add validate.Equal

equal.go

@@ -0,0 +1,11 @@
+package validate
+
+// Equal validates whether an input value is equal to a comparison value.
+func Equal[T comparable](cmp T) func(T) error {
+	return func(value T) error {
+		if value != cmp {
+			return ErrValueNotAllowed
+		}
+		return nil
+	}
+}