tagged defaults

[willbtlr]

Hey there! I'd love to be able to set a custom default for a type based on the tag like so:

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"`
}

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) }

This PR adds the interface and modifies the Set() logic to use it. Added tests to account for success and failure. All tests passing. Thanks!

[willbtlr]

Ah it looks like IsExported() needs go >= 1.17. I could just check the case of the first char of the field?

[servusdei2018]

See golang/go#41563, basically you'll want to check if t.Field(i).PkgPath != "" (exported). StructField.PkgPath is populated if and only if the field is unexported.

[servusdei2018]

if t.Field(i).IsExported() {

// Check whether the field is exported
if t.Field(i).PkgPath != "" {

[creasty]

func (d *Duration) SetTaggedDefaults(tag string) (err error) {

I would utilize https://pkg.go.dev/encoding/json#Unmarshaler or https://pkg.go.dev/encoding#TextUnmarshaler interfaces instead.