Recursive protocol method for a record never terminates

Question

When extending a protocol for a record, a recursive method can never terminate in some cases.

`
(defrecord R [a])

(defprotocol P
(f [x]))

(extend-protocol P
R
(f [x]

(if x
  (recur nil)
  x))

default
(f [x]

(if x
  (recur nil)
  x)))

(prn (f 1)) ; #1
(prn (f (R. 1))) ; #2
`

{{prn}} call #1 prints {{nil}} as expected, but {{prn}} call #2 never terminates.

It looks like the compiler creates a variable assigned to {{this}} within the {{while}} loop such that the test of "{{x}}" is always really testing {{this}} when it should be testing the value of {{x}} passed in by the call to {{recur}}.

Note, I'm testing ClojureScript 1.8.51. The "Affects Version/s" field above only gives 1.7.228 as the most recent version.

Answer 1

Comment made by: bstiles

Actually, "always really testing 'this' when it should be testing the value of x passed in by the call to recur" is only true if the type of the value represented by x remains the same. If, as in the example, the type changes, the call should be dispatched to the properly matching method implementation.

This seems to behave as expected in Clojure.

Answer 2

Comment made by: dnolen

Confirmed with Mike Fikes this is still an issue on master even with the recur enhancements.

Answer 3

Reference: https://clojure.atlassian.net/browse/CLJS-1644 (reported by alex+import)