fix: add RenderMessage() method for custom sinks (#64)

CHANGELOG.md

@@ -8,6 +8,13 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ## [Unreleased]
 
 ### Added
+- **RenderMessage() Method** - Public API for rendering message templates (#64)
+  - New `RenderMessage()` method on `core.LogEvent` for custom sinks
+  - Properly handles capturing operators (`{@Property}`), scalar hints (`{$Property}`), and format specifiers (`{Property:format}`)
+  - Enables custom sinks to render message templates without accessing internal parser
+  - Returns the original template as fallback if parsing fails
+  - Example: `message := event.RenderMessage()` renders the template with all properties
+
 - **Zed Extension** - Full editor support via Language Server Protocol (#56)
   - New `mtlog-lsp` command providing LSP wrapper for mtlog-analyzer
   - Native Zed extension using Rust/WASM (wasm32-wasip2 target)

README.md

@@ -1367,16 +1367,39 @@ The extension provides:
 Implement the `core.LogEventSink` interface for custom outputs:
 
 ```go
-type CustomSink struct{}
+type CustomSink struct {
+    output io.Writer
+}
+
+func (s *CustomSink) Emit(event *core.LogEvent) {
+    // Use RenderMessage() to properly render the message template
+    // This handles format specifiers, capturing operators, and scalar hints
+    message := event.RenderMessage()
+
+    // Format and write the log entry
+    timestamp := event.Timestamp.Format("15:04:05")
+    fmt.Fprintf(s.output, "[%s] %s: %s\n", timestamp, event.Level, message)
+
+    // Optionally include extra properties not in the template
+    for key, value := range event.Properties {
+        if !strings.Contains(event.MessageTemplate, "{"+key) {
+            fmt.Fprintf(s.output, "  %s: %v\n", key, value)
+        }
+    }
+}
 
-func (s *CustomSink) Emit(event *core.LogEvent) error {
-    // Process the log event
+func (s *CustomSink) Close() error {
+    // Cleanup if needed
     return nil
 }
 
+// Use your custom sink
 log := mtlog.New(
-    mtlog.WithSink(&CustomSink{}),
+    mtlog.WithSink(&CustomSink{output: os.Stdout}),
 )
+
+// This will properly render with RenderMessage():
+log.Info("User {@User} performed {Action} at {Time:HH:mm}", user, "login", time.Now())
 ```
 
 ### Custom Enrichers

core/event.go

@@ -1,6 +1,11 @@
 package core
 
-import "time"
+import (
+	"time"
+
+	"github.com/willibrandon/mtlog/internal/parser"
+	"github.com/willibrandon/mtlog/selflog"
+)
 
 // LogEvent represents a single log event with all its properties.
 type LogEvent struct {
@@ -31,3 +36,34 @@ func (e *LogEvent) AddPropertyIfAbsent(property *LogEventProperty) {
 func (e *LogEvent) AddProperty(name string, value any) {
 	e.Properties[name] = value
 }
+
+// RenderMessage renders the message template with the event's properties.
+// This method parses the MessageTemplate and replaces all placeholders with their
+// corresponding property values, handling format specifiers, capturing operators,
+// and scalar hints.
+//
+// If parsing fails, the original MessageTemplate is returned as a fallback.
+//
+// Example:
+//
+//	event := &LogEvent{
+//	    MessageTemplate: "User {UserId} logged in from {City}",
+//	    Properties: map[string]any{
+//	        "UserId": 123,
+//	        "City": "Seattle",
+//	    },
+//	}
+//	message := event.RenderMessage()  // "User 123 logged in from Seattle"
+func (e *LogEvent) RenderMessage() string {
+	tmpl, err := parser.Parse(e.MessageTemplate)
+	if err != nil {
+		// Log parsing error to selflog if enabled
+		if selflog.IsEnabled() {
+			selflog.Printf("[core] template parse error in RenderMessage: %v (template=%q)", err, e.MessageTemplate)
+		}
+		// Fallback to raw template on parse error
+		return e.MessageTemplate
+	}
+
+	return tmpl.Render(e.Properties)
+}

core/event_test.go

@@ -0,0 +1,77 @@
+package core
+
+import (
+	"testing"
+)
+
+// TestRenderMessage demonstrates that RenderMessage() fixes issue #64.
+// It shows that RenderMessage() correctly handles all template features.
+func TestRenderMessage(t *testing.T) {
+	t.Run("capturing operator", func(t *testing.T) {
+		event := &LogEvent{
+			MessageTemplate: "Configuration: {@Config}",
+			Properties: map[string]any{
+				"Config": map[string]any{
+					"debug": true,
+					"port":  8080,
+				},
+			},
+		}
+
+		got := event.RenderMessage()
+		expected := "Configuration: map[debug:true port:8080]"
+
+		if got != expected {
+			t.Errorf("RenderMessage() = %q, want %q", got, expected)
+		}
+	})
+
+	t.Run("scalar hint", func(t *testing.T) {
+		event := &LogEvent{
+			MessageTemplate: "Values: {$Values}",
+			Properties: map[string]any{
+				"Values": []int{1, 2, 3},
+			},
+		}
+
+		got := event.RenderMessage()
+		expected := "Values: [1 2 3]"
+
+		if got != expected {
+			t.Errorf("RenderMessage() = %q, want %q", got, expected)
+		}
+	})
+
+	t.Run("format specifier", func(t *testing.T) {
+		event := &LogEvent{
+			MessageTemplate: "Count: {Count:000}",
+			Properties: map[string]any{
+				"Count": 42,
+			},
+		}
+
+		got := event.RenderMessage()
+		expected := "Count: 042"
+
+		if got != expected {
+			t.Errorf("RenderMessage() = %q, want %q", got, expected)
+		}
+	})
+
+	t.Run("simple properties", func(t *testing.T) {
+		event := &LogEvent{
+			MessageTemplate: "User {UserId} logged in from {City}",
+			Properties: map[string]any{
+				"UserId": 123,
+				"City":   "Seattle",
+			},
+		}
+
+		got := event.RenderMessage()
+		expected := "User 123 logged in from Seattle"
+
+		if got != expected {
+			t.Errorf("RenderMessage() = %q, want %q", got, expected)
+		}
+	})
+}

examples/custom-sink/main.go

@@ -0,0 +1,121 @@
+package main
+
+import (
+	"fmt"
+	"os"
+	"strings"
+	"time"
+
+	"github.com/willibrandon/mtlog"
+	"github.com/willibrandon/mtlog/core"
+)
+
+// CustomSink demonstrates using RenderMessage() to properly render message templates
+type CustomSink struct {
+	output *os.File
+}
+
+func (s *CustomSink) Emit(event *core.LogEvent) {
+	// Use RenderMessage() to properly render the message template
+	// This handles format specifiers, capturing operators, and scalar hints
+	message := event.RenderMessage()
+
+	// Format and write the log entry
+	timestamp := event.Timestamp.Format("15:04:05")
+	levelStr := formatLevel(event.Level)
+
+	fmt.Fprintf(s.output, "[%s] %s: %s\n", timestamp, levelStr, message)
+
+	// Optionally include extra properties not in the template
+	for key, value := range event.Properties {
+		// Check for complete placeholder patterns to avoid false positives
+		// (e.g., "User" should not match "{UserId}")
+		placeholder1 := "{" + key + "}"        // Simple: {Key}
+		placeholder2 := "{" + key + ":"        // With format: {Key:format}
+		placeholder3 := "{@" + key + "}"       // Capturing: {@Key}
+		placeholder4 := "{$" + key + "}"       // Scalar: {$Key}
+		placeholder5 := "{" + key + ","        // Alignment: {Key,10}
+
+		if !strings.Contains(event.MessageTemplate, placeholder1) &&
+			!strings.Contains(event.MessageTemplate, placeholder2) &&
+			!strings.Contains(event.MessageTemplate, placeholder3) &&
+			!strings.Contains(event.MessageTemplate, placeholder4) &&
+			!strings.Contains(event.MessageTemplate, placeholder5) {
+			fmt.Fprintf(s.output, "  → %s: %v\n", key, value)
+		}
+	}
+}
+
+func (s *CustomSink) Close() error {
+	return nil
+}
+
+func formatLevel(level core.LogEventLevel) string {
+	switch level {
+	case core.VerboseLevel:
+		return "VRB"
+	case core.DebugLevel:
+		return "DBG"
+	case core.InformationLevel:
+		return "INF"
+	case core.WarningLevel:
+		return "WRN"
+	case core.ErrorLevel:
+		return "ERR"
+	case core.FatalLevel:
+		return "FTL"
+	default:
+		return "???"
+	}
+}
+
+func main() {
+	// Create logger with custom sink
+	log := mtlog.New(
+		mtlog.WithSink(&CustomSink{output: os.Stdout}),
+		mtlog.WithMinimumLevel(core.DebugLevel),
+	)
+
+	// Simple message with properties
+	log.Info("Application started on port {Port}", 8080)
+
+	// Capturing operator - renders the entire struct
+	config := map[string]any{
+		"debug":   true,
+		"port":    8080,
+		"timeout": 30,
+	}
+	log.Debug("Configuration: {@Config}", config)
+
+	// Scalar hint - renders as simple value
+	values := []int{10, 20, 30, 40, 50}
+	log.Info("Processing values: {$Values}", values)
+
+	// Format specifier - applies padding
+	log.Info("Order {OrderId:00000} processed", 42)
+
+	// Time formatting
+	log.Info("Event occurred at {Time:15:04:05}", time.Now())
+
+	// Multiple properties with format specifiers
+	log.Info("User {UserId:000} spent ${Amount:F2} on {ItemCount} items",
+		123, 49.95, 3)
+
+	// Properties not in template are shown separately
+	log.With("host", "api.example.com", "port", 443, "retry", 3).
+		Warning("Connection failed")
+
+	// Complex nested structures
+	user := struct {
+		ID   int
+		Name string
+		Tags []string
+	}{
+		ID:   789,
+		Name: "Alice",
+		Tags: []string{"premium", "verified"},
+	}
+	log.Info("User {@User} logged in", user)
+
+	fmt.Println("\n✅ Custom sink successfully using RenderMessage() to handle all template features!")
+}

internal/parser/token.go

@@ -3,12 +3,11 @@ package parser
 import (
 	"encoding/json"
 	"fmt"
+	"reflect"
 	"strconv"
 	"strings"
 	"time"
 	"unicode/utf8"
-
-	"github.com/willibrandon/mtlog/internal/capture"
 )
 
 // MessageTemplateToken represents a single token in a message template.
@@ -94,12 +93,13 @@ func (p *PropertyToken) formatValue(value any) string {
 
 	// Handle different value types with format strings
 	switch v := value.(type) {
-	case capture.Null:
-		// Null sentinel type renders as "nil" for strings
-		return v.String()
-	case *capture.CapturedStruct:
-		// Handle captured structs - render as struct notation
-		return formatStruct(v)
+	case time.Time:
+		// Handle time.Time explicitly to use custom formatting logic
+		// instead of its default String() method (time.Time implements fmt.Stringer)
+		if p.Format != "" {
+			return p.formatTime(v)
+		}
+		return formatValue(value)
 	case int, int8, int16, int32, int64:
 		if p.Format != "" {
 			return p.formatNumber(v)
@@ -115,11 +115,20 @@ func (p *PropertyToken) formatValue(value any) string {
 			return p.formatFloat(v)
 		}
 		return formatValue(value)
-	case time.Time:
-		if p.Format != "" {
-			return p.formatTime(v)
+	case fmt.Stringer:
+		// Handle types that implement Stringer (includes Null and CapturedStruct from capture package)
+		// This case is checked AFTER concrete types like time.Time to ensure proper formatting
+
+		// Use reflection to identify the capture.Null type without importing it
+		// This avoids import cycle issues while being more reliable than string comparison
+		typeName := reflect.TypeOf(v).String()
+		if typeName == "capture.Null" {
+			// This is the special Null sentinel type - preserve its "nil" representation
+			return v.String()
 		}
-		return formatValue(value)
+
+		// For other Stringers (like CapturedStruct), use their string representation
+		return v.String()
 	case string:
 		// Handle string formatting
 		if p.Format == "l" {
@@ -310,17 +319,11 @@ func (p *PropertyToken) applyAlignment(s string) string {
 	}
 }
 
-// formatStruct formats a CapturedStruct as Go struct notation.
-func formatStruct(cs *capture.CapturedStruct) string {
-	// Simply delegate to the CapturedStruct's String() method
-	return cs.String()
-}
-
 func formatValue(value any) string {
 	if value == nil {
-		return "nil"  // Go convention: nil without brackets
+		return "nil" // Go convention: nil without brackets
 	}
-	
+
 	switch v := value.(type) {
 	case []byte:
 		// Special handling for byte slices - render as string if valid UTF-8