Maybe something like...?
$ cat t.p6
sub infix:<rescue>(Callable $block, $otherwise) {
CATCH { return $otherwise; }
$block();
}
sub divide($a, $b) { die "Zero denominator" if $b == 0; $a / $b }
my $sixdivzero = { divide(6,0) } rescue -1;
say "6/0 = ", $sixdivzero;
my $sixdivtwo = { divide(6,2) } rescue -1;
say "6/2 = ", $sixdivtwo;
$ perl6 t.p6
6/0 = -1
6/2 = 3
Or if you prefer a prefix form, just declare "rescue" as a normal sub and then
do:
rescue { divide(6,2) }, -1;
Pm
On Fri, Aug 03, 2018 at 08:34:44PM +0100, Simon Proctor wrote:
> Hi Sean. I hope my second answer in stackoverflow gets closer to what you
> want.
>
> I am still trying to think of a more idiomatic way of handling to situation.
>
>
>
> On Fri, 3 Aug 2018, 19:29 Sean McAfee, <[email protected]> wrote:
>
> > I posted about this subject on Stack Overflow yesterday[1], but I chose a
> > poor example of something that raises an exception (dividing by zero, which
> > apparently doesn't necessarily do so) on which the answers have mostly
> > focused.
> >
> > I was looking for a way to evaluate an expression, and if the expression
> > threw an exception, for a default value to be provided instead. For
> > example, in Ruby:
> >
> > quotient = begin; a / b; rescue; -1; end
> >
> > Or in Lisp:
> >
> > (setq quotient (condition-case nil (/ a b) (error -1)))
> >
> > Not having written much exception-related code in Perl 6, I hoped that
> > this might work:
> >
> > sub divide($a, $b) { die "Zero denominator" if $b == 0; $a / $b }
> > my $quotient = do { divide($a, $b); CATCH { default { -1 } } };
> >
> > It doesn't, though. As far as I can tell, the value to which a CATCH
> > block evaluates is ignored; the only useful things one can do in such a
> > block are things with side effects. Long story short, I eventually came up
> > with this:
> >
> > my $quotient = do { my $q; { $q = divide($a, $b); CATCH { default { $q
> > = -1 } } }; $q };
> >
> > That's far more verbose than I've come to expect from Perl 6. Is there
> > some more concise way of expressing this logic?
> >
> > The doc page on exceptions mentions try, eg:
> >
> > my $quotient = try { divide($a, $b) } // -1;
> >
> > That works in this specific case, but it seems insufficient in general.
> > The function might validly return an undefined value, and this construction
> > can't distinguish between that and an exception. Also, it wouldn't let me
> > distinguish among various exception cases. I'd have to do something like:
> >
> > class EA is Exception { }
> > class EB is Exception { }
> > sub divide($a, $b) { (EA, EB).pick.new.throw if $b == 0; $a / $b }
> >
> > my $quotient = do { my $q; { $q = divide($a, $b); CATCH { when EA { $q
> > = -1 }; when EB { $q = -2 } } }; $q };
> >
> >
> > [1]
> > https://stackoverflow.com/questions/51644197/returning-values-from-exception-handlers-in-perl-6/51670573
> >
> --
> Simon Proctor
> Cognoscite aliquid novum cotidie
