Skip func type value in fields.

We skip those unprintable fields and an error field
instead of dropping the whole trace.

Fixes #642

entry.go

@@ -4,6 +4,7 @@ import (
 	"bytes"
 	"fmt"
 	"os"
+	"reflect"
 	"sync"
 	"time"
 )
@@ -43,6 +44,9 @@ type Entry struct {
 
 	// When formatter is called in entry.log(), a Buffer may be set to entry
 	Buffer *bytes.Buffer
+
+	// err may contain a field formatting error
+	err string
 }
 
 func NewEntry(logger *Logger) *Entry {
@@ -80,10 +84,18 @@ func (entry *Entry) WithFields(fields Fields) *Entry {
 	for k, v := range entry.Data {
 		data[k] = v
 	}
+	var field_err string
 	for k, v := range fields {
-		data[k] = v
+		if t := reflect.TypeOf(v); t != nil && t.Kind() == reflect.Func {
+			field_err = fmt.Sprintf("can not add field %q", k)
+			if entry.err != "" {
+				field_err = entry.err + ", " + field_err
+			}
+		} else {
+			data[k] = v
+		}
 	}
-	return &Entry{Logger: entry.Logger, Data: data, Time: entry.Time}
+	return &Entry{Logger: entry.Logger, Data: data, Time: entry.Time, err: field_err}
 }
 
 // Overrides the time of the Entry.