[m-rev.] IO error type

[Julien Fischer]

On Wed, 17 Aug 2022, Peter Wang wrote:

>    :- type system_error.
>
>    :- func make_io_error(system_error, string) = io.error.
>
>    :- pred get_system_error(io.error::in, system_error::out)
>        is semidet.
>
> That is, to introduce a backend/platform specific system_error type,
> and add a field for system_errors in the io.error type.
> (It would still be possible to construct an io.error without a
> system_error value.)

That was my preferred approach as well, however on Windows there is
samll wrinkle with this: some of the system error codes we return are
*not* errno values but instead are system error codes (e.g. of the kind
returned by GetLastError()). So at least on Windows, if you embed a raw
system error in an io.error, you have the additional problem of having
to know what kind of error code it is.

(This is not a problem with the existing code, since all error codes
are turned into error messages fairly close to the point where they
occur, and at those points we know what kind of error code it is.)

Looking a bit closer: the calls to GetLastError() occur pretty much
in the dir module, so maybe we can re-arrange how it handles errors
to avoid that?

> If we add your suggestion to allow converting an io.error to a name,
> we might have:
>
> 	% Returns a standard error name if the given error corresponds
> 	% to a system error known to the Mercury system, e.g. "ENOENT"
> 	% (or possibly the name of a Java or C# exception?).

Given that C# and Java exceptions are usually arranged in a class
hierarchy, which name would you return?

> 	% Fails if the error is not a system error, or if the error is
> 	% not one recognised by the Mercury system.
> 	%
>    :- pred get_error_name(io.error::in, string::out)
> 	is semidet.
>
> 	% As above, but takes a system_error instead.
> 	%
>    :- pred get_system_error_name(system_error::in, string::out)
> 	is semidet.

My preference would be for the standard library to simply give access
the the system error value and leave it for other system-specific
libraries (e.g. the POSIX binding) to provide further facilities for
examining it.

Julien.