Unify typed binding preparation

command.go

@@ -58,12 +58,21 @@ type Flag[I any] struct {
 	binding flagBinding[I]
 }
 
+type bindingIdentity struct{ marker byte }
+
+type preparedValue[I any] struct {
+	identity *bindingIdentity
+	supplied bool
+	apply    func(*I)
+}
+
 type flagBinding[I any] struct {
+	identity    *bindingIdentity
 	meta        FlagMeta
 	defaultText string
 	repeated    bool
 	boolean     bool
-	prepare     func([]string) (func(*I), error)
+	prepare     func([]string) (preparedValue[I], error)
 	clear       func(*I)
 	valid       bool
 }
@@ -74,11 +83,12 @@ type Argument[I any] struct {
 }
 
 type argumentBinding[I any] struct {
-	meta    ArgumentMeta
-	min     int
-	max     int
-	prepare func([]string) (func(*I), error)
-	valid   bool
+	identity *bindingIdentity
+	meta     ArgumentMeta
+	min      int
+	max      int
+	prepare  func([]string) (preparedValue[I], error)
+	valid    bool
 }
 
 // CommandSpec declares one command using typed input and handler callbacks.
@@ -319,7 +329,7 @@ func validateBindings[I any](flags []Flag[I], arguments []Argument[I]) error {
 			}
 			shortNames[binding.meta.Short] = struct{}{}
 		}
-		if !binding.valid || binding.prepare == nil {
+		if binding.identity == nil || !binding.valid || binding.prepare == nil {
 			return fmt.Errorf("flag %q is missing parser or setter wiring", binding.meta.Name)
 		}
 	}
@@ -332,7 +342,7 @@ func validateBindings[I any](flags []Flag[I], arguments []Argument[I]) error {
 		if binding.min < 0 || binding.max < -1 || (binding.max >= 0 && binding.min > binding.max) {
 			return fmt.Errorf("argument %q has invalid bounds %d..%d", binding.meta.Name, binding.min, binding.max)
 		}
-		if !binding.valid || binding.prepare == nil {
+		if binding.identity == nil || !binding.valid || binding.prepare == nil {
 			return fmt.Errorf("argument %q is missing parser or setter wiring", binding.meta.Name)
 		}
 	}