added a comment - - edited
Ah, I think I see the source of the confusion. Both EDN and the clojure reader spec both say something like "keywords are like symbols, except beginning with a colon." The confusion lies in whether we interpret that as meaning
- First character is a colon, then the second character and after are matched against the symbol definition.
- The first character is a colon, and the whole form is matched against the symbol definition.
CLJ-1003 CLJ-1252 and CLJ-1286 and myself all seem to understand the first meaning. This might be because when we say "the first character of a keyword" we typically mean the first character after the colon, as if the colon is "special" and not part of the keyword (e.g. like a reader macro character).
However clojure 1.6 seems to be following the second meaning (and explains why `:0/a` is ok but not `:0/0`), and I'm not sure from the cited tickets and google group discussions whether this is because of downstream breakage or if this is the intended interpretation and the patch from
CLJ-1252 was accepted by Alex Miller erroneously.
Note if we accept the second interpretation, then the restriction "A symbol can contain one or more non-repeating ':'s." from the clojure reader docs is incorrect for keywords. (EDN doesn't allow namespace-expanded keywords, it seems, so it's not an issue there.)
Also EDN allows contiguous colons in symbols, whereas clojure 1.6 and the reader spec do not.