Prolog中长度/2的逻辑推论数(swi-pl)

Question

我期望内置长度/2谓词在逻辑推理的数量上是线性的。然而，它似乎是不变的：

?- length(L,10),time(length(L,X)).
% 2 inferences, 0.000 CPU in 0.000 seconds (63% CPU, 142857 Lips)

?- length(L,20),time(length(L,X)).
% 2 inferences, 0.000 CPU in 0.000 seconds (62% CPU, 153846 Lips)

?- length(L,30),time(length(L,X)).
% 2 inferences, 0.000 CPU in 0.000 seconds (65% CPU, 111111 Lips)

这是因为这个过程被委托给C吗？我在SWIPL代码库中找不到相关的代码。

Answer 1

在init.pl中，length/2在第3230行附近有一个浅而强大的界面。在这里，它称'$skip_list'(Length0, List, Tail)为list C界面的“瑞士刀”。

您可以在src/pl-prims.c第2377行和以下内容中找到它：

/** '$skip_list'(-Length, +Xs0, -Xs) is det.

Xs0, Xs is a pair of list differences. Xs0   is the input list and Xs is
the minimal remaining list. Examination of   Xs  permits to classify the
list Xs0:

        Xs        | list type of Xs0   | Length
        []    ... | well formed        | length
        Var   ... | partial            | elements skipped
        [_|_] ... | infinite           | upper bound for cycle
        Term  ... | malformed          | elements skipped
*/
PRED_IMPL("$skip_list", 3, skip_list, 0)
...