How to detect undefined properties?

[fmarcia]

By default, the code var == 3 returns an error if var is not defined but obj.prop == 3 does not if prop is not a property of obj (obj.prop is evaluated to nil).
Is there a way to trigger an error in the latter case?

[antonmedv]

• If obj is map[string]any, prop or any other string may be there.
• If obj is a struct type Obj struct { Prop int `expr:"prop"` }, you will get an error.

[fmarcia]

For the record, here is how I got through it:

package main

import (
	"fmt"
	"strings"

	"github.com/expr-lang/expr/ast"
	"github.com/expr-lang/expr/parser"
)

type (
	T = map[string]any

	visitor struct {
		symbols []string
	}
)

func main() {

	env := T{"var": 1, "obj": T{"prop": T{"sub": "abc"}}}
	tests := []struct {
		code    string
		unknown string
	}{
		{`obj.prop.sub == "abc" || var == 1`, ""},
		{`obj2.prop.sub == "abc" || var == 1`, "obj2"},
		{`obj.prop2.sub == "abc" || var == 1`, "obj.prop2"},
		{`obj.prop.sub2 == "abc" || var == 1`, "obj.prop.sub2"},
		{`obj.prop2.sub2 == "abc" || var == 1`, "obj.prop2"},
		{`obj2.prop2.sub2 == "abc" || var == 1`, "obj2"},
		{`obj.prop.sub == "abc" || var2 == 1`, "var2"},
		{`obj2.prop.sub == "abc" || var2 == 1`, "obj2"},
	}

	fmt.Println("with", env)
	for _, t := range tests {

		tree, _ := parser.Parse(t.code)
		v := &visitor{}
		ast.Walk(&tree.Node, v)

		unknown := check(env, v.symbols)
		fmt.Printf("[%s]\t(unknown == %s) == %v\n", t.code, `"`+unknown+`"`, t.unknown == unknown)
	}
}

func (v *visitor) Visit(node *ast.Node) {
	if s, ok := (*node).(*ast.MemberNode); ok {
		v.symbols = append(v.symbols, s.String())
	} else if s, ok := (*node).(*ast.IdentifierNode); ok {
		v.symbols = append(v.symbols, s.String())
	}
}

func check(env T, symbols []string) string {

	for _, symbol := range symbols {

		var current any = env
		var acc []string

		parts := strings.Split(symbol, ".")
		last := len(parts) - 1

		for rank, part := range parts {

			current = current.(T)[part]
			acc = append(acc, part)
			if rank == last && current == nil {
				return strings.Join(acc, ".")
			}
		}
	}

	return ""
}