https://go.dev/play/p/pz02pSaVCyl
A function can return a value of a private type (or a pointer to such a
type, as you found). It's a way to return an opaque value that implements
an interface.
The receiver in a different package can assign that value, but can't
declare additional variables of that type. They can of course call public
methods on values of that type - or assign to a variable of a compatible
interface type.
On Sunday, 6 March 2022 at 19:46:18 UTC jlfo...@berkeley.edu wrote:
> (go version go1.17.7 linux/amd64)
>
> Consider the following trivial program:
> ------
> package main
>
> import (
> "fmt"
> "os"
> )
>
> func main() {
> file := "."
> fileinfo, _ := os.Stat(file)
> fmt.Printf("type of fileinfo = %T\n", fileinfo)
> }
> ------
>
> This runs and produces the output
>
> type of fileinfo = *os.fileStat
>
> Fine, but notice that "fileStat" isn't capitalized. This means this symbol
> isn't
> exported outside the "os" package. Yet, somehow the "fileinfo" variable is
> assigned
> this type.
>
> Indeed, if I try to explicitly use the "os.fileStat" type in the program,
> the program fails to compile, e.g.
>
> ------------
> package main
>
> import (
> "fmt"
> "os"
> )
>
> func main() {
> var fileinfo *os.fileStat
>
> file := "."
> fileinfo, _ = os.Stat(file)
> fmt.Printf("type of fileinfo = %T\n", fileinfo)
> }
> -----
> results in
>
> ./x3.go:9:16: cannot refer to unexported name os.fileStat
> ./x3.go:12:14: cannot assign fs.FileInfo to fileinfo (type *os.fileStat)
> in multiple assignment: need type assertion
> ./x3.go:12:14: cannot use fs.FileInfo value as type *os.fileStat in
> assignment: need type assertion
>
> Notice the first error message.
>
> I also don't understand why the other two error message are produced when
> all I did was to explicitly declare a variable that was previously assigned
> a value in a short declaration.
>
> What am I missing?
>
> Cordially,
> Jon Forrest
>
>
>
>
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