Interesting, thanks for the bug report.
"is" a synonym for "=", so here you've actually defined a function with, apparently, nothing to match to the left of the =, causing all kinds of nastiness.
I've fixed the bug by requiring that function definitions must have something on either side of the '=' (duh!).
To answer your question about it being like Siri, while it currently doesn't know how to provide weather reports, or to understand locations, the idea is that users can teach it (although it's not currently capable of this).