Does: > Keywords - Keywords are like symbols, except: > They can and must begin with a colon, e.g. :fred. Mean that the second character of a keyword (in this case 'f') follow the same rules as the second character of a symbol (e.g. can be a number) or that they follow the rules as the first character of a symbol (e.g. cannot be a number). Essentially, is :1 valid?

This question has been raised before, and it seems that reasonable people can interpret that statement in (at least) two different ways.

The Clojure reader accepts it now, and there was a patch that made it illegal for one or two alpha/beta releases several years ago, and then it was undone, because several projects used such keywords. Hyrum's Law is an interesting thing to know about as a software developer: https://www.hyrumslaw.com

Annoyingly, :foo/1 is not accepted by the reader.

heh, but ::1 is, so you can produce something that cannot then be read:

user=> (read-string (pr-str ::1))
Execution error at user/eval391 (REPL:1).
Invalid token: :user/1

Any kinds of changes in these things are likely to break something somewhere, unless extremely carefully done, and changes that don't break anything seem unlikely to make anyone happy in a "all of the invariants I wish were true, e.g. everything printable is readable" sense. That said, for all I know there could be changes planned in this area in a future release.

Mmhm, I'm not too worried about changing the reader itself (although :foo/1 should probably work as ::1 does, hyrums law and all :) ). Just curious about what is "proper" I suppose.

These are the questions that keep me up at night (or at least at 5:43)

FWIW, we have several places in our code where we do something akin to

(def m {(keyword foo) 'bar}
...
(get m (keyword baz))

@dominicm the original intent was that symbols and the non-colon parts of keywords should follow the same naming rules which would mean :1 is invalid (because 1 is an invalid symbol)

The regex in the reader for keywords is wrong. Back in 1.6 I think we fixed it and it broke a bunch of existing code

We saw no point in breaking stuff people relied on so we reverted it so it’s lived in kind of a gray area since then

These appear to be the questions that keep you up at 05:00 also :)

There are several tickets I could refer you to if you cared

I think it’s highly unlikely we will make stuff that works now stop working

We may make stuff that doesn’t work now start working :)