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Programming

Revert-world

Evaluate without (ultimately) changing the world

General Form:
(revert-world form)

where form evaluates to an error-triple.

Evaluation of (revert-world form) returns the same result, (mv erp val state), as the given form, except that the world of the returned state is the same as the world of the input state even if the evaluation of form modifies the world of the input state.

To see revert-world in action, consider the following definition.

(defun test-revert-world (state)
 (declare (xargs :mode :program :stobjs state))
 (er-progn
  (value (cw "Length of (w state) before defun: ~x0~%"
             (length (w state))))
  (revert-world (er-progn
                 (trans-eval '(with-output :off :all ; avoid output ;
                                (defun foo (x) x))
                             'my-ctx state nil)
                 (value (cw "Length of (w state) after defun: ~x0~%"
                            (length (w state))))))
  (value (cw "Length of (w state) after revert-world: ~x0~%"
             (length (w state))))))

Here is a log produced after admitting the definition above in a fresh session (for an ACL2 build circa November 2017). It shows that the definition lengthens the world, but that the world's length is back to its initial value after we return from revert-world.

ACL2 !>(test-revert-world state)
Length of (w state) before defun: 107133
Length of (w state) after defun: 107153
Length of (w state) after revert-world: 107133
 NIL
ACL2 !>:pbt 0
           0  (EXIT-BOOT-STRAP-MODE)
 P         1:x(DEFUN TEST-REVERT-WORLD (STATE) ...)
ACL2 !>

The macroexpansion of (revert-world form) contains a call of a program-mode function. It is thus illegal to call revert-world in the body of a logic-mode function. Contact the ACL2 implementors if you want them to consider working to lift this restriction.