From f7ef98fea662d1d43d9a79c8277762649e953dd1 Mon Sep 17 00:00:00 2001
From: will butler <will@btlr.dev>
Date: Mon, 1 May 2023 14:10:33 -0400
Subject: [PATCH] tagged defaults

---
 .gitignore       |  1 +
 defaults.go      | 10 ++++++++++
 defaults_test.go | 42 ++++++++++++++++++++++++++++++++++++++++++
 setter.go        | 11 +++++++++++
 4 files changed, 64 insertions(+)

diff --git a/.gitignore b/.gitignore
index e43b0f9..8d979a0 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1 +1,2 @@
 .DS_Store
+.idea/*
diff --git a/defaults.go b/defaults.go
index b5e7eb9..cf404a2 100644
--- a/defaults.go
+++ b/defaults.go
@@ -34,6 +34,16 @@ func Set(ptr interface{}) error {
 
 	for i := 0; i < t.NumField(); i++ {
 		if defaultVal := t.Field(i).Tag.Get(fieldName); defaultVal != "-" {
+			if t.Field(i).IsExported() {
+				iface := v.Field(i).Addr().Interface()
+				if taggedSetter, ok := iface.(TaggedSetter); ok {
+					if err := taggedSetter.SetTaggedDefaults(defaultVal); err != nil {
+						return err
+					}
+					continue
+				}
+			}
+
 			if err := setField(v.Field(i), defaultVal); err != nil {
 				return err
 			}
diff --git a/defaults_test.go b/defaults_test.go
index 350ad62..de9f18c 100644
--- a/defaults_test.go
+++ b/defaults_test.go
@@ -6,6 +6,7 @@ import (
 	"net"
 	"reflect"
 	"strconv"
+	"strings"
 	"testing"
 	"time"
 
@@ -757,3 +758,44 @@ func TestDefaultsSetter(t *testing.T) {
 		t.Errorf("expected 1 for MainInt, got %d", main.MainInt)
 	}
 }
+
+type Duration struct {
+	time.Duration
+}
+
+func (d *Duration) SetTaggedDefaults(tag string) (err error) {
+	d.Duration, err = time.ParseDuration(tag)
+	return
+}
+
+type TaggedDefaults struct {
+	Duration Duration `default:"1s"`
+}
+
+func TestTaggedDefaultsSetterHappy(t *testing.T) {
+	sample := &TaggedDefaults{}
+	err := Set(sample)
+	if err != nil {
+		t.Errorf("unexpected error: %s", err)
+	}
+
+	if sample.Duration.Duration != time.Second {
+		t.Errorf("expected 1s for Duration, got %s", sample.Duration.Duration)
+	}
+}
+
+type TaggedDefaultsSad struct {
+	Duration Duration `default:"invalid"`
+}
+
+func TestTaggedDefaultsSetterSad(t *testing.T) {
+	sample := &TaggedDefaultsSad{}
+	err := Set(sample)
+	if err == nil {
+		t.Error("unexpected success?!")
+	}
+
+	if !strings.HasPrefix(err.Error(), "time: invalid duration") {
+		t.Errorf("unexpected error: %s", err)
+	}
+}
diff --git a/setter.go b/setter.go
index 1f64aa6..05a27e4 100644
--- a/setter.go
+++ b/setter.go
@@ -10,3 +10,14 @@ func callSetter(v interface{}) {
 		ds.SetDefaults()
 	}
 }
+
+type TaggedSetter interface {
+	SetTaggedDefaults(tag string) error
+}
+
+func callTaggedSetter(v interface{}, tag string) error {
+	if ds, ok := v.(TaggedSetter); ok {
+		return ds.SetTaggedDefaults(tag)
+	}
+	return nil
+}