[m-dev.] diff: io__call_system/4

[Andrew Bromage]

G'day all.

I wrote:

> Perhaps a better interface for now is to return a type such as:
> 
> :- type io__system_result(T)
> 	--->	ok(T)
> 	;	signal(int)
> 	;	error(io__error).

I've been having a think about this.  Perhaps a better solution is to
turn io__error into a discriminated union type internally.

:- type io__error
	--->	message(string)
	;	signal(int).

io__error_message(message(Msg), Msg).
io__error_message(signal(S), Msg) :-
	string__format("Aborted with signal %i", [i(S)], Msg).

Then introduce some predicates to get the signal information out of
the io__error for those who need that information.

Zoltan wrote:

> The type conversion should be in Mercury code because allocating memory
> in C code should be avoided if possible.

Agreed.

Cheers,
Andrew Bromage