Prolog中缀运算符定义 [英] Prolog infix operator definition

问题描述

我最近正在学习Prolog，并且发现用于定义中缀运算符的三种类型令人困惑.

I am recently learning about Prolog and I find the three types used for defining infix operators confusing.

在指定运算符类型时，xfx，xfy和yfx有什么区别? 我已经搜索了这个问题，但没有发现任何有用的东西.

What are the differences between xfx, xfy and yfx when specifying the type of an operator? I have googled about the problem and haven't found anything useful.

我尝试在Prolog中键入以下代码:

I tried typing the following codes in Prolog:

:- op(500,yfx,is_alive).
is_alive(A,B) :- display([A,B]).
:- op(500,xfy,is_alive2).
is_alive2(A,B) :- display([A,B]).
:- op(500,xfx,is_alive3).
is_alive3(A,B) :- display([A,B]).

和输出:

| ?- 1 is_alive 2.
'.'(1,'.'(2,[]))

yes
| ?- 1 is_alive2 2.
'.'(1,'.'(2,[]))

yes
| ?- 1 is_alive3 2.
'.'(1,'.'(2,[]))

yes

结果对我来说没有区别.

The results showed no difference to me.

推荐答案

运算符的类型"控制关联性，例如表达式"5-4-3"应解释为(5-4)-3"，即左联想性还是"5-(4-3)"，即右联想性.

The 'type' of the operator controls associativity, e.g. whether expression "5 - 4 - 3" should be interpreted as "(5 - 4) - 3", which is known as left associativity or as "5 - (4 - 3)", which is right associativity.

如果减号运算符应按通常预期的方式工作，则需要将定义为'yfx'，使其与左侧关联.其他类型，例如"xfx"和"xfy"是对此的变体.有关更多信息，请参见 http://www.cse.unsw .edu.au/〜billw/cs9414/notes/prolog/op.html .

If the minus operator should work as usually expected, is needs to be defined as 'yfx' which makes it left-associative. Other types as 'xfx and 'xfy' are variations on this. See more at http://www.cse.unsw.edu.au/~billw/cs9414/notes/prolog/op.html.

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