How can I instantiate a non-nil pointer of type argument with generic Go?

Question

Now that type parameters are available on golang/go:master, I decided to give it a try. It seems that I'm running into a limitation I could not find in the Type Parameters Proposal. (Or I must have missed it).

I want to write a function which returns a slice of values of a generic type with the constraint of an interface type. If the passed type is an implementation with a pointer receiver, how can we instantiate it?

type SetGetter[V any] interface {
    Set(V)
    Get() V
}

// SetGetterSlice turns a slice of type V into a slice of type T,
// with T.Set() called for each entry in values.
func SetGetterSlice[V any, T SetGetter[V]](values []V) []T {
    out := make([]T, len(values))

    for i, v := range values {
        out[i].Set(v) // panic if T has pointer receiver!
    }

    return out
}

When calling the above SetGetterSlice() function with the *Count type as T, this code will panic upon calling Set(v). (Go2go playground) To no surprise, as basically the code created a slice of nil pointers:


// Count implements SetGetter interface
type Count struct {
    x int
}

func (c *Count) Set(x int) { c.x = x }
func (c *Count) Get() int  { return c.x }

func main() {
    ints := []int{1, 2, 3, 4, 5}

    sgs := SetGetterSlice[int, *Count](ints)
    
    for _, s := range sgs {
        fmt.Println(s.Get())
    }
}

Variations of the same problem

This ideas won't work, and I can't seem to find any simple way to instantiate the pointed value.

out[i] = new(T) will result in a compile failure, as it returns a *T where the type checker wants to see T.
Calling *new(T), compiles but will result in the same runtime panic because new(T) returns **Count in this case, where the pointer to Count is still nil.
Changing the return type to a slice of pointer to T will result in a compile failure:

func SetGetterSlice[V any, T SetGetter[V]](values []V) []*T {
    out := make([]*T, len(values))

    for i, v := range values {
        out[i] = new(T)
        out[i].Set(v) // panic if T has pointer receiver
    }

    return out
}

func main() {
    ints := []int{1, 2, 3, 4, 5}

    SetGetterSlice[int, Count](ints)
    // Count does not satisfy SetGetter[V]: wrong method signature
}

Workaround

The only solution I found until now, is to require a constructor function to be passed to the generic function. But this just feels wrong and a bit tedious. Why would this be required if func F(T interface{})() []T is perfectly valid syntax?

func SetGetterSlice[V any, T SetGetter[V]](values []V, constructor func() T) []T {
    out := make([]T, len(values))

    for i, v := range values {
        out[i] = constructor()
        out[i].Set(v)
    }

    return out
}

// ...
func main() {
    ints := []int{1, 2, 3, 4, 5}

    SetGetterSlice[int, *Count](ints, func() *Count { return new(Count) })
}

Summary

My questions, in order of priority:

Am I overlooking something obvious?
Is this a limitation of generics in Go and this is as good as it gets?
Is this limitation known or should I raise an issue at the Go project?

blackgreen · Accepted Answer · 2022-05-31T19:21:25.597

Basically you have to add one more type parameter to the constraint to make T convertible to its pointer type. In its most basic form, this technique looks like the following (with an anonymous constraint):

func Foo[T any, PT interface { *T; M() }]() {
    p := PT(new(T))
    p.M() // calling method on non-nil pointer
}

Playground: https://go.dev/play/p/L00tePwrDfx

Step by step solution

Your constraint SetGetter already declares a type param V, so we slightly modify the example above:

// V is your original type param
// T is the additional helper param
type SetGetter[V any, T any] interface {
    Set(V)
    Get() V
    *T
}

Then you define the SetGetterSlice function with the type parameter T any, whose purpose is just to instantiate the constraint SetGetter.

You will then be able to convert the expression &out[i] to the pointer type, and successfully call the method on the pointer receiver:

// T is the type with methods with pointer receiver
// PT is the SetGetter constraint with *T
func SetGetterSlice[V any, T any, PT SetGetter[V, T]](values []V) []T {
    out := make([]T, len(values))

    for i, v := range values {
        // out[i] has type T
        // &out[i] has type *T
        // PT constraint includes *T
        p := PT(&out[i]) // valid conversion!
        p.Set(v)         // calling with non-nil pointer receiver
    }

    return out
}

Full program:

package main

import (
    "fmt"
)

type SetGetter[V any, T any] interface {
    Set(V)
    Get() V
    *T
}

func SetGetterSlice[V any, T any, PT SetGetter[V, T]](values []V) []T {
    out := make([]T, len(values))

    for i, v := range values {
        p := PT(&out[i])
        p.Set(v)
    }

    return out
}

// Count implements SetGetter interface
type Count struct {
    x int
}

func (c *Count) Set(x int) { c.x = x }
func (c *Count) Get() int  { return c.x }

func main() {
    ints := []int{1, 2, 3, 4, 5}

    // instantiate with base type
    sgs := SetGetterSlice[int, Count](ints)

    for _, s := range sgs {
        fmt.Println(s.Get()) // prints 1,2,3,4,5 each in a newline
    }
}

This becomes more verbose because SetGetterSlice now requires three type parameters: the original V plus T (the type with pointer receivers) and PT (the new constraint). However when you call the function, you can omit the third one – with type inference, both type params V and T required to instantiate PT SetGetter[V,T] are already known:

SetGetterSlice[int, Count](ints)

Playground: https://go.dev/play/p/gcQZnw07Wp3

score 2 · Answer 2 · answered Nov 12 '22 at 23:43

Spent a few hours to understand it. So decided to add my example.

package main

import (
    "fmt"
)

type User struct {
    FullName string
    Removed  bool
}

type Account struct {
    Name    string
    Removed bool
}

type Scanner[T User | Account] interface {
    Scan()
    *T
}

type Model interface {
    User | Account
}

func (user *User) Scan() {
    user.FullName = `changed in scan method`
    user.Removed = true
}

func (account *Account) Scan() {
    account.Name = `changed in scan method`
    account.Removed = true
}

func setRemovedState[T Model, PT Scanner[T]](state bool) *T {
    var obj T

    pointer := PT(&obj)
    pointer.Scan() // calling method on non-nil pointer

    return &obj
}

func main() {
    user := setRemovedState[User](true)
    account := setRemovedState[Account](true)

    fmt.Printf("User: %v\n", *user)
    fmt.Printf("Account: %v\n", *account)
}

score 1 · Answer 3 · 2021-10-14T21:54:56.467

you can also try to attack the problem slighty differently, to keep it simple.

package main

import (
    "fmt"
)

func mapp[T any, V any](s []T, h func(T) V) []V {
    z := make([]V, len(s))
    for i, v := range s {
        z[i] = h(v)
    }
    return z
}

func mappp[T any, V any](s []T, h func(T) V) []V {
    z := make([]V, 0, len(s))
    for _, v := range s {
        z = append(z, h(v))
    }
    return z
}

// Count implements SetGetter interface
type Count struct {
    x int
}

func (c *Count) Set(x int) { c.x = x }
func (c *Count) Get() int  { return c.x }

func FromInt(x int) *Count {
    var out Count
    out.x = x
    return &out
}

func main() {
    ints := []int{1, 2, 3, 4, 5}

    sgs := mapp(ints, FromInt)
    fmt.Printf("%T\n",sgs)

    for _, s := range sgs {
        fmt.Println(s.Get())
    }

    fmt.Println()

    sgs = mappp(ints, FromInt)
    fmt.Printf("%T\n",sgs)

    for _, s := range sgs {
        fmt.Println(s.Get())
    }
}

https://go2goplay.golang.org/p/vzViKwiJJkZ

It is like your func SetGetterSlice[V any, T SetGetter[V]](values []V, constructor func() T) []T but without the complex verbosity. It also gave me zero pain to solve.

torek · Answer 4 · 2021-10-15T01:50:41.887

Edit: see blackgreen's answer, which I also found later on my own while scanning through the same documentation they linked. I was going to edit this answer to update based on that, but now I don't have to. :-)

There is probably a better way—this one seems a bit clumsy—but I was able to work around this with reflect:

if reflect.TypeOf(out[0]).Kind() == reflect.Ptr {
    x := reflect.ValueOf(out).Index(i)
    x.Set(reflect.New(reflect.TypeOf(out[0]).Elem()))
}

I just added the above four lines to your example. The temporary variable is left over from some debug and obviously can be removed. Playground link

How can I instantiate a non-nil pointer of type argument with generic Go?

Variations of the same problem

Workaround

Summary

4 Answers4

Linked

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