Mojo::Promise swallows exceptions in rejection handlers

[Rob Foehl]

Just ran into this on 7.55:

╶➤ perl -MMojo::Promise -le 'my $p = Mojo::Promise->new; $p->reject("die, please"); $p->catch(sub { die "oops: $_[0]\n" }); $p->wait'

Silence... Change the die to a warn, and:

╶➤ perl -MMojo::Promise -le 'my $p = Mojo::Promise->new; $p->reject("die, please"); $p->catch(sub { warn "oops: $_[0]\n" }); $p->wait'
oops: die, please

The promise is correctly rejected (confirmed by poking at $p->{status}),
but the exception disappears. I'm guessing that wasn't the intent, but
I'm not entirely sure after a brief glance at the implementation...

-Rob

[Dan Book]

The problem is that no exceptions in promise handlers are fatal, they occur in the event loop. The only way the exception could be propagated is if the call to ->wait checked for a failure and rethrew it as an exception afterward (this is how ->get works in Future). Also note that $p is not the last promise in the chain, that is returned by the ->catch call (so you would normally chain ->wait after that). I'm not sure what the idiomatic way is to accomplish this with promises.

-Dan

-Rob

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[sri]

I'm guessing that wasn't the intent, but 

I'm not entirely sure after a brief glance at the implementation...

It is intended, of course we welcome ideas for making

Mojo::Promise more dwim, as long as it doesn't conflict with the

specs.

--

sebastian