具体化的call_with_time_limit / call_with_inference_limit

Question

我正在尝试定义一个总是成功的关系callto_status(Goal, Status)，并根据调用Goal的结果统一状态(换句话说，我想实现call_with_inference_limit/3的一个具体化版本)。我的实现使用的是SWI的call_with_inference_limit/3，它具有与call_with_time_limit/3相同的接口(这应该也能使它在这种情况下工作)。call_with_..._limit的实现不会走回头路，所以我认为最好不要给人一种报告替代目标的答案的印象。

为了提高可读性，我引入了辅助谓词derivable_st。它处理成功和超时的情况，否则会失败。

% if Goal succeeds, succeed with Status = true,
% if Goal times out, succeed with Status = timeout
% if Goal fails, fail
derivable_st(Goal, Status) :-
    T = 10000,                               % set inference limit
%    copy_term(Goal, G),                    % work on a copy of Goal, we don't want to report an answer substitution
    call_with_inference_limit(G, T, R),     % actual call to set inference limit
    (  R == !
    -> Status = true                        % succeed deterministically, status = true
    ;  R == true
    -> Status = true                        % succeed non-deterministically, status = true
    ;  (  R == inference_limit_exceeded     % timeout
       -> (
              !,                            % make sure we do not backtrack after timeout
              Status = timeout              % status = timeout
          )
       ;  throw(unhandled_case)             % this should never happen
       )
    ).

主谓词包装在derivable_st周围，并处理失败情况和可能抛出的异常(如果有)。我们可能想要挑出堆栈溢出(在推理限制太高的情况下发生)，但现在我们只报告任何异常。

% if Goal succeeds, succeed with Status = true,
% if Goal times out, succeed with Status = timeout
% if Goal fails, succeed with Status = false
% if Goal throws an error, succeed with Status = exception(The_Exception)
% Goal must be sufficiently instantiated for call(Goal) but will stay unchanged
callto_status(Goal, Status) :-
    catch((  derivable_st(Goal, S)            % try to derive Goal
          -> Status = S                       % in case of success / timeout, pass status on
          ;  Status = false                   % in case of failure, pass failure status on, but succeed
          ),
          Exception,
          Status = exception(Exception)       % wrap the exception into a status term
    ).

该谓词适用于一些简单的测试用例：

?- callto_reif( length(N,X), Status).
Status = true.

?- callto_reif( false, Status).
Status = false.

?- callto_reif( (length(N,X), false), Status).
Status = timeout.

我现在的问题有点模糊:这个谓词是否如我所说的那样做？你看到任何错误/改进点了吗？我非常感谢您的任何意见！

编辑:根据@false的建议，注释掉了copy_term/2

Answer 1

这是一个较短的解决方案：

callto_status(Goal, Status) :-
    call_with_inference_limit(Goal, 10000, Status0),
    (Status0 = ! -> !, Status = true; Status = Status0).
callto_status(_, false).

你看到原来的有多有用了吧！状态是为了避免不必要的选择点：

?- callto_status(member(X,[1,2,3]), Y).
X = 1,
Y = true 
X = 2,
Y = true 
X = 3,
Y = true.

?- callto_status(fail, Y).
Y = false.

当然，您也可以仅将Status0 = ! -> !, Status = true替换为Status0 = ! -> Status = true。然后，您将始终获得剩余的选择点：

?- callto_status(member(X,[1,2,3]), Y).
X = 1,
Y = true 
X = 2,
Y = true 
X = 3,
Y = true 
Y = false.

从这个问题看，你到底想要什么并不清楚。