[lectures] Add 08-generics.
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lectures/08-generics/lecture.slide
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lectures/08-generics/lecture.slide
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generics
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Лекция 8
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Арсений Балобанов
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* Generics (draft)
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* New language features
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- Mechanism to parameterize a type or function by types.
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- Constraints mechanism to express requirements on type parameters.
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- Type inference (optional)
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* Parameter lists
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An ordinary parameter list
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(x, y aType, z anotherType)
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A type parameter list
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[P, Q aConstraint, R anotherConstraint]
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- Convention: Type parameter names are capitalized
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* Sorting in Go
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what we have
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func Sort(data Interface)
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type Interface interface {
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Len() int
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Less(i, j int) bool
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Swap(i, j int)
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}
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what we really want
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func Sort(list []Elem)
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// use
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Sort(myList)
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* Type parameters to the rescue
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func Sort[Elem ?](list []Elem)
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* Constraints
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- A constraint specifies the requirements which a type argument must satisfy.
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- In generic Go, constraints are interfaces
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- A type argument is valid if it implements its constraint.
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* Generic Sort
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func Sort[Elem interface{ Less(y Elem) bool }](list []Elem) {
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...
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}
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- The constraint is an interface, but the actual type argument can be any type that implements that interface.
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- The scope of a type parameter starts at the opening "[" and ends at the end of the generic type or function declaration
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* Using generic Sort
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Somewhere in library
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func Sort[Elem interface{ Less(y Elem) bool }](list []Elem)
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User code
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type book struct{...}
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func (x book) Less(y book) bool {...}
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var bookshelf []book
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...
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Sort[book](bookshelf) // generic function call
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* Type-checking the Sort call: Instantiation
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Sort[book] | (bookshelf)
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pass type argument
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Sort[Elem interface{ Less(y Elem) bool }] | (list []Elem)
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substitute book for elem
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Sort[book interface{ Less(y book) bool }] | (list []book)
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verify that book satisfies the book parameter constraint
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#Sort[book] | (list []book)
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A generic function or type must be instantiated before it can be used.
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* Type-checking a generic call
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Instantiation (new)
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- replace type parameters with type arguments in entire signature
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- verify that each type argument satisfies its constraintThen, using the instantiated signature.
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Invocation (as usual)
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- verify that each ordinary argument can be assigned to its parameter
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* Types can be generic, too
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type Lesser[T any] interface{
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Less(y T) bool}
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}
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any stands for "no constraint"(same as "interface{}")
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* Sort, decomposed
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type Lesser[T any] interface{
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Less(y T) bool
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}
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func Sort[Elem Lesser[Elem]](list []Elem)
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* Sort internals
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func Sort[Elem interface{ Less(y Elem) bool }](list []Elem) {
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...
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var i, j int
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...
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if list[i].Less(List[j]) {...}
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...
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}
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- type of list[i], list[j] is Elem
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- Elem is NOT an interface type!
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- A type parameter is a real type.It is not an interface type.
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* Argument type inference
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what we have
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Sort[book](bookshelf)
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what we want
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Sort(bookshelf)
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Type unification
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bookshelf -> []book
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Inference
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func Sort[Elem ...]([]Elem) => Elem == book
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* Problems
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what we want
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Sort([]int{1, 2, 3})
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int does not implement Elem constraint (no Less method)
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what we could do
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type myInt int
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func (x myInt) Less(y myInt) bool { return x < y }
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* Type lists
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A constraint interface may have a list of types (besides methods):
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type Float interface {
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type float32, float64
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}
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// Sin computes sin(x) for x of type float32 or float64.
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func Sin[T Float](x T) T
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Satisfying a type list
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An argument type satisfies a constraint with a type list if
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- The argument type implements the methods of the constraint
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- The argument type or its underlying type is found in the type list.
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As usual, the satisfaction check happens after substitution.
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* Generic min function
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type Ordered interface {
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type int, int8, int16, ..., uint, uint8, uint16, ...,
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float32, float64, string
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}
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min internals
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func min[T Ordered](x, y T) T {
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if x < y {
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return x
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}
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return y
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}
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* Different type parameters are different types
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func invalid[Tx, Ty Ordered](x Tx, y Ty) Tx {
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...
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if x < y { ...// INVALID
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...
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}
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- "<" requires that both operands have the same type
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* Relationships between type parameters
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type Pointer[T any] interface {
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type *T
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}
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func f[T any, PT Pointer[T]](x T)
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or with inlined constraint
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func foo[T any, PT interface{type *T}](x T)
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* When to use generics
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- Improved static type safety.
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- More efficient memory use.
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- (Significantly) better performance.
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* Summary
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Generics are type-checked macros.
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Declarations
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- Type parameter lists are like ordinary parameter lists with "[" "]".
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- Function and type declarations may have type parameter lists.
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- Type parameters are constrained by interfaces.
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Use
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- Generic functions and types must be instantiated when used.
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- Type inference (if applicable) makes function instantiation implicit.
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- Instantiation is valid if the type arguments satisfy their constraints.
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* How to try?
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.link https://go2goplay.golang.org/ - playground
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.link https://go.googlesource.com/go/+/refs/heads/dev.go2go/README.go2go.md - dev branch
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* Ссылки
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.link https://go.googlesource.com/proposal/+/refs/heads/master/design/go2draft-contracts.md - generics design draft
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.link https://blog.golang.org/why-generics - The Go Blog - Why Generics?
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.link https://www.youtube.com/watch?v=TborQFPY2IM - GopherCon 2020, Robert Griesemer - Typing [Generic] Go
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