纯 append/3 模式 (

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问题描述

限时送ChatGPT账号..

通常的 append/3 是纯的，但它为模式 (-,+,+) 留下了一个选择点:

The usually append/3 is pure, but it leaves a choice point for mode (-,+,+):

?- append(X, [3], [1,2,3]).
X = [1, 2] ;
false.

在上面的结果中你是一个选择点可以看出，例如 SWI-Prolog 提供提示，然后;"提供虚假.我想出了这个解决方案来避免选择点:

That thee is a choice point in the above result is seen in that for example SWI-Prolog offers the prompt and then ";" delivers false. I came up with this solution to avoid the choice point:

append2(X, Y, Z) :-
   reverse(Z, H),
   reverse(Y, J),
   append(J, K, H),
   reverse(K, X).

这个新的 append2/3 不再留下选择点:

This new append2/3 doesn't leave a choice point anymore:

?- append2(X, [3], [1,2,3]).
X = [1, 2].

但是有比 reverse/2 cludge 更好的解决方案吗?请注意，有一个谓词 last/3 可用于该示例，仅从末尾删除一个元素.但是模式 (-,+,+) 应该适用于第二个参数中的任意长列表.

But are there better solutions than the reverse/2 cludge? Note, that there is a predicate last/3 which could be used for the example with only one element removed from the end. But the mode (-,+,+) should work for arbitrary long lists in the second argument.

推荐答案

你的 append2 不会为 (-, +, +) 模式留下选择点，而是为其他模式引入它们.我不知道是否可以在不使用 var/1 或其他东西检查操作模式的情况下编写它.

Your append2 doesn't leave choice point for (-, +, +) mode but introduces them for other modes. I do not know if it is possible to write it without checking the mode of operation using var/1 or something.

这是 mercury library 手册中的评论关于所讨论的模式.

Here is a comment from the mercury library manual regarding the mode in question.

% The following mode is semidet in the sense that it doesn't
% succeed more than once - but it does create a choice-point,
% which means it's inefficient and that the compiler can't deduce
% that it is semidet. Use remove_suffix instead.
% :- mode append(out, in, in) is semidet.

remove_suffix 谓词在 Mercury 中实现如下:

The remove_suffix predicate is implemented in mercury as follows:

remove_suffix(List, Suffix, Prefix) :-
    list.length(List, ListLength),
    list.length(Suffix, SuffixLength),
    PrefixLength = ListLength - SuffixLength,
    list.split_list(PrefixLength, List, Prefix, Suffix).

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