improve handling of Fanout args (#42)

* feat: make Fanout a no-op when len(handlers) == 1

Updates #41

* feat: flatten FanoutHandlers in Fanout

Fixes #41

Author: josharian

multi.go

@@ -21,6 +21,8 @@ type FanoutHandler struct {
 }
 
 // Fanout creates a new FanoutHandler that distributes records to multiple slog.Handler instances.
+// If exactly one handler is provided, it returns that handler unmodified.
+// If you pass a FanoutHandler as an argument, its handlers are flattened into the new FanoutHandler.
 // This function is the primary entry point for creating a multi-handler setup.
 //
 // Example usage:
@@ -39,8 +41,20 @@ type FanoutHandler struct {
 //
 //	A slog.Handler that forwards all operations to the provided handlers
 func Fanout(handlers ...slog.Handler) slog.Handler {
+	var flat []slog.Handler
+	for _, handler := range handlers {
+		if fan, ok := handler.(*FanoutHandler); ok {
+			flat = append(flat, fan.handlers...)
+		} else {
+			flat = append(flat, handler)
+		}
+	}
+
+	if len(flat) == 1 {
+		return flat[0]
+	}
 	return &FanoutHandler{
-		handlers: handlers,
+		handlers: flat,
 	}
 }
 

multi_test.go

@@ -0,0 +1,56 @@
+package slogmulti
+
+import (
+	"io"
+	"log/slog"
+	"testing"
+)
+
+func TestFanoutIncrementalBuildFlattensHandlers(t *testing.T) {
+	t.Parallel()
+
+	h1 := slog.NewJSONHandler(io.Discard, nil)
+	h2 := slog.NewJSONHandler(io.Discard, nil)
+	h3 := slog.NewJSONHandler(io.Discard, nil)
+
+	var handler slog.Handler = h1
+	handler = Fanout(handler, h2)
+	handler = Fanout(handler, h3)
+
+	fanout, ok := handler.(*FanoutHandler)
+	if !ok {
+		t.Fatalf("expected FanoutHandler, got %T", handler)
+	}
+	if len(fanout.handlers) != 3 {
+		t.Fatalf("expected 3 handlers, got %d", len(fanout.handlers))
+	}
+	if fanout.handlers[0] != h1 || fanout.handlers[1] != h2 || fanout.handlers[2] != h3 {
+		t.Fatalf("handlers were not flattened correctly")
+	}
+}
+
+func TestFanoutFlattensMixedNestedHandlers(t *testing.T) {
+	t.Parallel()
+
+	h1 := slog.NewJSONHandler(io.Discard, nil)
+	h2 := slog.NewJSONHandler(io.Discard, nil)
+	h3 := slog.NewJSONHandler(io.Discard, nil)
+	h4 := slog.NewJSONHandler(io.Discard, nil)
+
+	handler := Fanout(Fanout(h1, h2), h3, Fanout(h4))
+
+	fanout, ok := handler.(*FanoutHandler)
+	if !ok {
+		t.Fatalf("expected FanoutHandler, got %T", handler)
+	}
+	if len(fanout.handlers) != 4 {
+		t.Fatalf("expected 4 handlers, got %d", len(fanout.handlers))
+	}
+
+	if fanout.handlers[0] != h1 ||
+		fanout.handlers[1] != h2 ||
+		fanout.handlers[2] != h3 ||
+		fanout.handlers[3] != h4 {
+		t.Fatalf("handlers were not flattened correctly")
+	}
+}