为什么 SWI-Prolog 将带引号和不带引号的字符串（不含空格）统一为同一规则？
假设我有以下规则：
unify('test', 'this is a test').
run :- write('Enter something: '),
read(X),
unify(X, Y),
write('The answer is '), write(Y).
然后我按如下方式运行它：
?- ['unify.pl'].
% unify.pl compiled 0.00 sec, -48 bytes
true.
?- run.
Enter something: test.
The answer is this is a test
true.
?- run.
Enter something: 'test'.
The answer is this is a test
true.
为什么 SWI-Prolog 将 test 和 'test' 统一为 unify('test' ，“这是一个测试”）。？我在回答有关 SO 的 Prolog 问题时遇到了这一点。虽然我能够回答这个人的问题，但我无法解释这种特殊行为，而且我想知道其他人是否可以。
Assume I have the following rules:
unify('test', 'this is a test').
run :- write('Enter something: '),
read(X),
unify(X, Y),
write('The answer is '), write(Y).
And then I run it as follows:
?- ['unify.pl'].
% unify.pl compiled 0.00 sec, -48 bytes
true.
?- run.
Enter something: test.
The answer is this is a test
true.
?- run.
Enter something: 'test'.
The answer is this is a test
true.
Why does SWI-Prolog unify both test and 'test' to unify('test', 'this is a test').? I came across this while answering a Prolog question on SO. While I was able to answer the person's question, I couldn't explain this particular behavior, and I was wondering if any one else could.
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虽然 SWI-PROLOG 中的原子可以使用单引号表示，例如，
'这是一个原子'，但当 SWI-PROLOG 解析器可以识别一个原子时，不需要单引号。字符序列中的原子，通常以小写字母字符开头，例如test。如果序列包含空格（或其他一些字符），则需要单引号来正确表示原子。字母数字字符和某些标点符号（例如下划线_）都可以，例如test5_6。如果不带单引号的字符序列以其他任何内容开头，例如数字
6k，解析器会将其视为数字；如果它是大写字母字符，例如Test，解析器会将其视为变量。While atoms in SWI-PROLOG can be denoted using single quotes, e.g,
'This is an atom', single quotes are not needed when the SWI-PROLOG parser can identify an atom from a sequence of characters, usually starting with a lowercase alphabetic character, such astest. If the sequence contained whitespace (or some other characters), you'd need the single quotes to denote an atom properly. Alphanumeric characters and certain punctuation characters like underscore_are fine, e.g.,test5_6.If the character sequence without single quotes were to start with anything else, such as a number
6k, the parser will treat it as anumber; if it were an uppercase alphabetic character such asTest, the parser will treat it as a variable.这不是 SWI 特定的行为 - 这是标准所要求的。
有一个简单的方法可以看到这一点。您也可以将其用于
语法不明显的任何其他术语。在顶层键入：
答案始终是有效的 Prolog 文本 - 这是特定于 SWI 的，但是
也适用于许多其他 Prolog 系统，如 YAP、GNU、B、IF、SICStus。
另一种查看此情况的方法是使用 write_canonical/1：
?- write_canonical('this is a test'). 'this is a test' true. ?- write_canonical([a,b,(c,d),{e,f}]). '.'(a,'.'(b,'.'(','(c,d),'.'({}(','(e,f)),[]))))This is not SWI specific behavior - it is required by the standard.
There is a simple way to see this. You can use this also for
any other term whose syntax is not evident. Either type at the toplevel:
The answer is always valid Prolog text - this is specific to SWI but
also to many other Prolog systems like YAP, GNU, B, IF, SICStus.
Another way to see this is to use write_canonical/1:
?- write_canonical('this is a test'). 'this is a test' true. ?- write_canonical([a,b,(c,d),{e,f}]). '.'(a,'.'(b,'.'(','(c,d),'.'({}(','(e,f)),[]))))