[mercury-users] Perl shift || $defalt

[Ralph Becket]

> In Perl, it is very easy to say the following:
> 
>   the value of x is equal to the first data item in ARGV,
>   or if ARGV is not defined then use the value 12:
> 
> $x = shift || 12;
> print "$x\n";
> 
> Thus, a program can either take values on STDIN or use hardcoded
> defaults with relative ease.
> 
> Could someone show how to do this same thing in Mercury?

main -->
	io__command_line_args(ArgV),
	{ if ArgV = [Arg1 | _] then X = Arg1 else X = "12" },
	io__format("%s\n", [s(X)]).

It's not quite the same since `shift' returns the first argument and
then removes it from argv; in Mercury you have to handle the list
explicitly.

-- Ralph
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