Namespaced map conversion fails when namespace keyword starts with a number

Question

It looks like Clojure has no problem with either :0 or :0/a, but when used together in a namesapced map it fails during some internal conversion.

Worst part: it prints back an unreadable form when if I feed it {:0/a :val}

user=> :0
:0
user=> :0/a
:0/a
user=> {:0/a :val}
#:0{:a :val}
user=> #:0{:a :val}
Syntax error reading source at (REPL:45:5).
Namespaced map must specify a valid namespace: 0
:a
:val
Syntax error reading source at (REPL:45:13).
Unmatched delimiter: }

Answer 1

It looks like Clojure has no problem with either :0 or :0/a

It does, it just doesn't complain - there are very few explicit checks and in general it's GIGO (garbage in, garbage out).

From https://clojure.org/reference/reader:

Symbols begin with a non-numeric character [...] [...]
Keywords are like symbols [...]

Comments

Correct, this is not valid.

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Interesting! I wonder if Rich meant to explicitly allow keywords that start with numbers (commit message seems unrelated), but he essentially did https://github.com/clojure/clojure/commit/00061735edd435995167ee7bbf01a17d8ae2cc66#diff-0a8564b06ce5d306c0449c3711a3df88720e392b411e469c2aa99041a96a4886L43

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Looks like keywords starting with a number are canon now https://clojure.atlassian.net/browse/CLJ-1252

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It is not valid Clojure, but it is not rejected in all situations, so it should not be relied on. There are some widely used Clojure libraries out there that assume it is not rejected in certain situations, so banning it is problematic—but also do not expect any changes that make it more widely accepted.