Added list tag

README.md

@@ -86,6 +86,13 @@ regexp
 
 nonnil
 	Validates that the given value is not nil. (Usage: nonnil)
+
+list
+	Validates that the given value is present in a list of
+	provided values. These values should be provided inside
+	square brackets separated by comma without any additional
+	spaces. It can be used with strings, integers and floating
+	point numbers. (Usage: list=[FOO,BAR,BAZ])
 ```
 
 Custom validators

builtins.go

@@ -20,6 +20,7 @@ import (
 	"reflect"
 	"regexp"
 	"strconv"
+	"strings"
 	"unicode/utf8"
 )
 
@@ -287,3 +288,66 @@ func nonnil(v interface{}, param string) error {
 	}
 	return nil
 }
+
+// list validates that the given value exists in the list of values
+func list(v interface{}, param string) error {
+	value := reflect.ValueOf(v)
+	if value.Kind() == reflect.Ptr {
+		if value.IsNil() {
+			return nil
+		}
+		value = value.Elem()
+	}
+
+	// get array of possible values
+	l := len(param)
+	if l < 3 || param[0] != '[' || param[l-1] != ']' {
+		return ErrBadParameter
+	}
+	param = param[1 : l-1]
+
+	possible := strings.Split(param, ",")
+
+	switch value.Kind() {
+	case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
+		for _, str := range possible {
+			i, err := strconv.ParseInt(str, 10, 64)
+			if err != nil {
+				return ErrBadParameter
+			}
+			if value.Int() == i {
+				return nil
+			}
+		}
+	case reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64, reflect.Uintptr:
+		for _, str := range possible {
+			i, err := strconv.ParseUint(str, 10, 64)
+			if err != nil {
+				return ErrBadParameter
+			}
+			if value.Uint() == i {
+				return nil
+			}
+		}
+	case reflect.Float32, reflect.Float64:
+		for _, str := range possible {
+			f, err := strconv.ParseFloat(str, 64)
+			if err != nil {
+				return ErrBadParameter
+			}
+			if value.Float() == f {
+				return nil
+			}
+		}
+	case reflect.String:
+		for _, str := range possible {
+			if value.String() == str {
+				return nil
+			}
+		}
+	default:
+		return ErrUnsupported
+	}
+
+	return ErrInvalid
+}

validator.go

@@ -140,6 +140,7 @@ func NewValidator() *Validator {
 			"max":     max,
 			"regexp":  regex,
 			"nonnil":  nonnil,
+			"list":    list,
 		},
 		printJSON: false,
 	}