2022-03-29

chapter1-why-generics.slide

@@ -1,8 +1,8 @@
 # Go 泛型介绍
 
 清和
-2022-01-22
-chenqh721@foxmail.com
+2022-03-31
+qinghe.chen@tuya.com
 
 
 ## Agenda
@@ -28,11 +28,11 @@ chenqh721@foxmail.com
 - 类型转换影响性能
 
 
-## Type parameters
+## How Generics
 
 ## Type parameters
 
-泛型代码中会用到抽象的数据类型，这个类型叫做`type parameters`. 在运行泛型代码的时候，类型形参会被替换为类型实参。
+泛型代码中会用到抽象的数据类型，这个类型叫做`type parameters`（类型参数）. 在运行泛型代码的时候，类型形参会被替换为类型实参。
 
 .code codes/print.go /PRINT OMIT/,/PRINT OMIT/
 

chapter3-others-about-generics.slide

@@ -65,8 +65,60 @@ Qinghe
 
 ## Type sets of embedded constraints
 
+.code codes/type_sets.go /EMBED INTERSECTION OMIT/,/EMBED INTERSECTION OMIT/
+
+
+
+## Type sets of embedded constraints
+.code codes/type_sets.go /EMBED UNION OMIT/,/EMBED UNION OMIT/
 
 ## Interface types in union elements
 
+.code codes/type_sets.go /INTERFACE IN UNION OMIT/,/INTERFACE IN UNION OMIT/
+
+- 约束的类型集包括string类型以及实现了fmt.Stringer接口的所有类型
+- 除所有类型都共有的操作外，无其他特殊的允许的操作
+- 必须通过断言或者反射来使用对应的值
+
+目前版本还未实现，see [issue#49054](https://github.com/golang/go/issues/49054)，[issue#45346 (comment)](https://github.com/golang/go/issues/45346#issuecomment-862505803)
+
+## Empty type sets
+
+.code codes/type_sets.go /EMPTY TYPE SETS OMIT/,/EMPTY TYPE SETS OMIT/
+
+- 可以正常编译，但无法实例化
+- go vet 对某些能够被检测出来的情况“应该”报出错误
+
+
+## Some Issues
+
+
+## The zero value
+
+.code codes/zero_value.go /ZERO VALUE OMIT/,/ZERO VALUE OMIT/
+
+一些可行的方案：
+
+- 使用`var zero T; return zero`，与现行方案不冲突，但是需要额外的代码
+- 使用`return *new(T)`，语法比较奇怪，但是也能正常执行
+- 使用命名返回值，返回值用return即可返回零值。此种方案仅对在返回值中生效
+- 允许使用nil作为参数类型的零值返回
+- 允许使用T{}作为参数类型的零值返回
+- 允许在赋值语句（包括return以及函数调用）的右边使用`_`作为零值
+- 允许使用`return ...`来返回对应类型的零值
+
+
+## Identifying the matched predeclared type
+
+.code codes/type_assertion.go /TYPE ASSERTION OMIT/,/TYPE ASSERTION OMIT/
+
+并不想知道v的真实类型，只想知道v的底层是float32还是float64，但目前无法做到。
+
+一种解决方法是扩展switch case语法，支持`case ~float32:`类型的语句，甚至在泛型函数之外也应该可以使用这样的语法。
+
+
+## No parameterized methods
+(略)
 
-## Empty type sets
+## No way to require pointer methods
+(略)

codes/atomic.go

@@ -1,6 +1,8 @@
-package example
+package main
 
 // OMIT
+package atomic 
+
 func AddInt32(addr *int32, delta int32) int32 {
 	// some codes ...
 	return 0

codes/type_assertion.go

@@ -0,0 +1,21 @@
+package main
+
+
+// TYPE ASSERTION OMIT
+func NewtonSqrt[T ~float32 | ~float64 ](v T) T {
+	var iterations int
+	switch (interface{})(v).(type) {
+	case float32:
+		iterations = 4
+	case float64:
+		iterations = 5
+	default:
+		panic(fmt.Sprintf("unexpected type %T", v))
+	}
+	// Code omitted.
+}
+
+type MyFloat float32
+
+var G = NewtonSqrt(MyFloat(64))
+// TYPE ASSERTION OMIT

codes/type_sets.go

@@ -154,4 +154,77 @@ func Add1024[T integer](s []T) {
 		s[i] = v + 1024 // INVALID: 1024 not permitted by int8/uint8
 	}
 }
-// UNTYPED CONSTANTS OMIT
+// UNTYPED CONSTANTS OMIT
+
+
+// EMBED INTERSECTION OMIT
+// Addable is types that support the + operator.
+type Addable interface {
+	~int | ~int8 | ~int16 | ~int32 | ~int64 |
+		~uint | ~uint8 | ~uint16 | ~uint32 | ~uint64 | ~uintptr |
+		~float32 | ~float64 | ~complex64 | ~complex128 |
+		~string
+}
+
+// Byteseq is a byte sequence: either string or []byte.
+type Byteseq interface {
+	~string | ~[]byte
+}
+
+// AddableByteseq is a byte sequence that supports +.
+// This is every type that is both Addable and Byteseq.
+// In other words, just the type set ~string.
+type AddableByteseq interface {
+	Addable
+	Byteseq
+}
+// EMBED INTERSECTION OMIT
+
+
+
+// EMBED UNION OMIT
+// Signed is a constraint with a type set of all signed integer
+// types.
+type Signed interface {
+	~int | ~int8 | ~int16 | ~int32 | ~int64
+}
+
+// Unsigned is a constraint with a type set of all unsigned integer
+// types.
+type Unsigned interface {
+	~uint | ~uint8 | ~uint16 | ~uint32 | ~uint64 | ~uintptr
+}
+
+// Integer is a constraint with a type set of all integer types.
+type Integer interface {
+	Signed | Unsigned
+}
+// EMBED UNION OMIT
+
+// INTERFACE IN UNION OMIT
+type Stringish interface {
+	string | fmt.Stringer
+}
+
+func ToString[T Stringish](v T) string {
+	switch x := (interface{})(v).(type) {
+	case string:
+	  return x
+  
+	case fmt.Stringer:
+	  return x.String()
+	}
+  
+	panic("impossible")
+  }
+// INTERFACE IN UNION OMIT
+
+// EMPTY TYPE SETS OMIT
+// Unsatisfiable is an unsatisfiable constraint with an empty type set.
+// No predeclared types have any methods.
+// If this used ~int | ~float32 the type set would not be empty.
+type Unsatisfiable interface {
+	int | float32
+	String() string
+}
+// EMPTY TYPE SETS OMIT

codes/zero_value.go

@@ -0,0 +1,13 @@
+package main
+
+
+// ZERO VALUE OMIT
+type Optional[T any] struct { p *T }
+func (o Optional[T]) Val() T {
+	if o.p != nil {
+		return *o.p
+	}
+	// how do we return default value of type T ?
+}
+
+// ZERO VALUE OMIT