add min/max length validation, improve tests

length.go

@@ -0,0 +1,32 @@
+package validate
+
+import (
+	"errors"
+	"fmt"
+)
+
+// MaxLength validates the length of a string as being less than or equal to a given maximum.
+func MaxLength(l int) func(string) error {
+	return func(value string) error {
+		if len(value) > l {
+			if l != 1 {
+				return fmt.Errorf("Must not be longer than %d characters", l)
+			}
+			return errors.New("Must not be longer than 1 character")
+		}
+		return nil
+	}
+}
+
+// MinLength validates the length of a string as being greater than or equal to a given minimum.
+func MinLength(l int) func(string) error {
+	return func(value string) error {
+		if len(value) < l {
+			if l != 1 {
+				return fmt.Errorf("Must not be shorter than %d characters", l)
+			}
+			return errors.New("Must not be shorter than 1 character")
+		}
+		return nil
+	}
+}