Allow arbitrary nesting of struct tag values.

This uses a pattern that leverages the idea that all of these values are
just maps, and an optional path can be provided to access a value at an
arbitrary nested level in the struct tags.

Why would you want to do this? I don't know, it's probably not a good
idea to go any further than one or two nestings.

However, it does make the signature a little nicer to work with as this
makes the second param optional.

Author: kevinfalting

stronf/field.go

@@ -68,24 +68,6 @@ func (f Field) set(val any) error {
 	return nil
 }
 
-// LookupTag will return the value associated with the key and optional tag. See
-// examples for supported formats. The bool reports if the key or tag was
-// explicitly found in the struct tag.
-func (f Field) LookupTag(key, tag string) (string, bool) {
-	value, ok := f.rStructField.Tag.Lookup(key)
-	if !ok {
-		return "", false
-	}
-
-	if tag == "" {
-		return value, true
-	}
-
-	tags := parseStructTag(value)
-	val, ok := tags[tag]
-	return val, ok
-}
-
 // Parse will call the handler against the field and set the field to the
 // returned handler value. If the handler returns nil, no change is made to the
 // field.

stronf/field_test.go

@@ -50,7 +50,7 @@ func TestField_LookupTag(t *testing.T) {
 		}
 
 		field := fields[0]
-		v, ok := field.LookupTag("key", "")
+		v, ok := field.LookupTag("key")
 		if !ok {
 			t.Error("expected key to exist")
 		}
@@ -74,7 +74,7 @@ func TestField_LookupTag(t *testing.T) {
 		}
 
 		field := fields[0]
-		v, ok := field.LookupTag("key", "")
+		v, ok := field.LookupTag("key")
 		if !ok {
 			t.Error("expected key to exist")
 		}
@@ -140,7 +140,7 @@ func ExampleField_LookupTag() {
 		log.Println("should not recieve a value for a key that doesn't exist")
 	}
 
-	if val, ok := field.LookupTag("emptyKey", ""); ok {
+	if val, ok := field.LookupTag("emptyKey"); ok {
 		fmt.Printf("val: %q\n", val)
 	}
 

stronf/structtag.go

@@ -2,18 +2,40 @@ package stronf
 
 import "strings"
 
-func parseStructTag(tag string) map[string]string {
-	options := make(map[string]string)
-	for _, option := range strings.Split(tag, ",") {
-		optionParts := strings.SplitN(option, ":", 2)
-		key := optionParts[0]
-		var value string
-		if len(optionParts) > 1 {
-			value = optionParts[1]
+// LookupTag will return the value associated with the tag and optional path.
+// The tag arg is the only required argument and uses the reflect package's
+// Lookup semantics. An optional path can be provided to lookup nested values.
+// Nested values can themselves be maps, each key/val pair separated by a comma.
+// See examples for supported formats. The bool reports if the value was
+// explicitly found at the struct tag path.
+func (f Field) LookupTag(tag string, path ...string) (string, bool) {
+	value, ok := f.rStructField.Tag.Lookup(tag)
+	if !ok {
+		return "", false
+	}
+
+	if len(path) == 0 {
+		return value, true
+	}
+
+	parseStructTag := func(tag string) map[string]string {
+		subTags := make(map[string]string)
+		for _, subTag := range strings.Split(tag, ",") {
+			key, val, _ := strings.Cut(subTag, ":")
+			subTags[key] = val
 		}
 
-		options[key] = value
+		return subTags
+	}
+
+	val, ok := "", false
+	for _, p := range path {
+		tags := parseStructTag(value)
+		val, ok = tags[p]
+		if !ok {
+			return "", false
+		}
 	}
 
-	return options
+	return val, ok
 }