Make len() return runes count for string

Fixes #746

Author: antonmedv

builtin/lib.go

@@ -5,15 +5,18 @@ import (
 	"math"
 	"reflect"
 	"strconv"
+	"unicode/utf8"
 
 	"github.com/expr-lang/expr/internal/deref"
 )
 
 func Len(x any) any {
 	v := reflect.ValueOf(x)
 	switch v.Kind() {
-	case reflect.Array, reflect.Slice, reflect.Map, reflect.String:
+	case reflect.Array, reflect.Slice, reflect.Map:
 		return v.Len()
+	case reflect.String:
+		return utf8.RuneCountInString(v.String())
 	default:
 		panic(fmt.Sprintf("invalid argument for len (type %T)", x))
 	}

expr_test.go

@@ -871,6 +871,18 @@ func TestExpr(t *testing.T) {
 			`len("hello, world")`,
 			12,
 		},
+		{
+			`len('北京')`,
+			2,
+		},
+		{
+			`len('👍🏻')`, // one grapheme cluster, two code points
+			2,
+		},
+		{
+			`len('👍')`, // one grapheme cluster, one code point
+			1,
+		},
 		{
 			`len(ArrayOfInt)`,
 			5,