ability to ignore undefined flags per parser

flagutil.go

@@ -37,7 +37,8 @@ func (fn PrinterFunc) Name(ctx context.Context, fs parse.FlagSet) (func(*flag.Fl
 
 type parser struct {
 	Parser
-	stash func(*flag.Flag) bool
+	stash           func(*flag.Flag) bool
+	ignoreUndefined bool
 }
 
 type config struct {
@@ -72,6 +73,10 @@ func Parse(ctx context.Context, flags *flag.FlagSet, opts ...ParseOption) (err e
 	for _, p := range c.parsers {
 		parse.NextLevel(fs)
 		parse.Stash(fs, p.stash)
+		if p.ignoreUndefined {
+			// User may ask to ignore undefined per parser as well.
+			parse.IgnoreUndefined(fs)
+		}
 
 		if err = p.Parse(ctx, fs); err != nil {
 			if err == flag.ErrHelp {

options.go

@@ -28,8 +28,8 @@ func (fn ParseOptionFunc) setupParseConfig(c *config) { fn(c) }
 
 type ParserOptionFunc func(*config, *parser)
 
-func (fn ParserOptionFunc) setupParserConfig(p *parser) { fn(nil, p) }
 func (fn ParserOptionFunc) setupParseConfig(c *config)  { fn(c, nil) }
+func (fn ParserOptionFunc) setupParserConfig(p *parser) { fn(nil, p) }
 
 func stashFunc(check func(*flag.Flag) bool) (opt ParserOptionFunc) {
 	return ParserOptionFunc(func(c *config, p *parser) {
@@ -79,9 +79,14 @@ func WithParser(p Parser, opts ...ParserOption) ParseOptionFunc {
 }
 
 // WithIgnoreUndefined makes Parse() to not fail on setting undefined flag.
-func WithIgnoreUndefined() ParseOptionFunc {
-	return ParseOptionFunc(func(c *config) {
-		c.ignoreUndefined = true
+func WithIgnoreUndefined() ParserOptionFunc {
+	return ParserOptionFunc(func(c *config, p *parser) {
+		switch {
+		case c != nil:
+			c.ignoreUndefined = true
+		case p != nil:
+			p.ignoreUndefined = true
+		}
 	})
 }
 

parse/flagset.go

@@ -39,6 +39,11 @@ func Stash(fs FlagSet, fn func(*flag.Flag) bool) {
 	fset.stash = fn
 }
 
+func IgnoreUndefined(fs FlagSet) {
+	fset := fs.(*flagSet)
+	fset.ignoreUndefined = true
+}
+
 type flagSet struct {
 	dest            *flag.FlagSet
 	ignoreUndefined bool