Author here. I ran into this while comparing Pendulum with `whenever`, the datetime library I maintain. I sent a patch that removes the hack, but the post is about why the underlying design can't be patched the same way. Happy to answer questions.

OMGeeeee!

I too have plumbed the call stack for hidden-maybe-forbidden knowledge from the caller, and it is always fraught. No matter how good one's intentions. Forget dynamic type testing. Forget Perl's `wantarray` dynamic introspection of "what does the caller want?" Those are small potatoes. This is full on "who's calling me and what do I want to send back as a result?" The people who will never forgive you for type testing are going to straight-up excommunicate whoever wrote this! And yet...it makes complete sense. Based on the promises made, you have to do something... and you're already in "what is the least of the evils?" territory, because you're going to have to do something that someone—maybe a lot of someones—will call evil.

My admiration for Pendulum just increased. This is sin-eating.

"In the face of ambiguity, refuse the temptation to guess."