traefik/yaegi

Yaegi is Another Elegant Go Interpreter. It powers executable Go scripts and plugins, in embedded interpreters or interactive shells, on top of the Go runtime.

• Complete support of Go specification
• Written in pure Go, using only the standard library
• Simple interpreter API: New(), Eval(), Use()
• Works everywhere Go works
• All Go & runtime resources accessible from script (with control)
• Security: unsafe and syscall packages neither used nor exported by default
• Support the latest 2 major releases of Go (Go 1.21 and Go 1.22)

import "github.com/traefik/yaegi/interp"

go install github.com/traefik/yaegi/cmd/yaegi@latest

Note that you can use rlwrap (install with your favorite package manager), and alias the yaegi command in alias yaegi='rlwrap yaegi' in your ~/.bashrc, to have history and command line edition.

curl -sfL https://raw.githubusercontent.com/traefik/yaegi/master/install.sh | bash -s -- -b $GOPATH/bin v0.9.0

Create an interpreter with New(), run Go code with Eval():

package main

import (
	"github.com/traefik/yaegi/interp"
	"github.com/traefik/yaegi/stdlib"
)

func main() {
	i := interp.New(interp.Options{})

	i.Use(stdlib.Symbols)

	_, err := i.Eval(`import "fmt"`)
	if err != nil {
		panic(err)
	}

	_, err = i.Eval(`fmt.Println("Hello Yaegi")`)
	if err != nil {
		panic(err)
	}
}

Go Playground

The following program is compiled ahead of time, except bar() which is interpreted, with the following steps:

1. use of i.Eval(src) to evaluate the script in the context of interpreter
2. use of v, err := i.Eval("foo.Bar") to get the symbol from the interpreter context, as a reflect.Value
3. application of Interface() method and type assertion to convert v into bar, as if it was compiled

package main

import "github.com/traefik/yaegi/interp"

const src = `package foo
func Bar(s string) string { return s + "-Foo" }`

func main() {
	i := interp.New(interp.Options{})

	_, err := i.Eval(src)
	if err != nil {
		panic(err)
	}

	v, err := i.Eval("foo.Bar")
	if err != nil {
		panic(err)
	}

	bar := v.Interface().(func(string) string)

	r := bar("Kung")
	println(r)
}

Go Playground

The Yaegi command can run an interactive Read-Eval-Print-Loop:

$ yaegi
> 1 + 2
3
> import "fmt"
> fmt.Println("Hello World")
Hello World
>

Note that in interactive mode, all stdlib package are pre-imported, you can use them directly:

$ yaegi
> reflect.TypeOf(time.Date)
: func(int, time.Month, int, int, int, int, int, *time.Location) time.Time
>

Or interpret Go packages, directories or files, including itself:

$ yaegi -syscall -unsafe -unrestricted github.com/traefik/yaegi/cmd/yaegi
>

Or for Go scripting in the shebang line:

$ cat /tmp/test
#!/usr/bin/env yaegi
package main

import "fmt"

func main() {
	fmt.Println("test")
}
$ ls -la /tmp/test
-rwxr-xr-x 1 dow184 dow184 93 Jan  6 13:38 /tmp/test
$ /tmp/test
test

Documentation about Yaegi commands and libraries can be found at usual godoc.org.

Beside the known bugs which are supposed to be fixed in the short term, there are some limitations not planned to be addressed soon:

• Assembly files (.s) are not supported.
• Calling C code is not supported (no virtual "C" package).
• Directives about the compiler, the linker, or embedding files are not supported.
• Interfaces to be used from the pre-compiled code can not be added dynamically, as it is required to pre-compile interface wrappers.
• Representation of types by reflect and printing values using %T may give different results between compiled mode and interpreted mode.
• Interpreting computation intensive code is likely to remain significantly slower than in compiled mode.

Go modules are not supported yet. Until that, it is necessary to install the source into $GOPATH/src/github.com/traefik/yaegi to pass all the tests.

Contributing guide.

Apache 2.0.