Is there an equivalent to JavaScript's `eval()` function for dynamic evaluation of Go code?

Evaluating Go Code with eval()

Introduction

In JavaScript, the eval() function allows you to execute code or expressions dynamically. Is there an equivalent function in Go that can evaluate Go code?

Answer

Yes, it is possible to evaluate Go expressions in a dynamic manner. The key component for this is the go/types package. Here's how you can do it:

Creating a Package and Scope

First, create a package object to hold the code to be evaluated, and a scope.Scope object to define the scope in which the code will execute.

package eval

import (
    "go/ast"
    "go/constant"
    "go/parser"
    "go/token"
    "go/types"
)

var (
    // Create a new package object to hold the evaluated code.
    pkg = types.NewPackage("eval", "example.com/eval")

    // Create a new scope object to define the scope of evaluation.
    scope = types.NewScope(nil, token.NewFileSet())
)

Inserting Constants into Scope

The eval() function typically allows evaluating expressions that reference defined variables or constants. To simulate this behavior in Go, we can insert constants into the scope of evaluation.

// Insert a constant named "x" with value 10 into the scope.
scope.Insert(scope.Lookup("x"), &types.Const{
    Val:    constant.MakeInt64(10),
    Type:   pkg.Scope().Lookup("int").Type(), // Lookup the "int" type from the package's scope.
    Pkg:    pkg,
    Name:   "x",
    Kind:   types.Const,
    Obj:    nil, // We don't need an Object for introducing constants directly.
    Alias:  false,
})

Parsing and Evaluating Expressions

Next, you need to parse the Go expression to be evaluated and create an AST (Abstract Syntax Tree) for it. Once you have the AST, you can evaluate the expression with the help of the go/types package.

// Parse the input Go expression.
expr, err := parser.ParseExpr("x + 17")
if err != nil {
    panic(err)
}

// Evaluate the expression in the defined scope.
result, err := types.Eval(expr, scope)
if err != nil {
    panic(err)
}

Result and Conclusion

The result of the evaluation will be stored in the result variable as a constant.Value. You can convert it to the desired type as needed. In your example, you can obtain the result using:

intResult, ok := constant.Int64Val(result)
if !ok {
    panic("failed to convert result to int")
}

By following these steps, you can achieve dynamic evaluation of Go code, similar to the eval() function in JavaScript.

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