Dear Marc,
How can I make Gambit always generate unique closure objects for (lambda () ...) ?
A (declare) to enforce this would do the job.
I'm having a table with test: eq? where the key is a closure and thus needs to guaranteedly be unique. This should be a perfectly valid usecase.
I noted that currently in compiled code (lambda () ...) not guaranteedly returns a unique closure object in certain circumstances: if a closure closes over no variables, or Gambit optimizes away all closed over variables so no variables are closed over that way, then it does not generate a unique closure.
This was quite surprising to me as my best understanding of R5RS spec (
here &
here) is that eq? should work on procedures:
"Each procedure created as the result of evaluating a lambda expression is (conceptually) tagged with a storage location, in order to make eqv? and eq? work on procedures (see section 6.1)."
Perhaps the wording could have been even more precise.
I acknowledge this Gambit optimization is really useful many times. Though there should be a way to disable it as it wouldn't be a reliable coding strategy to need to outsmart the optimizer for correct function.
Thanks!
Mikael
Example:
$ gsc
> (let loop () (define x (lambda () 'myvalue)) (print x "\n") (loop))
#<procedure #2 x>
#<procedure #3 x>
#<procedure #4 x>
#<procedure #5 x>
[...]
[ctrl+c],q
$ echo (let loop () (define x (lambda () 'myvalue)) (print x "\n") (loop)) > test.scm
$ gsc
> (compile-file "test.scm")
"test.o1"
> (load "test.o1")
#<procedure #2>
#<procedure #2>
#<procedure #2>
#<procedure #2>
#<procedure #2>
#<procedure #2>
#<procedure #2>
[...]
This
(let loop ((v 0)) (define x (lambda () v 'myvalue)) (print x "\n") (loop 1))
gives the same behavior. Note that the lambda closes over v, though the compiler optimizes it away.
Neither
(let loop ((v 0)) (define x (lambda (z) (declare (not optimize-dead-local-variables)) v 'myvalue)) (print x "\n") (loop 1))
works, which may appear a bit surprising!
Here's a hack around the optimizer though:
(let loop ((v 0)) (define x (lambda (z) (case z ((never-happens) v) (else 'myvalue)))) (print x "\n") (loop 1))