It's because os.FileInfo is an interface, not a concrete type.
Specifically, it's an alias for io/fs.FileInfo. The concrete type returned
by os.Stat() is os.fileStat; a struct that implements the os.FileInfo
interface. While you can't refer to a private type like os.fileStat there
is nothing prohibiting the os package from returning a private type that
satisfies an interface. See, for example,
https://github.com/golang/go/blob/45f45444b307cea7c8330b100b30382e642e010f/src/os/stat_unix.go#L15-L26
On Sun, Mar 6, 2022 at 11:47 AM jlfo...@berkeley.edu <jlforr...@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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> .
>
--
Kurtis Rader
Caretaker of the exceptional canines Junior and Hank
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