has two dependencies beyond the Go standard library: and

Gomacro can be used as:

  • a standalone executable with interactive Go REPL, line editing and code completion: just run gomacro from your command line, then type Go code. Example:

    $ gomacro
    [greeting message...]
    gomacro> import "fmt"
    gomacro> fmt.Println("hello, world!")
    hello, world!
    14      // int
    <nil>   // error

    press TAB to autocomplete a word, and press it again to cycle on possible completions.

    Line editing follows mostly Emacs: Ctrl+A or Home jumps to start of line, Ctrl+E or End jumps to end of line, Ald+D deletes word starting at cursor... For the full list of key bindings, see

  • a tool to experiment with Go generics: see Generics

  • a Go source code debugger: see Debugger

  • an interactive tool to make science more productive and more fun. If you use compiled Go with scientific libraries (physics, bioinformatics, statistics...) you can import the same libraries from gomacro REPL (immediate on Linux and Mac OS X, requires restarting on other platforms, see Importing packages below), call them interactively, inspect the results, feed them to other functions/libraries, all in a single session. The imported libraries will be compiled, not interpreted, so they will be as fast as in compiled Go.

    For a graphical user interface on top of gomacro, see Gophernotes. It is a Go kernel for Jupyter notebooks and nteract, and uses gomacro for Go code evaluation.

  • a library that adds Eval() and scripting capabilities to your Go programs in few lines of code:

     package main
     import (
     func RunGomacro(toeval string) reflect.Value {
     	interp := fast.New()
     	vals, _ := interp.Eval(toeval)
     	// for simplicity, only use the first returned value
     	return vals[0].ReflectValue()
     func main() {

    Also, github issue #13 explains how to have your application's functions, variable, constants and types available in the interpreter.

    Note: gomacro license is MPL 2.0, which imposes some restrictions on programs that use gomacro. See MPL 2.0 FAQ for common questions regarding the license terms and conditions.

  • a way to execute Go source code on-the-fly without a Go compiler: you can either run gomacro FILENAME.go (works on every supported platform)

    or you can insert a line #!/usr/bin/env gomacro at the beginning of a Go source file, then mark the file as executable with chmod +x FILENAME.go and finally execute it with ./FILENAME.go (works only on Unix-like systems: Linux, *BSD, Mac OS X ...)

  • a Go code generation tool: gomacro was started as an experiment to add Lisp-like macros to Go, and they are extremely useful (in the author's opinion) to simplify code generation. Macros are normal Go functions, they are special only in one aspect: they are executed before compiling code, and their input and output is code (abstract syntax trees, in the form of go/ast.Node)

    Don't confuse them with C preprocessor macros: in Lisp, Scheme and now in Go, macros are regular functions written in the same programming language as the rest of the source code. They can perform arbitrary computations and call any other function or library: they can even read and write files, open network connections, etc... as a normal Go function can do.

    See doc/code_generation.pdf for an introduction to the topic.



Supported platforms

Gomacro is pure Go, and in theory it should work on any platform supported by the Go compiler. The following combinations are tested and known to work:

  • Linux: amd64, 386, arm64, arm, mips, ppc64le
  • Mac OS X: amd64, 386 (386 binaries running on amd64 system)
  • Windows: amd64, 386
  • FreeBSD: amd64, 386
  • Android: arm64, arm (tested with Termux and the Go compiler distributed with it)

How to install

The command

go install

downloads, compiles and installs gomacro and its dependencies

Current Status

Almost complete.

The main limitations and missing features are:

  • importing 3rd party libraries at runtime currently only works on Linux, Mac OS X and *BSD. On other systems as Windows and Android it is cumbersome and requires recompiling - see Importing packages.
  • when importing packages, both from standard library or from 3rd party libraries, generics are not imported.
  • defining generic functions and types in interpreted code is experimental and incomplete - see Generics
  • conversions from/to unsafe.Pointer are not supported.
  • some corner cases using interpreted interfaces, as interface -> interface type assertions and type switches, are not implemented yet.
  • some corner cases using recursive types may not work correctly.
  • goto can only jump backward, not forward
  • out-of-order code is under testing - some corner cases, as for example out-of-order declarations used in keys of composite literals, are not supported. Clearly, at REPL code is still executed as soon as possible, so it makes a difference mostly if you separate multiple declarations with ; on a single line. Example: var a = b; var b = 42
    Support for "batch mode" is in progress - it reads as much source code as possible before executing it, and it's useful mostly to execute whole files or directories.

The documentation also contains the full list of features and limitations


Compared to compiled Go, gomacro supports several extensions:

  • an integrated debugger, see Debugger

  • configurable special commands. Type :help at REPL to list them, and see cmd.go:37 for the documentation and API to define new ones.

  • untyped constants can be manipulated directly at REPL. Examples:

    gomacro> 1<<100
    {int 1267650600228229401496703205376}	// untyped.Lit
    gomacro> const c = 1<<100; c * c / 100000000000
    {int 16069380442589902755419620923411626025222029937827}	// untyped.Lit

    This provides a handy arbitrary-precision calculator.

    Note: operations on large untyped integer constants are always exact, while operations on large untyped float constants are implemented with go/constant.Value, and are exact as long as both numerator and denominator are <= 5e1232.

    Beyond that, go/constant.Value switches from *big.Rat to *big.Float with precision = 512, which can accumulate rounding errors.

    If you need exact results, convert the untyped float constant to *big.Rat (see next item) before exceeding 5e1232.

  • untyped constants can be converted implicitly to *big.Int, *big.Rat and *big.Float. Examples:

    import "math/big"
    var i *big.Int = 1<<1000                 // exact - would overflow int
    var r *big.Rat = 1.000000000000000000001 // exact - different from 1.0
    var s *big.Rat = 5e1232                  // exact - would overflow float64
    var t *big.Rat = 1e1234                  // approximate, exceeds 5e1232
    var f *big.Float = 1e646456992           // largest untyped float constant that is different from +Inf

    Note: every time such a conversion is evaluated, it creates a new value - no risk to modify the constant.

    Be aware that converting a huge value to string, as typing f at REPL would do, can be very slow.

  • zero value constructors: for any type T, the expression T() returns the zero value of the type

  • macros, quoting and quasiquoting: see doc/code_generation.pdf

and slightly relaxed checks:

  • unused variables and unused return values never cause errors


Some short, notable examples - to run them on non-Linux platforms, see Importing packages first.

plot mathematical functions

  • install libraries: go get
  • start the interpreter: gomacro
  • at interpreter prompt, paste the whole Go code listed at (the source code starts after the picture under the section "Functions", and ends just before the section "Histograms")
  • still at interpreter prompt, enter main() If all goes well, it will create a file named "functions.png" in current directory containing the plotted functions.

simple mandelbrot web server

  • install libraries: go get
  • chdir to mandelbrot-go source folder: cd; cd go/src/
  • start interpreter with arguments: gomacro -i mbrot.go
  • at interpreter prompt, enter init(); main()
  • visit http://localhost:8090/ Be patient, rendering and zooming mandelbrot set with an interpreter is a little slow.

Further examples are listed by Gophernotes

Importing packages

Gomacro supports the standard Go syntax import, including package renaming. Examples:

import "fmt"
import (
    r "reflect"

Third party packages - i.e. packages not in Go standard library - can also be imported with the same syntax.

Extension: unpublished packages can also be imported from a local filesystem directory (implemented on 2022-05-28). Supported syntaxes are:

import (
     "."                             // imports the package in current directory
     ".."                            // imports the package in parent directory
     "./some/relative/path"          // "./"  means relative to current directory
     "../some/other/relative/path"   // "../" means relative to parent directory
     "/some/absolute/path"           // "/"   means absolute

For an import to work, you usually need to follow its installation procedure: sometimes there are additional prerequisites to install, and the typical command go get PACKAGE-PATH may or may not be needed.

The next steps depend on the system you are running gomacro on:

Linux, Mac OS X and *BSD

If you are running gomacro on Linux, Mac OS X or *BSD, import will then just work: it will automatically download, compile and import a package. Example:

$ gomacro
[greeting message...]

gomacro> import ( ""; "" )
// debug: running "go get" ...
go: downloading v0.12.0
go: downloading v0.12.0
[ more "go: downloading " messages for dependencies...]
go: added v0.12.0
go: added v0.12.0
// debug: running "go mod tidy" ...
go: downloading v0.0.0-20220827204233-334a2380cb91
go: downloading v0.2.0
go: downloading v0.1.1
go: downloading v0.1.0
// debug: compiling plugin "/home/max/go/src/gomacro.imports/gomacro_pid_44092/import_1" ...

gomacro> floats.Sum([]float64{1,2,3})
6       // float64

Note: internally, gomacro will compile and load a single Go plugin containing the exported declarations of all the packages listed in import ( ... ).

The command go mod tidy is automatically executed before compiling the plugin, and it tries - among other things - to resolve any version conflict due to different versions of the same package being imported directly (i.e. listed in import ( ... )) or indirectly (i.e. as a required dependency).

Go plugins are currently supported only on Linux and Mac OS X.

WARNING On Mac OS X, never execute strip gomacro: it breaks plugin support, and loading third party packages stops working.

Other systems

On all other systems as Windows, Android and *BSD you can still use import, but there are more steps: you need to manually download the package, and you also need to recompile gomacro after the import (it will tell you). Example:

$ go get
$ gomacro
[greeting message...]

gomacro> import ""
// warning: created file "/home/max/go/src/", recompile gomacro to use it

Now quit gomacro, recompile and reinstall it:

gomacro> :quit
$ go install

Finally restart it. Your import is now linked inside gomacro and will work:

$ gomacro
[greeting message...]

gomacro> import ""
gomacro> plot.New()
&{...} // *plot.Plot
<nil>  // error

Note: if you need several packages, you can first import all of them, then quit and recompile gomacro only once.


gomacro contains two alternative, experimental versions of Go generics:

  • the first version is modeled after C++ templates, and is appropriately named "C++ style"
    See doc/ for how to enable and use them.

  • the second version is named "contracts are interfaces" - or more briefly "CTI". It is modeled after several preliminary proposals for Go generics, most notably Ian Lance Taylor's Type Parameters in Go It has some additions inspired from Haskell generics and original contributions from the author - in particular to create a simpler alternative to Go 2 contracts

    For their design document and reasoning behind some of the design choices, see doc/

The second version of generics "CTI" is enabled by default in gomacro.

They are in beta status, and at the moment only generic types and functions are supported. Syntax and examples:

// declare a generic type with two type arguments T and U
type Pair#[T,U] struct {
	First T
	Second U

// instantiate the generic type using explicit types for T and U,
// and create a variable of such type.
var pair Pair#[complex64, struct{}]

// equivalent:
pair := Pair#[complex64, struct{}] {}

// a more complex example, showing higher-order functions
func Transform#[T,U](slice []T, trans func(T) U) []U {
	ret := make([]U, len(slice))
	for i := range slice {
		ret[i] = trans(slice[i])
	return ret
Transform#[string,int] // returns func([]string, func(string) int) []int

// returns []int{3, 2, 1} i.e. the len() of each string in input slice:
Transform#[string,int]([]string{"abc","xy","z"}, func(s string) int { return len(s) })

Contracts specify the available methods of a generic type. For simplicity, they do not introduce a new syntax or new language concepts: contracts are just (generic) interfaces. With a tiny addition, actually: the ability to optionally indicate the receiver type.

For example, the contract specifying that values of type T can be compared with each other to determine if the first is less, equal or greater than the second is:

type Comparable#[T] interface {
	// returns -1 if a is less than b
	// returns  0 if a is equal to b
	// returns  1 if a is greater than b
	func (a T) Cmp(b T) int

A type T implements Comparable#[T] if it has a method func (T) Cmp(T) int. This interface is carefully chosen to match the existing methods of *math/big.Float, *math/big.Int and *math/big.Rat. In other words, *math/big.Float, *math/big.Int and *math/big.Rat already implement it.

What about basic types as int8, int16, int32, uint... float*, complex* ... ? Gomacro extends them, automatically adding many methods equivalent to the ones declared on *math/big.Int to perform arithmetic and comparison, including Cmp which is internally defined as (no need to define it yourself):

func (a int) Cmp(b int) int {
	if a < b {
		return -1
	} else if a > b {
		return 1
	} else {
		return 0

Thus the generic functions Min and Max can be written as

func Min#[T: Comparable] (a, b T) T {
	if a.Cmp(b) < 0 { // also <= would work
		return a
	return b
func Max#[T: Comparable] (a, b T) T {
	if a.Cmp(b) > 0 { // also >= would work
		return a
	return b

Where the syntax #[T: Comparable] or equivalently #[T: Comparable#[T]] indicates that T must satisfy the contract (implement the interface) Comparable#[T]

Such functions Min and Max will then work automatically for every type T that satisfies the contract (implements the interface) Comparable#[T]:
all basic integers and floats, plus *math/big.Float, *math/big.Int and *math/big.Rat, plus every user-defined type T that has a method func (T) Cmp(T) int

If you do not specify the contract(s) that a type must satisfy, generic functions cannot access the fields and methods of a such type, which is then treated as a "black box", similarly to interface{}.

Two values of type T can be added if T has an appropriate method. But which name and signature should we choose to add values? Copying again from math/big, the method we choose is func (T) Add(T,T) T If receiver is a pointer, it will be set to the result - in any case, the result will also be returned. Similarly to Comparable, the contract Addable is then

type Addable#[T] interface {
	// Add two values a, b and return the result.
	// If recv is a pointer, it must be non-nil
	// and it will be set to the result
	func (recv T) Add(a, b T) T

With such a contract, a generic function Sum is quite straightforward:

func Sum#[T: Addable] (args ...T) T {
	// to create the zero value of T,
	// one can write 'var sum T' or equivalently 'sum := T()'
	// Unluckily, that's not enough for math/big numbers, which require
	// the receiver of method calls to be created with a function `New()`
	// Once math/big numbers have such method, the following
	// will be fully general - currently it works only on basic types.
	sum := T().New()

	for _, elem := range args {
		// use the method T.Add(T, T)
		// as an optimization, relevant at least for math/big numbers,
		// also use sum as the receiver where result of Add will be stored
		// if the method Add has pointer receiver.
		// To cover the case where method Add has instead value receiver,
		// also assign the returned value to sum
		sum = sum.Add(sum, elem)
	return sum
Sum#[int]         // returns func( int
Sum#[int] (1,2,3) // returns int(6)

Sum#[complex64]                 // returns func(...complex64) complex64
Sum#[complex64] (1.1+2.2i, 3.3) // returns complex64(4.4+2.2i)

Sum#[string]                         // returns func(...string) string
Sum#[string]("abc.","def.","xy","z") // returns ""

Partial and full specialization of generics is not supported in CTI generics, both for simplicity and to avoid accidentally providing Turing completeness at compile-time.

Instantiation of generic types and functions is on-demand.

Current limitations:

  • type inference on generic arguments #[...] is not yet implemented, thus generic arguments #[...] must be explicit.
  • generic methods are not yet implemented.
  • types are not checked to actually satisfy contracts.


Since version 2.6, gomacro also has an integrated debugger. There are three ways to enter it:

  • hit CTRL+C while interpreted code is running.
  • type :debug STATEMENT-OR-FUNCTION-CALL at the prompt.
  • add a statement (an expression is not enough) "break" or _ = "break" to your code, then execute it normally.

In all cases, execution will be suspended and you will get a debug> prompt, which accepts the following commands:
step, next, finish, continue, env [NAME], inspect EXPR, list, print EXPR-OR-STATEMENT


  • commands can be abbreviated.
  • print fully supports expressions or statements with side effects, including function calls and modifying local variables.
  • env without arguments prints all global and local variables.
  • an empty command (i.e. just pressing enter) repeats the last command.

Only interpreted statements can be debugged: expressions and compiled code will be executed, but you cannot step into them.

The debugger is quite new, and may have some minor glitches.

Why it was created

First of all, to experiment with Go :)

Second, to simplify Go code generation tools (keep reading for the gory details)

Problem: "go generate" and many other Go tools automatically create Go source code from some kind of description - usually an interface specifications as WSDL, XSD, JSON...

Such specification may be written in Go, for example when creating JSON marshallers/unmarshallers from Go structs, or in some other language, for example when creating Go structs from JSON sample data.

In both cases, a variety of external programs are needed to generate Go source code: such programs need to be installed separately from the code being generated and compiled.

Also, Go is currently lacking generics (read: C++-like templates) because of the rationale "we do not yet know how to do them right, and once you do them wrong everybody is stuck with them"

The purpose of Lisp-like macros is to execute arbitrary code while compiling, in particular to generate source code.

This makes them very well suited (although arguably a bit low level) for both purposes: code generation and C++-like templates, which are a special case of code generation - for a demonstration of how to implement C++-like templates on top of Lisp-like macros, see for example the project from the same author.

Building a Go interpreter that supports Lisp-like macros, allows to embed all these code-generation activities into regular Go source code, without the need for external programs (except for the interpreter itself).

As a free bonus, we get support for Eval()


Gomacro is distributed under the terms of Mozilla Public License 2.0 or any later version.