Consider the following function:
func NilCompare[T comparable](a *T, b *T) bool {
if a == nil && b == nil {
// if both nil, we consider them equal
return true
}
if a == nil || b == nil {
// if either is nil, then they are clearly not equal
return false
}
return *a == *b
}
This function works. However, when I call it, I must supply the type, as Go cannot infer (cannot infer T) it, e.g. NilCompare[string](a, b), where a and b are *string.
If I modify T to be *comparable and a and b to be T, I get this error instead:
cannot use type comparable outside a type constraint: interface is (or embeds) comparable
I am using Go 1.19.2.
$ go version
go version go1.19.2 linux/amd64
Ironically, my IDE (GoLand 2022.2.3) believes that the above function should be inferrable.
Is there a way to make a function that take nillable comparable and make it inferrable? Or am I doing it correct, but I need to help the go function along?
>Solution :
Type inference just works, in this case. You simply can’t infer T using literal nil, as NilCompare(nil, nil) because that doesn’t really carry type information.
To test your function with nils do this:
package main
import "fmt"
func main() {
var a *string = nil
var b *string = nil
// a and b are explicitly typed
res := NilCompare(a, b) // T inferred
fmt.Println(res) // true
}
this also would work:
func main() {
// literal nil converted to *string
res := NilCompare((*string)(nil), (*string)(nil)) // T inferred
fmt.Println(res) // true
}