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code {
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<!-- _class: chapter -->
# Introduzione alle Generics in Go
---
## Chi sono?
Antonio De Lucreziis, studente di Matematica e macchinista del PHC
### Cos'è il PHC?
Il PHC è un gruppo di studenti di Matematica con interessi per, open source, Linux, self-hosting e soprattutto smanettare sia con hardware e software (veniteci pure a trovare!)
< div style = "display: flex; align-items: center; justify-content: center; gap: 2rem;" >
< img src = "./assets/devfest-logo.png" height = "100" / >
< img src = "./assets/logo-circuit-board.svg" height = "100" / >
< / div >
---
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_The Go 1.18 release adds support for generics. Generics are the biggest change we’ ve made to Go since the first open source release_
:link: < https: / / go . dev / blog / intro-generics >
---
## Il Problema
---
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```go
func Min(x, y int) int {
if x < y {
return x
}
return y
}
```
---
```go
func MinInt8(x, y int8) int8 {
if x < y {
return x
}
return y
}
func MinInt16(x, y int16) int16 {
if x < y {
return x
}
return y
}
func MinFloat32(x, y float32) float32 {
if x < y {
return x
}
return y
}
```
---
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< / style >
```go
...
if x < y {
return x
}
return y
...
```
---
## Soluzioni Pre-Generics
- Fare una funzione che prende `any` ed usare degli switch sul tipo
- Copia incollare tante volte la funzione per ogni tipo
- Utilizzare tool come `go generate`
---
## Soluzione Post-Generics
#### Type Parameters
```go
import "golang.org/x/exp/constraints"
func Min[T constraints.Ordered](x, y T) T {
if x < y {
return x
}
return y
}
```
```go
var a, b int = 0, 1
Min[int](a, b)
...
var a, b float32 = 3.14, 2.71
Min[float32](a, b)
```
---
#### Type Inference
```go
var a, b int = 0, 1
Min(a, b)
...
var a, b float32 = 3.14, 2.71
Min(a, b)
```
---
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```
[T Vincolo1, R interface{ Method(), ... }, ...]
```
---
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## Type Sets
< img src = "./assets/method-sets.png" / >
:anchor: < https: / / go . dev / blog / intro-generics >
---
< style scoped > section { justify-content : space-between ; } < / style >
## Type Sets
< img src = "./assets/type-sets.png" / >
:fish: < https: / / go . dev / blog / intro-generics >
---
< style scoped > section { justify-content : space-between ; } < / style >
## Type Sets
< img src = "./assets/type-sets-2.png" / >
:fish: < https: / / go . dev / blog / intro-generics >
---
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#### Type Sets (Sintassi)
- `[T interface{}]` si può anche scrivere `[T any]`
- `[T interface{ int | float32 }]` si può anche scrivere `[T int | float32]`
---
#### Type Sets
```go
func Somma[T float32|float64](x, y T) T {
return x + y
}
```
```go
type Liter float64
```
```go
var a, b int = 1, 2
Somma(a, b) // Ok
var a, b Liter = 1, 2
Somma(a, b) // Errore
```
---
#### Type Sets
```go
func Somma[T ~float32|~float64](x, y T) T {
return x + y
}
```
```go
type Liter float64
```
```go
var a, b int = 1, 2
Somma(a, b) // Ok
var a, b Liter = 1, 2
Somma(a, b) // Ok
```
---
#### Type Sets
```go
package constraints
...
type Float interface {
~float32 | ~float64
}
...
```
---
#### Type Sets
```go
package constraints
...
type Ordered interface {
Integer | Float | ~string
}
type Float interface {
~float32 | ~float64
}
type Integer interface {
Signed | Unsigned
}
type Signed interface {
~int | ~int8 | ~int16 | ~int32 | ~int64
}
type Unsigned interface {
~uint | ~uint8 | ~uint16 | ~uint32 | ~uint64 | ~uintptr
}
...
```
---
<!-- _class: chapter -->
# Tipi Generici
---
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```go
type Stack[T any] []T
```
```go
func (s *Stack[T]) Push(value T) {
*s = append(*s, value)
}
func (s Stack[T]) Peek() T {
return s[len(s)-1]
}
func (s Stack[T]) Len() int {
return len(s)
}
```
---
```go
func (s *Stack[T]) Pop() (T, bool) {
items := *s
if len(items) == 0 {
var zero T
return zero, false
}
newStack, poppedValue := items[:len(items)-1], items[len(items)-1]
*s = newStack
return poppedValue, true
}
```
---
Per ora ci tocca utilizzare questa funzione di _utility_
```go
func Zero[T any]() T {
var zero T
return zero
}
```
:link: [43651-type-parameters.md#the-zero-value ](https://go.googlesource.com/proposal/+/refs/heads/master/design/43651-type-parameters.md#the-zero-value )
---
<!-- _class: chapter -->
# Pattern: Tipi Contenitore
---
### Tipi generici nativi
- `[n]T`
Array di `n` elementi per il tipo `T`
- `[]T`
Slice per il tipo `T`
- `map[K]V`
Mappe con chiavi `K` e valori `V`
- `chan T`
Canali per elementi di tipo `T`
---
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## `golang.org/x/exp/slices`
- `func Index[E comparable](s []E, v E) int`
- `func Equal[E comparable](s1, s2 []E) bool`
- `func Sort[E constraints.Ordered](x []E)`
- `func SortFunc[E any](x []E, less func(a, b E) bool)`
- e molte altre...
---
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font-size: 140%;
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}
< / style >
## `golang.org/x/exp/maps`
- `func Keys[M ~map[K]V, K comparable, V any](m M) []K`
- `func Values[M ~map[K]V, K comparable, V any](m M) []V`
- e molte altre...
---
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font-size: 140%;
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}
< / style >
## Strutture Dati Generiche
Esempio notevole: < https: / / github . com / zyedidia / generic > (1K:star: su GitHub)
- `mapset.Set[T comparable]` , set basato su un dizionario.
- `multimap.MultiMap[K, V]` , dizionario con anche più di un valore per chiave.
- `stack.Stack[T]` , slice ma con un'interfaccia più simpatica rispetto al modo idiomatico del Go.
- `cache.Cache[K comparable, V any]` , dizionario basato su `map[K]V` con una taglia massima e rimuove gli elementi usando la strategia LRU.
- `bimap.Bimap[K, V comparable]` , dizionario bi-direzionale.
- `hashmap.Map[K, V any]` , implementazione alternativa di `map[K]V` con supporto per _copy-on-write_ .
- e molte altre...
---
<!-- _class: chapter -->
# Anti-Pattern (1)
Utility HTTP
---
```go
// library code
type Validator interface {
Validate() error
}
func DecodeAndValidateJSON[T Validator](r *http.Request) (T, error) {
var value T
if err := json.NewDecoder(r.Body).Decode(&value); err != nil {
var zero T
return zero, err
}
if err := value.Validate(); err != nil {
var zero T
return zero, err
}
return value, nil
}
```
---
```go
// client code
type FooRequest struct {
A int `json:"a"`
B string `json:"b"`
}
func (foo FooRequest) Validate() error {
if foo.A < 0 {
return fmt.Errorf(`parameter "a" cannot be lesser than zero`)
}
if !strings.HasPrefix(foo.B, "baz-") {
return fmt.Errorf(`parameter "b" has wrong prefix`)
}
return nil
}
```
```go
foo, err := DecodeAndValidateJSON[FooRequest](r)
if err != nil {
http.Error(w, err.Error(), http.StatusInternalServerError)
return
}
```
---
```go
func DecodeAndValidateJSON(r *http.Request, target Validator) error {
err := json.NewDecoder(r.Body).Decode(target)
if err != nil {
return err
}
if err := target.Validate(); err != nil {
return err
}
return nil
}
```
```go
var foo FooRequest
if err := DecodeAndValidateJSON(r, &foo); err != nil {
http.Error(w, err.Error(), http.StatusInternalServerError)
return
}
```
---
<!-- _class: chapter -->
# Anti-Pattern (2)
Generics vs Interfacce
---
## Momento Quiz
```go
func WriteOneByte(w io.Writer, data byte) {
w.Write([]byte{data})
}
...
d := & bytes.Buffer{}
WriteOneByte(d, 42)
```
```go
func WriteOneByte[T io.Writer](w T, data byte) {
w.Write([]byte{data})
}
...
d := & bytes.Buffer{}
WriteOneByte[*bytes.Buffer](d, 42)
```
---
```
BenchmarkInterface
BenchmarkInterface-4 135735110 9.017 ns/op
BenchmarkGeneric
BenchmarkGeneric-4 50947912 22.26 ns/op
```
---
```go
//go:noinline
func WriteOneByte(w io.Writer, data byte) {
w.Write([]byte{data})
}
...
d := & bytes.Buffer{}
WriteOneByte(d, 42)
```
---
```
BenchmarkInterface
BenchmarkInterface-4 135735110 9.017 ns/op
BenchmarkInterfaceNoInline
BenchmarkInterfaceNoInline-4 46183813 23.64 ns/op
BenchmarkGeneric
BenchmarkGeneric-4 50947912 22.26 ns/op
```
---
```go
d := & bytes.Buffer{} /* (*bytes.Buffer) */
WriteOneByte(d /* (io.Writer) */, 42)
```
< div style = "font-size: 45px;" > ↓< / div >
```go
d := & bytes.Buffer{} /* (*bytes.Buffer) */
(io.Writer).Write(d /* (io.Writer) */, []byte{ 42 })
```
< div style = "font-size: 45px;" > ↓< / div >
```go
d := & bytes.Buffer{} /* (*bytes.Buffer) */
(*bytes.Buffer).Write(d /* (*bytes.Buffer) */, []byte{ 42 })
```
---
#### Go 1.18 Implementation of Generics via Dictionaries and Gcshape Stenciling
- _A **gcshape** (or gcshape grouping) is a collection of types that all **share the same instantiation of a generic function/method**_ .
- _Two concrete types are in the same gcshape grouping if and only if they have the **same underlying type** or they are **both pointer types** ._
- _To avoid creating a different function instantiation for each generic call with distinct type arguments (which would be pure stenciling), we **pass a dictionary along with every call**_.
:link: [generics-implementation-dictionaries-go1.18.md ](https://github.com/golang/proposal/blob/master/design/generics-implementation-dictionaries-go1.18.md )
<!-- :link: [Go 1.18 implementation of generics via dictionaries and gcshape stenciling ](https://github.com/golang/proposal/blob/master/design/generics-implementation-dictionaries-go1.18.md ) -->
---
Quindi nella maggior parte dei casi se ci ritroviamo a scrivere una funzione generica con un **parametro vincolato ad un'interfaccia** forse dobbiamo porci qualche domanda
---
<!-- _class: chapter -->
# Pattern: Type-safe Database
Vediamo un analogo di `PhantomData<T>` dal Rust per rendere _type-safe_ l'interfaccia di una libreria
---
Proviamo ad usare questa tecnica per rendere _type-safe_ l'interfaccia con `*sql.DB`
```go
type DatabaseRef[T any] string
```
```go
package tables
// tables metadata
var Users = database.Table[User]{ ... }
var Products = database.Table[Product]{ ... }
```
```go
userRef1 := DatabaseRef[User]("j.smith@example.org")
...
// Ok
user1, err := database.Read(dbConn, tables.Users, userRef1)
// Errore
user2, err := database.Read(dbConn, tables.Products, userRef1)
```
---
```go
package database
type WithPK interface {
PrimaryKey() *string
}
type Ref[T WithPK] string
type Table[T WithPK] struct {
Name string
PkColumn string
Columns func(*T) []any
}
...
func Read[T WithPK](d DB, t Table[T], ref Ref[T]) (*T, error)
```
---
```go
package database
func Create[T WithPK](d DB, t Table[T], row T) (Ref[T], error)
func Insert[T WithPK](d DB, t Table[T], row T) (Ref[T], error)
func Read[T WithPK](d DB, t Table[T], ref Ref[T]) (*T, error)
func Update[T WithPK](d DB, t Table[T], row T) error
func Delete[T WithPK](d DB, t Table[T], id Ref[T]) error
```
---
```go
func Read[T WithPK](d DB, t Table[T], ref Ref[T]) (*T, error) {
result := d.QueryRow(
fmt.Sprintf(
`SELECT * FROM %s WHERE %s = ?` ,
t.Name, t.PkColumn,
),
string(ref),
)
var value T
if err := result.Scan(t.Columns(&value)...); err != nil {
return nil, err
}
return & value, nil
}
```
---
```go
package model
type User struct {
Username string
FullName string
Age int
}
func (u *User) PrimaryKey() *string {
return & u.Username
}
```
```go
package tables
var Users = Table[User]{
Name: "users",
PkColumn: "username",
Columns: func(u *User) []any {
return []any{ & u.Username, & u.FullName, & u.Age }
}
}
```
---
Quindi possiamo anche utilizzare le **generics** per rendere **type-safe** l'interfaccia di qualcosa che inizialmente non lo era.
---
<!-- _class: chapter -->
# Pattern: _Channels_
Alcune utility per lavorare meglio con i _channel_
---
```go
func trySend[T any](c chan< - T , v T ) bool {
select {
case c < - v:
return true
default:
return false
}
}
```
---
```go
func raceSame[T any](cs ...< -chan T ) T {
done := make(chan T)
defer close(done)
for _, c := range cs {
go func(c < -chan T ) {
trySend(done, < -c )
}(c)
}
return < -done
}
```
---
```go
type Awaiter interface {
Await()
}
type awaiterChan[T any] < -chan T
func (ac awaiterChan[T]) Await() { < -ac }
```
---
```go
type targetChan[T any] struct {
c < -chan T
target *T
}
func (tc targetChan[T]) Await() { *tc.target = < -tc.c }
```
---
```go
func race(rs ...Awaiter) {
done := make(chan struct{})
defer close(done)
for _, r := range rs {
go func(r Awaiter) {
r.Await()
trySend(done, struct{}{})
}(r)
}
< -done
}
```
---
```go
var result2 int
var result3 float64
raceAny(
awaiterChan[string](c1),
targetChan[int]{c2, & result2},
targetChan[float64]{c3, & result3},
)
fmt.Println(result2, result3)
```
---
```go
var result2 int
var result3 float64
// Variante più pulita di questa utility
channels.Race(
channels.Awaiter(c1),
channels.Awaiter(c2, channels.WithTarget(& result2)),
channels.Awaiter(c3, channels.WithTarget(& result3)),
)
fmt.Println(result2, result3)
```
---
<!-- _class: chapter -->
# 1 + 1 = 2
_Proof checking_ in Go
---
## Premesse
Definiamo i possibili "tipi" delle nostre espressioni
```go
type Bool interface{ isBool() }
type Nat interface{ isNat() }
type Nat2Nat interface{ isNat2Nat() }
```
---
## Premesse
Trick per codificare higher-kinded types in Go
```go
type V[ H Nat2Nat, T Nat ] Nat
```
---
## Assiomi dei Naturali
```go
type Zero Nat
type Succ Nat2Nat
// Alcuni alias utili
type One = V[Succ, Zero]
type Two = V[Succ, V[Succ, Zero]]
type Three = V[Succ, V[Succ, V[Succ, Zero]]]
```
---
## Uguaglianza
```go
type Eq[A, B any] Bool
// Eq_Refl ovvero l'assioma
// forall x : x = x
func Eq_Reflexive[T any]() Eq[T, T] {
panic("axiom: comptime only")
}
// Eq_Symmetric ovvero l'assioma
// forall a, b: a = b => b = a
func Eq_Symmetric[A, B any](_ Eq[A, B]) Eq[B, A] {
panic("axiom: comptime only")
}
// Eq_Transitive ovvero l'assioma
// forall a, b, c: a = b e b = c => a = c
func Eq_Transitive[A, B, C any](_ Eq[A, B], _ Eq[B, C]) Eq[A, C] {
panic("axiom: comptime only")
}
```
---
## Uguaglianza e Sostituzione
Per ogni funzione `F` , ovvero tipo vincolato all'interfaccia `Nat2Nat` vorremmo dire che
```
F
Eq[ A , B ] ------> Eq[ F[A] , F[B] ]
```
---
## Uguaglianza e Sostituzione
Data una funzione ed una dimostrazione che due cose sono uguali allora possiamo applicare la funzione ed ottenere altre cose uguali
```go
// Function_Eq ovvero l'assioma
// forall f function, forall a, b nat: a = b => f(a) = f(b)
func Function_Eq[F Nat2Nat, A, B Nat](_ Eq[A, B]) Eq[V[F, A], V[F, B]] {
panic("axiom: comptime only")
}
```
---
## Assiomi dell'addizione
```go
type Plus[L, R Nat] Nat
// "n + 0 = n"
// Plus_Zero ovvero l'assioma
// forall n, m: n + succ(m) = succ(n + m)
func Plus_Zero[N Nat]() Eq[Plus[N, Zero], N] {
panic("axiom: comptime only")
}
// "n + (m + 1) = (n + m) + 1"
// Plus_Sum ovvero l'assioma
// forall n, m: n + succ(m) = succ(n + m)
func Plus_Sum[N, M Nat]() Eq[
Plus[N, V[Succ, M]],
V[Succ, Plus[N, M]],
] { panic("axiom: comptime only") }
```
---
## 1 + 1 = 2
```go
func Theorem_OnePlusOneEqTwo() Eq[Plus[One, One], Two] {
// 1 + 0 = 1
// en1 :: Eq[ Plus[One, Zero], One ]
en1 := Plus_Zero[One]()
// (1 + 0) + 1 = 2
// en2 :: Eq[ V[Succ, Plus[One, Zero]], Two ]
en2 := Function_Eq[Succ](en1)
// 1 + 1 = (1 + 0) + 1
// en3 :: Eq[ Plus[One, One], V[Succ, Plus[One, Zero]] ]
en3 := Plus_Sum[One, Zero]()
return Eq_Transitive(en3, en2)
}
```
<!--
---
## 1 + 1 = 2
```go
func Theorem_OnePlusOneEqTwo() Eq[Plus[One, One], Two] {
return Eq_Transitive(
Plus_Sum[One, Zero](),
Function_Eq[Succ](
Plus_Zero[One](),
),
)
}
``` -->
---
<!-- _class: chapter -->
# Conclusione
---
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text-align: left;
}
< / style >
### Regole generali
Per scrivere _codice generico_ in Go
- Se l'implementazione dell'operazione che vogliamo supportare non dipende del tipo usato allora conviene usare dei **type-parameter**
- Se invece dipende dal tipo usato allora è meglio usare delle **interfacce**
- Se invece dipende sia dal tipo e deve anche funzionare per tipi che non supportano metodi (ad esempio per i tipi primitivi) allora conviene usare **reflection**
---
# Fine :C
_Domande_
---
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li {
font-size: 80%;
}
< / style >
## Bibliografia
- < https: // go . dev / blog / intro-generics >
- < https: // go . dev / blog / when-generics >
- < https: // github . com / golang / proposal / blob / master / design / generics-implementation-dictionaries-go1 . 18 . md >