content: add generic option

content/assets/generic-option/1pattern/option.go

@@ -0,0 +1,53 @@
+// Copyright 2022 The golang.design Initiative Authors.
+// All rights reserved. Use of this source code is governed
+// by a MIT license that can be found in the LICENSE file.
+//
+// Written by Changkun Ou <changkun.de>
+
+package main
+
+import "fmt"
+
+type A struct {
+	v1 int
+	// Removed, now moved to v3.
+	// v2 int
+	v3 int
+}
+
+type Option func(*A)
+
+func V1(v1 int) Option {
+	return func(a *A) {
+		a.v1 = v1
+	}
+}
+
+// Deprecated: Use V3 instead.
+func V2(v2 int) Option {
+	return func(a *A) {
+		// no effects anymore
+		// a.v2 = v2
+	}
+}
+
+func V3(v3 int) Option {
+	return func(a *A) {
+		a.v3 = v3
+	}
+}
+
+func NewA(opts ...Option) *A {
+	a := &A{}
+	for _, opt := range opts {
+		opt(a)
+	}
+	return a
+}
+
+func main() {
+	fmt.Printf("%#v\n", NewA())               // &main.A{v1:0, v2:0}
+	fmt.Printf("%#v\n", NewA(V1(42)))         // &main.A{v1:42, v2:0}
+	fmt.Printf("%#v\n", NewA(V2(42)))         // &main.A{v1:0, v2:0}
+	fmt.Printf("%#v\n", NewA(V1(42), V2(42))) // &main.A{v1:42, v2:0}
+}

content/assets/generic-option/2problem/option.go

@@ -0,0 +1,66 @@
+// Copyright 2022 The golang.design Initiative Authors.
+// All rights reserved. Use of this source code is governed
+// by a MIT license that can be found in the LICENSE file.
+//
+// Written by Changkun Ou <changkun.de>
+
+package main
+
+import "fmt"
+
+type A struct {
+	v1 int
+}
+
+type B struct {
+	v1 int
+	v2 int
+}
+
+type OptionA func(a *A)
+type OptionB func(a *B)
+
+func V1ForA(v1 int) OptionA {
+	return func(a *A) {
+		a.v1 = v1
+	}
+}
+
+func V1ForB(v1 int) OptionB {
+	return func(b *B) {
+		b.v1 = v1
+	}
+}
+
+func V2ForB(v2 int) OptionB {
+	return func(b *B) {
+		b.v2 = v2
+	}
+}
+
+func NewA(opts ...OptionA) *A {
+	a := &A{}
+
+	for _, opt := range opts {
+		opt(a)
+	}
+	return a
+}
+
+func NewB(opts ...OptionB) *B {
+	b := &B{}
+
+	for _, opt := range opts {
+		opt(b)
+	}
+	return b
+}
+
+func main() {
+	fmt.Printf("%#v\n", NewA())                       // &main.A{v1:0}
+	fmt.Printf("%#v\n", NewA(V1ForA(42)))             // &main.A{v1:42}
+	fmt.Printf("%#v\n", NewB())                       // &main.B{v1:0, v2:0}
+	fmt.Printf("%#v\n", NewB(V1ForB(42)))             // &main.B{v1:42, v2:0}
+	fmt.Printf("%#v\n", NewB(V2ForB(42)))             // &main.B{v1:0, v2:42}
+	fmt.Printf("%#v\n", NewB(V1ForB(42), V2ForB(42))) // &main.B{v1:42, v2:42}
+}

content/assets/generic-option/3interface/option.go

@@ -0,0 +1,77 @@
+// Copyright 2022 The golang.design Initiative Authors.
+// All rights reserved. Use of this source code is governed
+// by a MIT license that can be found in the LICENSE file.
+//
+// Written by Changkun Ou <changkun.de>
+
+package main
+
+import "fmt"
+
+type A struct {
+	v1 int
+}
+
+type B struct {
+	v1 int
+	v2 int
+}
+
+type Common interface {
+	/* ... */
+}
+
+type Option func(c Common)
+
+func V1(v1 int) Option {
+	return func(c Common) {
+		switch x := c.(type) {
+		case *A:
+			x.v1 = v1
+		case *B:
+			x.v1 = v1
+		default:
+			panic("unexpected use")
+		}
+	}
+}
+
+func V2(v2 int) Option {
+	return func(c Common) {
+		switch x := c.(type) {
+		case *B:
+			x.v2 = v2
+		default:
+			panic("unexpected use")
+		}
+	}
+}
+
+func NewA(opts ...Option) *A {
+	a := &A{}
+
+	for _, opt := range opts {
+		opt(a)
+	}
+	return a
+}
+
+func NewB(opts ...Option) *B {
+	b := &B{}
+
+	for _, opt := range opts {
+		opt(b)
+	}
+	return b
+}
+
+func main() {
+	fmt.Printf("%#v\n", NewA())               // &main.A{v1:0}
+	fmt.Printf("%#v\n", NewA(V1(42)))         // &main.A{v1:42}
+	fmt.Printf("%#v\n", NewB())               // &main.B{v1:0, v2:0}
+	fmt.Printf("%#v\n", NewB(V1(42)))         // &main.B{v1:42, v2:0}
+	fmt.Printf("%#v\n", NewB(V2(42)))         // &main.B{v1:0, v2:42}
+	fmt.Printf("%#v\n", NewB(V1(42), V2(42))) // &main.B{v1:42, v2:42}
+
+	// fmt.Printf("%#v\n", NewA(V2(42))) // ERROR: panic
+}

content/assets/generic-option/4generic/option.go

@@ -0,0 +1,74 @@
+// Copyright 2022 The golang.design Initiative Authors.
+// All rights reserved. Use of this source code is governed
+// by a MIT license that can be found in the LICENSE file.
+//
+// Written by Changkun Ou <changkun.de>
+
+package main
+
+import "fmt"
+
+type A struct {
+	v1 int
+}
+
+type B struct {
+	v1 int
+	v2 int
+}
+
+type Option[T A | B] func(a *T)
+
+func V1[T A | B](v1 int) Option[T] {
+	return func(a *T) {
+		switch x := any(a).(type) {
+		case *A:
+			x.v1 = v1
+		case *B:
+			x.v1 = v1
+		default:
+			panic("unexpected use")
+		}
+	}
+}
+
+func V2[T B](v2 int) Option[T] {
+	return func(a *T) {
+		switch x := any(a).(type) {
+		case *B:
+			x.v2 = v2
+		default:
+			panic("unexpected use")
+		}
+	}
+}
+
+func NewA[T A](opts ...Option[T]) *T {
+	t := new(T)
+	for _, opt := range opts {
+		opt(t)
+	}
+	return t
+}
+
+func NewB[T B](opts ...Option[T]) *T {
+	t := new(T)
+	for _, opt := range opts {
+		opt(t)
+	}
+	return t
+}
+
+func main() {
+	fmt.Printf("%#v\n", NewA())                     // &main.A{v1:0}
+	fmt.Printf("%#v\n", NewA(V1[A](42)))            // &main.A{v1:42}
+	fmt.Printf("%#v\n", NewB())                     // &main.B{v1:0, v2:0}
+	fmt.Printf("%#v\n", NewB(V1[B](42)))            // &main.B{v1:42, v2:0}
+	fmt.Printf("%#v\n", NewB(V2[B](42)))            // &main.B{v1:0, v2:42}
+	fmt.Printf("%#v\n", NewB(V1[B](42), V2[B](42))) // &main.B{v1:42, v2:42}
+
+	// _ = NewA(V2[B](42))            // ERROR: B does not implement A
+	// _ = NewA(V2[A](42))            // ERROR: A does not implement B
+	// _ = NewB(V1[A](42), V2[B](42)) // ERROR: type Option[B] of V2[B](42) does not match inferred type Option[A] for Option[T]
+	// _ = NewB(V1[B](42), V2[A](42)) // ERROR: type Option[A] of V2[A](42) does not match inferred type Option[B] for Option[T]
+}

content/assets/generic-option/go.mod

@@ -0,0 +1,3 @@
+module generic-option
+
+go 1.18