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[VM] Tail recursion and multiple values

From: Ludovic Courtès
Subject: [VM] Tail recursion and multiple values
Date: Sat, 28 Feb 2009 15:27:09 +0100
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/23.0.90 (gnu/linux)


Use of multiple values breaks tail recursion in VM-compiled code:

  (let loop ((x 1000000))
    (and (> x 0)
             (lambda ()
               (values (1+ x) (1- x)))
           (lambda (next prev)
             (loop prev)))))

This example yields a stack overflow with the VM, whereas tail recursion
is correctly handled by the interpreter.  Tail recursion for
`call-with-values' is mandated by R5RS (info "(r5rs)Proper tail

  Certain built-in procedures are also required to perform tail calls.
  The first argument passed to `apply' and to
  `call-with-current-continuation', and the second argument passed to
  `call-with-values', must be called via a tail call.  Similarly, `eval'
  must evaluate its argument as if it were in tail position within the
  `eval' procedure.

I'll look into it and report back if no one beats me.  ;-)


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