114 lines
3.0 KiB
Go
114 lines
3.0 KiB
Go
// Starting with version 1.18, Go has added support for
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// _generics_, also known as _type parameters_.
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package main
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import "fmt"
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type MyInt16 int16
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type MyInt int
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func (m MyInt16) String() string {
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return fmt.Sprintf("{MyInt16: %d}", m)
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}
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func (m MyInt) String() string {
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return fmt.Sprintf("{MyInt: %d}", m)
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}
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// `~` the underlying type of T must be itself, and T cannot be an interface.
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type Number interface {
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int | ~int16
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}
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// An interface representing all types with underlying type int that implement the String method.
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type IntString interface {
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~int16
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String() string
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}
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func SumNumber[T Number](a, b T) T {
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return a + b
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}
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func SumIntString[T IntString](a, b T) (T, string) {
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return a + b, a.String() + ", " + b.String()
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}
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// As an example of a generic function, `MapKeys` takes
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// a map of any type and returns a slice of its keys.
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// This function has two type parameters - `K` and `V`;
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// `K` has the `comparable` _constraint_, meaning that
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// we can compare values of this type with the `==` and
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// `!=` operators. This is required for map keys in Go.
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// `V` has the `any` constraint, meaning that it's not
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// restricted in any way (`any` is an alias for `interface{}`).
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func MapKeys[K comparable, V any](m map[K]V) []K {
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r := make([]K, 0, len(m))
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for k := range m {
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r = append(r, k)
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}
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return r
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}
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// As an example of a generic type, `List` is a
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// singly-linked list with values of any type.
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type List[T any] struct {
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head, tail *element[T]
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}
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type element[T any] struct {
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next *element[T]
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val T
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}
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// We can define methods on generic types just like we
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// do on regular types, but we have to keep the type
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// parameters in place. The type is `List[T]`, not `List`.
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func (lst *List[T]) Push(v T) {
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if lst.tail == nil {
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lst.head = &element[T]{val: v}
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lst.tail = lst.head
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} else {
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lst.tail.next = &element[T]{val: v}
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lst.tail = lst.tail.next
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}
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}
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func (lst *List[T]) GetAll() []T {
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var elems []T
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for e := lst.head; e != nil; e = e.next {
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elems = append(elems, e.val)
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}
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return elems
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}
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func main() {
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var m = map[int]string{1: "2", 2: "4", 4: "8"}
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// When invoking generic functions, we can often rely
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// on _type inference_. Note that we don't have to
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// specify the types for `K` and `V` when
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// calling `MapKeys` - the compiler infers them
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// automatically.
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fmt.Println("keys:", MapKeys(m))
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// ... though we could also specify them explicitly.
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_ = MapKeys[int, string](m)
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lst := List[int]{}
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lst.Push(10)
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lst.Push(13)
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lst.Push(23)
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fmt.Println("list:", lst.GetAll())
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// We can use `MyInt16` as the parameter of the function `SumNumber`, because its underlying type is int16.
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fmt.Println(SumNumber(MyInt16(1), MyInt16(2)))
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fmt.Println(SumNumber(1, 2))
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// We can't use `MyInt` as the parameter of the function `SumInString`, because underlying type of `MyInt` is int not int16.
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// also int16 does not implement `IntString` (missing method String).
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result, str := SumIntString(MyInt16(1), MyInt16(2))
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fmt.Printf("result: %d, output: %s\n", result, str)
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}
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