"this" as optional second argument in functions

[philrz]

tl;dr

A user asked in a community Slack thread:

I noticed that the grep function takes this as the 2nd argument by default, and [...] most(?) / "all other"(?) functions don't. But now that I see they could, why don't they?

Details

At the time this issue is being opened, Zed is at commit 343ac63.

In chatting with @mccanne about why the grep function currently behaves this way, he pointed out "that's how grep works" (i.e., at the Unix command line, grep just works off unnamed incoming lines of input). That said, the user's observation seems valid since we've already had some reports of user confusion between functions and operators, and seen through the eyes of a new user, I can see how this behavior of grep feels operator-like and could add to that confusion.