[m-users.] Error handling in io module

[Volker Wysk]

Hi.

I suggest to add a test to the io module, for an error being zero. Meaning
no error / success. This could be a constant, say "error_success", or a
predicate "error_is_success/1".

I'm in the situation that I need to test a system_error (an errno) for being
a success. The only way I've found is to create an io.error from it and call
get_system_error_name/2. For the zero error, this creates the "errno 0"
string. The errno.h header doesn't define an errno value for "success".

The created io.error is always a system error, and thus the call of
get_system_error_name/2 always succeeds. But the compiler doesn't get that
and it's necessary to do an ugly, non-needed unexpected/2 call or something
like that.

Cheers,
Volker
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