如何修复SWI-Prolog代码中的Syntax error: operator expected错误

第一个错误（第165行） #

问题出在retract(at(you,maze(2)),这一行，括号未正确闭合，Prolog无法解析后续的assert和!，触发"Operator expected"错误。

修正后的代码：

/*does nothing if you aren't there*/
key:-
at(you,key),
write('you have found the key'),
retract(at(you,maze(2))),  % 补全闭合括号
assert(at(you,key)),
!.

/*If you are at the same place as the magic pants*/
magic_pants:-
at(magic_pants,Loc),
at(you,Loc),
write('Magic pants?! I wonder what these do!'),
!.

第二个错误（第291行） #

问题出在go谓词的前几行，多个谓词调用的括号未闭合：

• retract(at(you, maze(2)), 缺少闭合括号
• move(at(you, treasure), 缺少闭合括号
• move(at(you, mountaintop), 缺少闭合括号

未闭合的括号导致Prolog解析到main时语法混乱，触发错误。

修正后的代码：

go :-
retractall(at(_,_)), /* clean up from previous runs */
retract(at(you, maze(2))),  % 补全闭合括号
move(at(you, treasure)),     % 补全闭合括号
move(at(you, mountaintop)),  % 补全闭合括号
assert(at(you,valley)),
assert(at(ogre,maze(3))),
assert(at(key,maze(2))),
assert(at(gate_pants,meadow)),
assert(at(treasure,mountaintop)),
write('This is an adventure game.\n'),
write('Legal moves are left, right, forward, or backward.\n'),
write('To pick up items type take(x).\n'),
write('To drop items drop(x).\n'),
write('End each move with a period.\n\n'),
report,
main.

内容的提问来源于stack exchange，提问作者Gene Holt