strings: add StringsOr for optional slice of strings

forms.go

@@ -123,13 +123,26 @@ func (p *stringsParser[T]) Parse(values []string) error {
 }
 
 // Strings is used to extract a form data value into a slice of Go strings.
+//
+// If the form value is missing, then an error is returned during parsing.
 func Strings[T StringType](s *[]T) Parser {
 	return &stringsParser[T]{
 		required:    true,
 		destination: s,
 	}
 }
 
+// StringsOr is used to extract multiple form values for a given key into a slice of Go strings.
+//
+// If the form value is missing, then the given alt value is used instead.
+func StringsOr[T StringType](s *[]T, alt []T) Parser {
+	*s = alt
+	return &stringsParser[T]{
+		required:    false,
+		destination: s,
+	}
+}
+
 // Secret is used to extract a form data value into a Go conceal.Text. If the
 // value is missing then an error is returned during parsing.
 func Secret(s **conceal.Text) Parser {

forms_test.go

@@ -364,7 +364,7 @@ func Test_Parse_IntType_IntOr(t *testing.T) {
 	must.Eq(t, 100, age)
 }
 
-func Test_Parse_StringType_Strings(t *testing.T) {
+func Test_Parse_StringType_Strings_present(t *testing.T) {
 	t.Parallel()
 
 	data := url.Values{
@@ -379,3 +379,50 @@ func Test_Parse_StringType_Strings(t *testing.T) {
 	must.NoError(t, err)
 	must.Eq(t, []string{"alice", "bob", "carol"}, names)
 }
+
+func Test_Parse_StringType_Strings_missing(t *testing.T) {
+	t.Parallel()
+
+	data := url.Values{
+		"names": []string{"alice", "bob", "carol"},
+	}
+
+	var jobs []string
+
+	err := ParseValues(data, Schema{
+		"jobs": Strings(&jobs),
+	})
+	must.Error(t, err)
+}
+
+func Test_Parse_StringType_StringsOr_present(t *testing.T) {
+	t.Parallel()
+
+	data := url.Values{
+		"names": []string{"alice", "bob", "carol"},
+	}
+
+	var names []string
+
+	err := ParseValues(data, Schema{
+		"names": StringsOr(&names, []string{"zed", "yulia"}),
+	})
+	must.NoError(t, err)
+	must.Eq(t, []string{"alice", "bob", "carol"}, names)
+}
+
+func Test_Parse_StringType_StringsOr_missing(t *testing.T) {
+	t.Parallel()
+
+	data := url.Values{
+		"names": []string{"alice", "bob", "carol"},
+	}
+
+	var jobs []string
+
+	err := ParseValues(data, Schema{
+		"jobs": StringsOr(&jobs, []string{"janitor", "cashier"}),
+	})
+	must.NoError(t, err)
+	must.Eq(t, []string{"janitor", "cashier"}, jobs)
+}