prolog dcg 限制 [英] prolog dcg restriction

问题描述

我想使用 DCG 作为生成器.截至目前，语法是

I would like to use DCGs as a generator. As of now, the syntax is

s-->a,b.
a-->[].
a-->a,c.
c-->[t1].
c-->[t2].
b-->[t3].
b-->[t4].

我想生成所有s，其中a的长度是.

I would like to generate all s where the length of a is < someNumber.

使用?-短语(a,X),length(X,Y),Y<4.我可以得到少于4个项目的所有a.但是，当所有组合都用完时，系统(SWI-Prolog 6.2.5)似乎会停滞.有时以前，在这里提出了类似的问题.但是，作为 Prolog 的新手，我无法使其与上述语法一起使用.有什么想法吗?

Using ?- phrase(a,X),length(X,Y),Y<4. i can get all a with less than 4 items. However, when all combinations are exhausted, the system (SWI-Prolog 6.2.5) seems to stall. Sometimes ago, a similar question was asked here. However, being new to Prolog i am not able to make it work with the grammar above. Any ideas?

更新:(canrememberthename) 的评论不知何故被删除了.无论如何，建议使用 between(1,4,Y),length(X,Y),phrase(a,X). 来设置限制.在将我的代码更改为 a-->c,a.

Update: There was a comment by (canrememberthename) which got deleted, somehow. Anyway, it was suggested to use between(1,4,Y),length(X,Y),phrase(a,X). to set limits. This worked nicely, after changing my code to a-->c,a.

推荐答案

非终结符 a//0 both 是左递归和 'epsilon'(生成空序列)，phrase/2 将空生产后立即循环.

the nonterminal a//0 is both left recursive and 'epsilon' (generate the empty sequence), and phrase/2 will loop immediately after the empty production.

您可以解决边界列表的长度问题:

You can solve your problem bounding list' length:

?- between(1,4,Y),length(X,Y),phrase(a,X).

并且，正如您已经完成的那样，删除左递归.

and, as you have already done, removing the left recursion.

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