7.2.9 precaution ¶

Syntax ¶

(precaution traversal [(beforehand condition)|(afterwards condition)])

or

(carefully traversal [(beforehand condition)|(afterwards condition)])

Description ¶

Execute a traversal, but ensure that certain conditions hold either before or after executing the traversal (or both). If a condition does not hold, then abort the traversal, considering it to have failed.

Each of the conditions can be any predicate — either a built-in predicate form, or an arbitrary traversal, including a lambda. See Predicates for details.

Note that, as the predicates could be lambdas, you could also perform effects resembling effect (say, setting an ELisp variable) using precaution, either before or after the traversal. If you’d like to do this, just remember that the effect function you use, as a predicate, must return a truthy value. And further, as a traversal, it must accept the two standard arguments to traversals (See Lambdas).

Examples ¶

"Go down but don’t descend to the root node."

(symex-eval
  (symex-traversal
    (carefully (move down)
               (afterwards (not (at root))))))

"Go backward as long as we aren’t at the first node at this level."

(symex-eval
  (symex-traversal
    (carefully (move backward)
               (beforehand (not (at first))))))

Note that this executes a *single* traversal while taking precautions. It is not repeated unless wrapped in a circuit or employed as a detour.