go generics: processing different struct types with same data member types [duplicate] - go

Question

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How can I access a struct field with generics (type T has no field or method)?
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Generic function to work on different structs with common members from external package?
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Closed 3 months ago.
There are two struct types, Foo and Bar, with an int data member val. I am trying to write a generic function that can handle both types. I tried the following and this did not work.
package main
import "fmt"
type Foo struct {
val int
}
type Bar struct {
val int
}
func Add[T any](slice []T) int {
var sum int
for _, elem := range slice {
sum += elem.val
}
return sum
}
func Test() {
f1 := Foo{val: 2}
f2 := Foo{val: 2}
fslice := []Foo{f1, f2}
fsum := Add(fslice)
fmt.Printf("fsum = %d\n", fsum)
b1 := Bar{val: 3}
b2 := Bar{val: 3}
bslice := []Bar{b1, b2}
bsum := Add(bslice)
fmt.Printf("bsum = %d\n", bsum)
}
func main() {
Test()
}
The compiler throws the following error.
$ go run generics1.go
# command-line-arguments
./generics1.go:16:15: elem.val undefined (type T has no field or method val)
Go playground link: https://go.dev/play/p/mdOMH3xuwu7
What could be a possible way to approach this?

Answer 1

Per golang 1.18 release note
The Go compiler does not support accessing a struct field x.f where x is of type parameter type even if all types in the type parameter's type set have a field f. We may remove this restriction in a future release.
You could define one GetVal() interface method to retrieve the val, and use this method as part of type constraint of generic.
Sample codes
type Foo struct {
val int
}
func (f Foo) GetVal() int {
return f.val
}
type Bar struct {
val int
}
func (b Bar) GetVal() int {
return b.val
}
type MyType interface {
Foo | Bar
GetVal() int
}
func Add[T MyType](slice []T) int {
var sum int
for _, elem := range slice {
sum += elem.GetVal()
}
return sum
}
https://go.dev/play/p/0eJZpqy7q8f