# The Universe of Discourse

Tue, 21 May 2019

Say $dt is a Perl DateTime object. You are allowed to say $dt->add( days => 2 )
$dt->subtract( days => 2 )  Today Jeff Boes pointed out that I had written a program that used $dt->add({ days => 2 })


which as far as I can tell is not documented to work. But it did work. (I wrote it in 2016 and would surely have noticed by now if it hadn't.) Jeff told me he noticed when he copied my code and got a warning. When I tried it, no warning.

It turns out that

  $dt->add({ days => 2 })$dt->subtract({ days => 2 })


both work, except that:

1. The subtract call produces a warning (add doesn't! and Jeff had changed my add to subtract)

2. If you included an end_of_month => \$mode parameter in the arguments to subtract, it would get lost.

Also, the working-ness of what I wrote is a lucky fluke. It is undocumented (I think) and works only because of a quirk of the implementation. ->add passes its arguments to DateTime::Duration->new, which passes them to Params::Validate::validate. The latter is documented to accept either form. But its use by DateTime::Duration is an undocumented implementation detail.

->subtract works the same way, except that it does a little bit of preprocessing on the arguments before calling DateTime::Duration->new. That's where the warning comes from, and why end_of_month won't work with the hashref form.

(All this is as of version 1.27. The current version is 1.51. Matthew Horsfall points out that 1.51 does not raise a warning, because of a different change to the same interface.)

This computer stuff is amazingly complicated. I don't know how anyone gets anything done.