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cm: implement JSON [un]marshaling for Option types #301

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ce4ca98
cm: implement JSON [un]marshaling for Option types
brooksmtownsend Mar 7, 2025
e755c07
cm: test JSON [un]marshaling for Option types
brooksmtownsend Mar 7, 2025
450b98c
cm: implement JSON [un]marshaling for option types
brooksmtownsend Mar 10, 2025
c7d2e56
cm: test JSON [un]marshaling for named and nested options
brooksmtownsend Mar 11, 2025
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40 changes: 40 additions & 0 deletions cm/option.go
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Original file line number Diff line number Diff line change
@@ -1,5 +1,7 @@
package cm

import "encoding/json"

// Option represents a Component Model [option<T>] type.
//
// [option<T>]: https://component-model.bytecodealliance.org/design/wit.html#options
Expand Down Expand Up @@ -57,3 +59,41 @@ func (o option[T]) Value() T {
}
return o.some
}

// MarshalJSON implements the json.Marshaler interface for the public option type.
func (o Option[T]) MarshalJSON() ([]byte, error) {
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Methods should be on option[T] so named option types inherit the methods.

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Is there an example I can follow for a named option type? I assumed that if external implementations named a type it would be for Option[T], perhaps I should implement the un/marshaling for both?

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// This type would not have the JSON methods.
type OptionalI32 cm.Option[int32]

This is why the methods are implemented on option, which is embedded in Option, so methods are preserved.

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@brooksmtownsend brooksmtownsend Mar 11, 2025 •
edited
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@ydnar added a few more test cases that show that this works for named and nested options 🫡

I did find that I could remove the Option[T].MarshalJSON implementation and still have tests work, but removing Option[T].UnmarshalJSON did fail the cases that we have, so I left both in.

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This is not necessary.

return json.Marshal(o.Some())
}

// UnmarshalJSON implements the json.Unmarshaler interface for the public option type.
func (o *Option[T]) UnmarshalJSON(data []byte) error {
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This is not necessary.

if string(data) == "null" {
*o = None[T]()
return nil
}
var v T
if err := json.Unmarshal(data, &v); err != nil {
return err
}
*o = Some(v)
return nil
}

// MarshalJSON implements the json.Marshaler interface for the internal option type.
func (o option[T]) MarshalJSON() ([]byte, error) {
return json.Marshal(o.Some())
}

// UnmarshalJSON implements the json.Unmarshaler interface for the internal option type.
func (o *option[T]) UnmarshalJSON(data []byte) error {
if string(data) == "null" {
o = &option[T]{isSome: false}
return nil
}
var v T
if err := json.Unmarshal(data, &v); err != nil {
return err
}
o = &option[T]{isSome: true, some: v}
return nil
}
118 changes: 117 additions & 1 deletion cm/option_test.go
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Original file line number Diff line number Diff line change
@@ -1,6 +1,9 @@
package cm

import "testing"
import (
"encoding/json"
"testing"
)

func TestOption(t *testing.T) {
o1 := None[string]()
Expand Down Expand Up @@ -65,3 +68,116 @@ func TestOption(t *testing.T) {
t.Errorf("Value: %v, expected %v", got, want)
}
}

func TestOptionMarshalJSON(t *testing.T) {
type TestStruct struct {
Field Option[string] `json:"field"`
}

// Test marshaling None
ts1 := TestStruct{Field: None[string]()}
data1, err := json.Marshal(ts1)
if err != nil {
t.Fatalf("json.Marshal failed: %v", err)
}
expected1 := `{"field":null}`
if string(data1) != expected1 {
t.Errorf("json.Marshal: got %s, expected %s", data1, expected1)
}

// Test marshaling Some
ts2 := TestStruct{Field: Some("hello")}
data2, err := json.Marshal(ts2)
if err != nil {
t.Fatalf("json.Marshal failed: %v", err)
}
expected2 := `{"field":"hello"}`
if string(data2) != expected2 {
t.Errorf("json.Marshal: got %s, expected %s", data2, expected2)
}

// Test marshaling custom option type
type OptionalI32 Option[int32]
ts3 := OptionalI32(Some(int32(42)))
data3, err := json.Marshal(ts3)
if err != nil {
t.Fatalf("json.Marshal failed: %v", err)
}
expected3 := `42`
if string(data3) != expected3 {
t.Errorf("json.Marshal: got %s, expected %s", data3, expected3)
}

// Test marshaling nested option type
type NestedStruct struct {
Field Option[Option[int32]] `json:"field"`
}
ts4 := NestedStruct{Field: Some(Some(int32(42)))}
data4, err := json.Marshal(ts4)
if err != nil {
t.Fatalf("json.Marshal failed: %v", err)
}
expected4 := `{"field":42}`
if string(data4) != expected4 {
t.Errorf("json.Marshal: got %s, expected %s", data4, expected4)
}
}

func TestOptionUnmarshalJSON(t *testing.T) {
type TestStruct struct {
Field Option[string] `json:"field"`
}

// Test unmarshaling None
data1 := []byte(`{"field":null}`)
var ts1 TestStruct
if err := json.Unmarshal(data1, &ts1); err != nil {
t.Fatalf("json.Unmarshal failed: %v", err)
}
if got, want := ts1.Field.None(), true; got != want {
t.Errorf("ts1.Field.None: %t, expected %t", got, want)
}

// Test unmarshaling None (not present)
data2 := []byte(`{}`)
var ts2 TestStruct
if err := json.Unmarshal(data2, &ts2); err != nil {
t.Fatalf("json.Unmarshal failed: %v", err)
}
if got, want := ts2.Field.None(), true; got != want {
t.Errorf("ts1.Field.None: %t, expected %t", got, want)
}

// Test unmarshaling Some
data3 := []byte(`{"field":"hello"}`)
var ts3 TestStruct
if err := json.Unmarshal(data3, &ts3); err != nil {
t.Fatalf("json.Unmarshal failed: %v", err)
}
if got, want := ts3.Field.isSome, true; got != want {
t.Errorf("ts2.Field.Some: %t, expected %t", got, want)
}
if got, want := ts3.Field.Value(), "hello"; got != want {
t.Errorf("ts2.Field.Value: %v, expected %v", got, want)
}

// Test unmarshaling nested option type
type NestedStruct struct {
Field Option[Option[int32]] `json:"field"`
}
data5 := []byte(`{"field":42}`)
var ns NestedStruct
if err := json.Unmarshal(data5, &ns); err != nil {
t.Fatalf("json.Unmarshal failed: %v", err)
}
if got, want := ns.Field.isSome, true; got != want {
t.Errorf("ns.Field.Some: %t, expected %t", got, want)
}
if got, want := ns.Field.Value(), Some(int32(42)); got != want {
t.Errorf("ns.Field.Value: %v, expected %v", got, want)
}
// Deref the inner option to get the value
if got, want := ns.Field.Value().Value(), int32(42); got != want {
t.Errorf("ns.Field.Value.Value: %v, expected %v", got, want)
}
}