非常基本的 dcg prolog 语法 [英] Very basic dcg prolog syntax

问题描述

我正在尝试理解序言和定语从句语法，但我很难理解它们.

I am trying to understand prolog and definite clause grammar but I am having a very hard time understanding both of them.

我真的很想了解如何使用 dcg 语法...

I am really trying to understand how to use the dcg syntax...

这里有两个例子:

第一个实际上是来自本论坛另一个问题的代码，但还有一个问题:

The first is actually the code from another question on this forum but with an additional question:

代码如下:

s --> first, operator, second.
first --> [X].
operator --> ['+'].
second --> [X].

当 Prolog 被问到这个时，它返回真/假，但我一生都无法弄清楚如何实际修改它以绑定"值，所以如果问 s(X, [2,+,2],[]).它会返回 first 的值，所以它不会返回 true 而是说 X = 2

And when Prolog is asked about this, it returns true/false but I can't for the life of me figure out how to actually modify this to "bind" the value so if asked s(X, [2,+,2],[]). it would return the value of first, so instead of returning true it'd say X = 2

无论如何回到实际问题.我在普通序言中有一些规则，这是其中之一；它实际上并没有做任何事情，只是作为示例而已.

Anyway back to the actual question. I have a few rules in normal prolog and this is one of them; it doesn't actually do anything and was merely made up as an example.

do(X, Y, [H|T], Sum):-
    H == 1, %check if H is 1
    X = H,
    Y = T,
    Additional is H+5,
    Sum is Additional+Additional.

基本上，我是在问是否有人可以将其翻译成 DCG，以便我可以尝试理解 DCG 的基本语法！我试过阅读一些教程，但我觉得我没有变得更聪明......

Basically, I am asking if someone could translate this to DCG so that I could try and understand the basic syntax of DCG! I've tried reading some tutorials but I feel like I haven't gotten any wiser...

推荐答案

DCG:foo(A1, A2, A3, ... , An) --> bar.

DCG: foo(A1, A2, A3, ... , An) --> bar.

序言:foo(A1, A2, A3, ... , An, X, Y) :- bar(X,Y)

Prolog: foo(A1, A2, A3, ... , An, X, Y) :- bar(X,Y)

所以，s 应该改为:

s(X) --> first(X), operator, second.
first(X) --> [X].
operator --> ['+'].
second --> [X].

当然，返回实际结果可能更好；为此，您应该将 prolog 代码封装在 DCG 子句中，该子句使用 {} 完成:

Of course, it might be better to return the actual result; to do this you should encapsulate prolog code in the DCG clause which is done with {}:

s(Z) --> first(X), operator, second(Y), {Z is X+Y}.
first(X) --> [X].
operator --> ['+'].
second(X) --> [X].

(当然，如果你有更多的操作符，prolog 代码不会那么简单).

(naturally, if you have more operators, the prolog code won't be that simple).

关于 do/4 谓词，它应该是这样的:

Regarding the do/4 predicate, it should be something like this:

do(X,Y,[H|T],Sum) -->
   {H == 1, %check if H is 1
    X = H,
    Y = T,
    Additional is H+5,
    Sum is Additional+Additional}.

但我不明白你为什么想要那个.

but I don't see why you would want that.

最后一个提示:建议使用 phrase/3 而不是在 DCG 谓词中添加最后两个参数.

One last tip: it's recommended to use phrase/3 instead of adding the last two arguments in a DCG predicate.

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