cannot close over mutable fields (in deftype)

Question

Simplest case:

`
user=>
(deftype Bench [#^{:unsynchronized-mutable true} val]
Runnable
(run [_]

(fn [] (set! val 5))))

java.lang.IllegalArgumentException: Cannot assign to non-mutable: val (NO_SOURCE_FILE:5)
`

Functions should be able to mutate mutable fields in their surrounding deftype (just like inner classes do in Java).

Filed as bug, because the loop special form expands into a fn form sometimes:

`
user=>
(deftype Bench [#^{:unsynchronized-mutable true} val]
Runnable
(run [_]

(let [x (loop [] (set! val 5))])))

java.lang.IllegalArgumentException: Cannot assign to non-mutable: val (NO_SOURCE_FILE:9)
`

Answer 1

Comment made by: importer

donmullen said: Updated each run to (link: _) for new syntax.

Now gives exception listed.

Answer 2

Comment made by: importer

richhickey said: We're not going to allow closing over mutable fields. Instead we'll have to generate something other than fn for loops et al used as expressions. Not going to come before cinc

Answer 3

Comment made by: importer

Converted from http://www.assembla.com/spaces/clojure/tickets/274

Answer 4

Comment made by: bronsa

The patch for CLJ-1226 makes this work:

(deftype Bench [#^{:unsynchronized-mutable true} val] Runnable (run [_] (let [x (loop [] (set! (.val _) 5))])))

If there's interest, I could provide a patch that converts closed over mutable field access by generated fns (for loop/try) into field access on closed over "this", i.e. val -> (.val this)

Answer 5

Comment made by: bronsa

Related tickets: CLJ-1075 CLJ-1023

Answer 6

Comment made by: bronsa

CLJ-701 could probably make the loop case working

Answer 7

Reference: https://clojure.atlassian.net/browse/CLJ-274 (reported by alex+import)