Thanks, that's really useful.
It never occurred to me that you could cast it to a type after the error
has been appended - my attempts to print out the type that was being
returned didn't show this as an option.
I'll have a play.
Thanks again
On Monday, 2 October 2017 17:20:41 UTC+1, Marvin Renich wrote:
>
> * Marvin Renich <mr...@renich.org <javascript:>> [171002 11:31]:
> > * Chris Hopkins <cbeho...@gmail.com <javascript:>> [171002 10:52]:
> > > out, err := os.Create(potential_file_name)
> > > switch t := err.(type) {
> > > case *os.PathError:
> > > switch t.Err {
> > > case os.ErrNotExist:
> > > log.Fatal("Invalid filename", potential_file_name)
> > > case os.ErrInvalid:
> > > log.Fatal("Invalid argument", potential_file_name)
> > > default :
>
> Additionally, os.Create, os.Open, and os.OpenFile all guarantee (via
> their documentation and the Go 1 Compatibility Guarantee) that if they
> return a non-nil error, it will be of type *os.PathError, so you could,
> if you wish, replace the "switch t := err.(type)" with
> t = err.(*os.PathError) and if it panics, you can file a bug.
> Furthermore, because the current implementation is extremely unlikely to
> change, you can simplify even more. I would write this as:
>
> var out, err = os.Create(potential_file_name)
> if err != nil {
> var pe = err.(*os.PathError) // let it panic or use the ,ok trick
> as below
> var en, ok = pe.Err.(syscall.Errno) // not a Go 1 Compat
> guarantee, so handle failed type assertion
> if !ok {
> log.Fatalf("Unexpected error from os.Create: %s\n", pe)
> }
> switch en {
> case syscall.EEXIST:
> ...
> case syscall.EISDIR:
> ...
> case syscall.EINVAL:
> ...
> }
> }
>
> This code is much more readable than the nested type switch that you are
> using. Note that os.ErrNotExist and os.ErrInvalid are not of type
> *os.PathError and os.Create will never return either of them (promised
> by Go 1 Compat).
>
> If the !ok test ever succeeds, you know that the implementation of
> os.OpenFile has changed.
>
> ...Marvin
>
>
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