Read time evaluation support

[shantanu-sardesai]

Currently, unlike the Clojure reader, jank can't evaluate code at read time.

Clojure:

user=> #=(* 2 21)
42

jank:

user=> #=(* 2 21)
─ parse/invalid-reader-symbolic-value ──────────────────────────────────────────────────────────────
error: This reader tag is not supported. '#uuid', '#inst' and '#cpp' are the only default tags
       currently supported.
─────┬──────────────────────────────────────────────────────────────────────────────────────────────
     │ /var/folders/xd/9v1dcg0d46l7d19830vq0lhc0000gn/T/jank-repl-RDlZ3C
─────┼──────────────────────────────────────────────────────────────────────────────────────────────
  1  │ #=(* 2 21)
     │  ^ Found here.
─────┴──────────────────────────────────────────────────────────────────────────────────────────────

Honestly I've not been able to find any documentation on this behaviour in the Reader conditional docs of Clojure.

Other than the first place I saw this behaviour mentioned:

user=> (doc *read-eval*)
-------------------------
clojure.core/*read-eval*
  ...

  When set to logical false in the thread-local binding,
  the eval reader (#=) and record/type literal syntax are disabled in read/load.
  Example (will fail): (binding [*read-eval* false] (read-string "#=(* 2 21)"))

  The default binding can be controlled by the system property
  'clojure.read.eval' System properties can be set on the command line
  like this:

  java -Dclojure.read.eval=false ...

  ...

nil

[shantanu-sardesai]

I think we can add this as an issue to our backlog for the moment.

---

Turns out the purpose is primarily for:

1. Printing data structures in a way to allow for them to be read back in.

 user=> (pr-str (sorted-set 1))   ;; The result of this expression when passed to `read-string` won't give back the same data structure.
 "#{1}"

 user=> (binding [*print-dup* true] (pr-str (sorted-set 1)))
 "#=(clojure.lang.PersistentTreeSet/create [1])"

 ;; ...

 user=> (pr-str #'read)
 "#'clojure.core/read"

 user=> (binding [*print-dup* true] (pr-str #'read))
 "#=(var clojure.core/read)"

2. The other usage is the Java constructor reader:

   user=> #java.util.Date[1772690057789]
   #inst "2026-03-05T05:54:17.789-00:00"

---

Slack thread.