ERROR-TRIPLES-AND-PARALLELISM.html -- ACL2 Version 6.3

### ERROR-TRIPLES-AND-PARALLELISM

how to avoid error triples in ACL2(p)

    Major Section:  PARALLEL-PROGRAMMING

This documentation topic relates to the experimental extension of ACL2 supporting parallel execution and proof; see parallelism.

ACL2 supports the use of error triples in many features; e.g., `computed-hints`. (For background on error triples, see programming-with-state.) However, ACL2(p) does not support the use of error triples in some of these features (e.g., `computed-hints`) while waterfall-parallelism is enabled.

You may see an error message like the following when running ACL2(p) with waterfall-parallelism enabled:

    ACL2 Error in ( THM ...):  Since we are translating a form in ACL2(p)
    intended to be executed with waterfall parallelism enabled, the form
    (MY-STATE-MODIFYING-COMPUTED-HINT ID STATE) was expected to represent
    an ordinary value, not an error triple (mv erp val state), as would
    be acceptable in a serial execution of ACL2.  Therefore, the form returning
    a tuple of the form (* * STATE) is an error.  See :DOC unsupported-
    waterfall-parallelism-features and :DOC error-triples-and-parallelism
    for further explanation.

In this particular example, the cause of the error was trying to use a computed hint that returned state, which is not allowed when executing the waterfall in parallel (see unsupported-waterfall-parallelism-features for other related information).

Often, the only reason users need to return state is so they can perform some output during the proof process. In this case, we suggest using one of the state-free output functions, like `cw` or `observation-cw`. If the user is concerned about the interleaving of their output with other output, these calls can be surrounded with the macro `with-output-lock`.

Another frequent reason users return state is so they can cause a `soft` error and halt the proof process. In this case, we suggest instead calling `er` with the `hard` or `hard?` severity. By using these mechanisms, the user avoids modifying `state`, a requirement for much of the code written in ACL2(p).

You may encounter other similar error messages when using computed-hints, custom-keyword-hints, or override-hints. Chances are that you are somehow returning an error triple when an ordinary value is needed. If this turns out not to be the case, please let the ACL2 implementors know.