From b1f97d8f1ce1e769990fc68049e6cb85547eca2a Mon Sep 17 00:00:00 2001
From: Anmol Sethi <hi@nhooyr.io>
Date: Tue, 10 Dec 2019 11:35:03 -0600
Subject: [PATCH 1/2] Add reflection encoder for lists

---
 map.go      | 29 +++++++++++++++++++++++------
 map_test.go |  2 +-
 2 files changed, 24 insertions(+), 7 deletions(-)

diff --git a/map.go b/map.go
index 2a50c37..034a61b 100644
--- a/map.go
+++ b/map.go
@@ -13,6 +13,11 @@ import (
 // Map represents an ordered map of fields.
 type Map []Field
 
+// SlogValue implements Value.
+func (m Map) SlogValue() interface{} {
+	return ForceJSON(m)
+}
+
 var _ json.Marshaler = Map(nil)
 
 // MarshalJSON implements json.Marshaler.
@@ -55,14 +60,14 @@ func (v jsonVal) MarshalJSON() ([]byte, error) {
 	return json.Marshal(v.v)
 }
 
-func marshalArray(a []interface{}) []byte {
+func marshalList(rv reflect.Value) []byte {
 	b := &bytes.Buffer{}
 	b.WriteByte('[')
-	for i, v := range a {
+	for i := 0; i < rv.Len(); i++ {
 		b.WriteByte('\n')
-		b.Write(encode(v))
+		b.Write(encode(rv.Index(i).Interface()))
 
-		if i < len(a)-1 {
+		if i < rv.Len()-1 {
 			b.WriteByte(',')
 		}
 	}
@@ -75,13 +80,25 @@ func encode(v interface{}) []byte {
 	switch v := v.(type) {
 	case Value:
 		return encode(v.SlogValue())
-	case []interface{}:
-		return marshalArray(v)
 	case xerrors.Formatter:
 		return encode(errorChain(v))
 	case error, fmt.Stringer:
 		return encode(fmt.Sprint(v))
 	default:
+		rv := reflect.Indirect(reflect.ValueOf(v))
+		if rv.IsValid() {
+			switch rv.Type().Kind() {
+			case reflect.Slice:
+				if rv.IsNil() {
+					b, _ := json.Marshal(nil)
+					return b
+				}
+				fallthrough
+			case reflect.Array:
+				return marshalList(rv)
+			}
+		}
+
 		b, err := json.Marshal(v)
 		if err != nil {
 			return encode(M(
diff --git a/map_test.go b/map_test.go
index 26907d2..bea594a 100644
--- a/map_test.go
+++ b/map_test.go
@@ -93,7 +93,7 @@ func TestMap(t *testing.T) {
 					{
 						"msg": "failed to marshal to JSON",
 						"fun": "cdr.dev/slog.encode",
-						"loc": "`+mapTestFile+`:88"
+						"loc": "`+mapTestFile+`:105"
 					},
 					"json: unsupported type: func(*testing.T, string) string"
 				],

From 1dec25abb04e89bf3a0b67aed0c116f2bbb3ce09 Mon Sep 17 00:00:00 2001
From: Anmol Sethi <hi@nhooyr.io>
Date: Tue, 10 Dec 2019 11:44:50 -0600
Subject: [PATCH 2/2] Document encoding

---
 map.go      | 18 ++++++++++++++----
 map_test.go | 12 +++++++++++-
 2 files changed, 25 insertions(+), 5 deletions(-)

diff --git a/map.go b/map.go
index 034a61b..435b406 100644
--- a/map.go
+++ b/map.go
@@ -23,8 +23,19 @@ var _ json.Marshaler = Map(nil)
 // MarshalJSON implements json.Marshaler.
 //
 // It is guaranteed to return a nil error.
-// Any error marshalling a field will
-// become the field's value.
+// Any error marshalling a field will become the field's value.
+//
+// Every field value is encoded with the following process:
+//
+// 1. slog.Value is handled to allow any type to replace its representation for logging.
+//
+// 2. xerrors.Formatter is handled.
+//
+// 3. error and fmt.Stringer are handled.
+//
+// 4. slices and arrays are handled to go through the encode function for every value.
+//
+// 5. json.Marshal is invoked as the default case.
 func (m Map) MarshalJSON() ([]byte, error) {
 	b := &bytes.Buffer{}
 	b.WriteByte('{')
@@ -90,8 +101,7 @@ func encode(v interface{}) []byte {
 			switch rv.Type().Kind() {
 			case reflect.Slice:
 				if rv.IsNil() {
-					b, _ := json.Marshal(nil)
-					return b
+					break
 				}
 				fallthrough
 			case reflect.Array:
diff --git a/map_test.go b/map_test.go
index bea594a..150b354 100644
--- a/map_test.go
+++ b/map_test.go
@@ -93,7 +93,7 @@ func TestMap(t *testing.T) {
 					{
 						"msg": "failed to marshal to JSON",
 						"fun": "cdr.dev/slog.encode",
-						"loc": "`+mapTestFile+`:105"
+						"loc": "`+mapTestFile+`:115"
 					},
 					"json: unsupported type: func(*testing.T, string) string"
 				],
@@ -164,6 +164,16 @@ func TestMap(t *testing.T) {
 			"error": "xdxd"
 		}`)
 	})
+
+	t.Run("nilSlice", func(t *testing.T) {
+		t.Parallel()
+
+		test(t, slog.M(
+			slog.F("slice", []string(nil)),
+		), `{
+			"slice": null
+		}`)
+	})
 }
 
 type meow struct {