Yes; but then I think that something like infix:<rescue> probably just ends up
as a macro somehow. I just didn't know the state of macros in Perl 6 well
enough to be able to head down that route. :)
Pm
On Fri, Aug 03, 2018 at 10:32:41PM +0200, Elizabeth Mattijsen wrote:
> Sometimes I wish we could use Thunk as a type:
>
> sub infix:<rescue>(Thunk:D $block, $otherwise) { }
>
> which would then allow you to do:
>
> my $sixdivzero = divide(6,0) rescue -1; # note absence of curlies
>
>
>
> One can wish, can’t one?
>
>
> Liz
>
> > On 3 Aug 2018, at 22:18, Patrick R. Michaud <[email protected]> wrote:
> >
> > 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
