Add support for custom struct tags via environment variable

builtin/lib.go

@@ -2,6 +2,7 @@ package builtin
 
 import (
 	"fmt"
+	"github.com/expr-lang/expr/environment"
 	"math"
 	"reflect"
 	"strconv"
@@ -421,7 +422,7 @@ func get(params ...any) (out any, err error) {
 		fieldName := i.(string)
 		value := v.FieldByNameFunc(func(name string) bool {
 			field, _ := v.Type().FieldByName(name)
-			switch field.Tag.Get("expr") {
+			switch field.Tag.Get(environment.GetGoTag()) {
 			case "-":
 				return false
 			case fieldName:

checker/nature/utils.go

@@ -1,13 +1,14 @@
 package nature
 
 import (
+	"github.com/expr-lang/expr/environment"
 	"reflect"
 
 	"github.com/expr-lang/expr/internal/deref"
 )
 
 func fieldName(field reflect.StructField) (string, bool) {
-	switch taggedName := field.Tag.Get("expr"); taggedName {
+	switch taggedName := field.Tag.Get(environment.GetGoTag()); taggedName {
 	case "-":
 		return "", false
 	case "":

environment/environment.go

@@ -0,0 +1,16 @@
+package environment
+
+import "os"
+
+const (
+	ExprEnvGotag = "expr__gotag"
+	DefaultGotag = "expr"
+)
+
+func GetGoTag() string {
+	if v := os.Getenv(ExprEnvGotag); v != "" {
+		return v
+	} else {
+		return DefaultGotag
+	}
+}

expr_test.go

@@ -2890,3 +2890,52 @@ func TestIssue807(t *testing.T) {
 		t.Fatalf("expected 'in' operator to return false for unexported field")
 	}
 }
+
+func TestJsonGoTag(t *testing.T) {
+	type Embed struct {
+		Field string `json:"jsonField"`
+	}
+
+	type MyStruct struct {
+		Field string `json:"jsonField"`
+		Embed Embed  `json:"embedField"`
+	}
+
+	type EmbedNoTag struct {
+		Field string
+	}
+
+	type MyStructNoTag struct {
+		Field string
+		Embed EmbedNoTag
+	}
+
+	env := map[string]any{
+		"obj":      MyStruct{Field: "value", Embed: Embed{Field: "embedFieldValue"}},
+		"objNoTag": MyStructNoTag{Field: "value", Embed: EmbedNoTag{Field: "embedFieldValue"}},
+	}
+
+	if err := os.Setenv("expr__gotag", "json"); err != nil {
+		t.Fatalf("error setting env var: %v", err)
+	}
+	defer os.Unsetenv("expr__gotag")
+
+	code := `
+	{
+"fieldValue": obj.jsonField, 
+"embedFieldValue": obj.embedField.jsonField,
+"fieldValueNoTag": objNoTag.Field,
+"embedFieldValueNoTag": objNoTag.Embed.Field
+}
+`
+	prog, err := expr.Compile(code, expr.Env(env))
+	require.NoError(t, err)
+
+	out, err := expr.Run(prog, env)
+	require.NoError(t, err)
+	require.NotNil(t, out)
+	require.Equal(t, "value", out.(map[string]any)["fieldValue"])
+	require.Equal(t, "value", out.(map[string]any)["fieldValueNoTag"])
+	require.Equal(t, "embedFieldValue", out.(map[string]any)["embedFieldValue"])
+	require.Equal(t, "embedFieldValue", out.(map[string]any)["embedFieldValueNoTag"])
+}

vm/runtime/runtime.go

@@ -4,6 +4,7 @@ package runtime
 
 import (
 	"fmt"
+	"github.com/expr-lang/expr/environment"
 	"math"
 	"reflect"
 
@@ -65,7 +66,7 @@ func Fetch(from, i any) any {
 		fieldName := i.(string)
 		value := v.FieldByNameFunc(func(name string) bool {
 			field, _ := v.Type().FieldByName(name)
-			switch field.Tag.Get("expr") {
+			switch field.Tag.Get(environment.GetGoTag()) {
 			case "-":
 				return false
 			case fieldName:
@@ -223,7 +224,7 @@ func In(needle any, array any) bool {
 			panic(fmt.Sprintf("cannot use %T as field name of %T", needle, array))
 		}
 		field, ok := v.Type().FieldByName(n.String())
-		if !ok || !field.IsExported() || field.Tag.Get("expr") == "-" {
+		if !ok || !field.IsExported() || field.Tag.Get(environment.GetGoTag()) == "-" {
 			return false
 		}
 		value := v.FieldByIndex(field.Index)