[m-rev.] IO error type

[Julien Fischer]

On Thu, 18 Aug 2022, Peter Wang wrote:

>> 5. We also add the functions you listed below
>>
>>>>>    :- 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.
>>
>> except that the signature of the latter one would be:
>>
>>      :- pred get_system_error_name(io.error::in, string::out)
>>         is semidet.
>
> Right, if a system_error could be value from multiple sets of errors
> then we can't have a predicate that converts an system_error (only) to a
> string.
>
>> (I hope there's a way of getting Windows error code name, that doesn't
>> involve switching over all 5000 or so Windows error codes; perhaps
>> FormatMessage can do that?)
>>
>
> I couldn't find one :(

Neither could I.

> The MinGW-w64 winerror.h defines ~1765 ERROR_* constants, which should
> be far beyond what we need to recognise (practically).

At the moment the only ones we generate are those in the dir module.
I think it would suffice to handle those plus the most common other
cases and for every other value construct a string of the form:

     "System error: 0xNNNNNNN"

(That's no worse than what Microsoft does in various places; the number
is hexadecimal since that what Microsoft's documentation does.)

> Both the switch for the POSIX and Windows errors would be
> auto-generated, so it's just a matter of code size.

For "unrecognised" errno values, I suggest we create a string
of the form:

     "errno: NNNNN"

(In decimal this time.)

Julien.