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 <pmich...@pobox.com> 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, <eef...@gmail.com> 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