From e7ccd89cad2e09f1fe7ff590dbdc8a2d6e350fbb Mon Sep 17 00:00:00 2001
From: Joel Nordell <joel@unit410.com>
Date: Wed, 30 Aug 2023 14:58:26 -0500
Subject: [PATCH] Add MarshalJSON and UnmarshalJSON for Result[T]

---
 result.go      | 53 +++++++++++++++++++++++++++-
 result_test.go | 93 ++++++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 145 insertions(+), 1 deletion(-)

diff --git a/result.go b/result.go
index cba761c..a9f58da 100644
--- a/result.go
+++ b/result.go
@@ -1,6 +1,10 @@
 package mo
 
-import "fmt"
+import (
+	"encoding/json"
+	"errors"
+	"fmt"
+)
 
 // Ok builds a Result when value is valid.
 // Play: https://go.dev/play/p/PDwADdzNoyZ
@@ -162,3 +166,50 @@ func (r Result[T]) FlatMap(mapper func(value T) Result[T]) Result[T] {
 
 	return Err[T](r.err)
 }
+
+// MarshalJSON encodes Result into json, following the JSON-RPC specification for results,
+// with one exception: when the result is an error, the "code" field is not included.
+// Reference: https://www.jsonrpc.org/specification
+func (o Result[T]) MarshalJSON() ([]byte, error) {
+	if o.isErr {
+		return json.Marshal(map[string]any{
+			"error": map[string]any{
+				"message": o.err.Error(),
+			},
+		})
+	}
+
+	return json.Marshal(map[string]any{
+		"result": o.value,
+	})
+}
+
+// UnmarshalJSON decodes json into Result. If "error" is set, the result is an
+// Err containing the error message as a generic error object. Otherwise, the
+// result is an Ok containing the result. If the JSON object contains netiher
+// an error nor a result, the result is an Ok containing an empty value. If the
+// JSON object contains both an error and a result, the result is an Err. Finally,
+// if the JSON object contains an error but is not structured correctly (no message
+// field), the unmarshaling fails.
+func (o *Result[T]) UnmarshalJSON(data []byte) error {
+	var result struct {
+		Result T `json:"result"`
+		Error  struct {
+			Message string `json:"message"`
+		} `json:"error"`
+	}
+
+	if err := json.Unmarshal(data, &result); err != nil {
+		return err
+	}
+
+	if result.Error.Message != "" {
+		o.err = errors.New(result.Error.Message)
+		o.isErr = true
+		return nil
+	}
+
+	o.value = result.Result
+	o.isErr = false
+	return nil
+}
diff --git a/result_test.go b/result_test.go
index 4a4a1ab..0130d64 100644
--- a/result_test.go
+++ b/result_test.go
@@ -1,6 +1,8 @@
 package mo
 
 import (
+	"encoding/json"
+	"fmt"
 	"testing"
 
 	"github.com/stretchr/testify/assert"
@@ -201,3 +203,94 @@ func TestResultFlatMap(t *testing.T) {
 	is.Equal(Result[int]{value: 42, isErr: false, err: nil}, opt1)
 	is.Equal(Result[int]{value: 0, isErr: true, err: assert.AnError}, opt2)
 }
+
+func TestResultMarshalJSON(t *testing.T) {
+	is := assert.New(t)
+
+	result1 := Ok("foo")
+	result2 := Err[string](fmt.Errorf("an error"))
+	result3 := Ok("")
+
+	value, err := result1.MarshalJSON()
+	is.NoError(err)
+	is.Equal(`{"result":"foo"}`, string(value))
+
+	value, err = result2.MarshalJSON()
+	is.NoError(err)
+	is.Equal(`{"error":{"message":"an error"}}`, string(value))
+
+	value, err = result3.MarshalJSON()
+	is.NoError(err)
+	is.Equal(`{"result":""}`, string(value))
+
+	type testStruct struct {
+		Field Result[string]
+	}
+
+	resultInStruct := testStruct{
+		Field: result1,
+	}
+	var marshalled []byte
+	marshalled, err = json.Marshal(resultInStruct)
+	is.NoError(err)
+	is.Equal(`{"Field":{"result":"foo"}}`, string(marshalled))
+}
+
+func TestResultUnmarshalJSON(t *testing.T) {
+	is := assert.New(t)
+
+	result1 := Ok("foo")
+	result2 := Err[string](fmt.Errorf("an error"))
+	result3 := Ok("")
+
+	err := result1.UnmarshalJSON([]byte(`{"result":"foo"}`))
+	is.NoError(err)
+	is.Equal(Ok("foo"), result1)
+
+	var res Result[string]
+	err = json.Unmarshal([]byte(`{"result":"foo"}`), &res)
+	is.NoError(err)
+	is.Equal(res, result1)
+
+	err = result2.UnmarshalJSON([]byte(`{"error":{"message":"an error"}}`))
+	is.NoError(err)
+	is.Equal(Err[string](fmt.Errorf("an error")), result2)
+
+	err = result3.UnmarshalJSON([]byte(`{"result":""}`))
+	is.NoError(err)
+	is.Equal(Ok(""), result3)
+
+	type testStruct struct {
+		Field Result[string]
+	}
+
+	unmarshal := testStruct{}
+	err = json.Unmarshal([]byte(`{"Field":{"result":"foo"}}`), &unmarshal)
+	is.NoError(err)
+	is.Equal(testStruct{Field: Ok("foo")}, unmarshal)
+
+	unmarshal = testStruct{}
+	err = json.Unmarshal([]byte(`{"Field":{"error":{"message":"an error"}}}`), &unmarshal)
+	is.NoError(err)
+	is.Equal(testStruct{Field: Err[string](fmt.Errorf("an error"))}, unmarshal)
+
+	unmarshal = testStruct{}
+	err = json.Unmarshal([]byte(`{}`), &unmarshal)
+	is.NoError(err)
+	is.Equal(testStruct{Field: Ok("")}, unmarshal)
+
+	// Both result and error are set; unmarshal to Err
+	unmarshal = testStruct{}
+	err = json.Unmarshal([]byte(`{"Field":{"result":"foo","error":{"message":"an error"}}}`), &unmarshal)
+	is.NoError(err)
+	is.Equal(testStruct{Field: Err[string](fmt.Errorf("an error"))}, unmarshal)
+
+	// Bad structure for error; cannot unmarshal
+	unmarshal = testStruct{}
+	err = json.Unmarshal([]byte(`{"Field":{"result":"foo","error":true}}`), &unmarshal)
+	is.Error(err)
+
+	unmarshal = testStruct{}
+	err = json.Unmarshal([]byte(`{"Field": "}`), &unmarshal)
+	is.Error(err)
+}