Prolog（Sicstus）

Prolog（Sicstus） - setof和findall组合问题(Prolog (Sicstus) - setof and findall combination issues)

给定一个给定站点的路由，例如我们：

route(TubeLine, ListOfStations). route(green, [a,b,c,d,e,f]). route(blue, [g,b,c,h,i,j]). ...

我需要查找具有特定站点的行的名称。 结果必须是非重复的站点，如果没有结果，必须返回一个空列表。 所以，查询

| ?- lines(i, Ls).

应该给：

Ls = [blue,red,silver] ? ; no

我尝试过以下操作：

lines(X, L) :- setof(L1, findall(W, (route(W, Stations),member(X, Stations)),L1), L).

但是，它给出了以下答案：

Is = [[blue,silver,red]]; no

双括号无序。 我尝试使用findall，但结果没有订购。 我知道我可以编写sort函数并通过它，但是我想知道在这个实例中是否可以使用findall和setof？

Given a set of routes a given station has, such us :

route(TubeLine, ListOfStations). route(green, [a,b,c,d,e,f]). route(blue, [g,b,c,h,i,j]). ...

I am required to find names of lines that have a specific station in common. The result must be ordered, with non-repeated stations and must return an empty list, if there were no results. So, querying

| ?- lines(i, Ls).

Should give:

Ls = [blue,red,silver] ? ; no

I tried doing the following:

lines(X, L) :- setof(L1, findall(W, (route(W, Stations),member(X, Stations)),L1), L).

However, it gives the following as an answer:

Is = [[blue,silver,red]]; no

So unordered with double braces. I tried using just findall, but the result is not ordered. I know I could then write sort function and pass that through, however I was wondering if it is possible to use just findall and setof in this instance?

最满意答案

实际上，它比你的尝试更容易，但是你需要掌握自由变量的特殊行为，并考虑到需要一个未知站的可能性（如果没有解决方案，setof / 3就会失败）。

lines(X, Ls) :- setof(L, Stations^(route(L, Stations), member(X, Stations)), Ls) -> true ; Ls = [].

如你所说，更容易的替代方法是使用findall / 3，就像你正在做的那样（没有setof！），并对输出进行排序。

Actually, it's easier than your attempt, but you need to grasp the peculiar setof' behaviour wrt free variables, and account for the eventuality that an unknown station was required (setof/3 fails if there are no solutions).

lines(X, Ls) :- setof(L, Stations^(route(L, Stations), member(X, Stations)), Ls) -> true ; Ls = [].

An easier alternative, as you said, use findall/3 exactly as you're doing (without setof!), and sort the output.