While playing around on the swipl repl, I tried out the following:
?- [X|X].
X = [].
?- [[]|[]].
true.
?- [[]].
true.
I don’t really understand this behavior. What is happening here? Why is a list containing one empty list true?
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I think the reason is in here:
See consult/1. I can’t be certain, but the sentence:
Abbreviation using
?- [file1,file2].does not work for the empty list ([]).
probably means that this doesn’t work:
?- [].
If I read the code in init.pl correctly:
?- [[]].
becomes
?- consult(user:[]).
which calls load_files/2 which calls ‘$load_files’/3 which has the clause:
'$load_files'([], _, _) :- !.
which is why ?- [[]]. succeeds without choice points.
This is the “what happens”. The answer to “why” is beyond me.
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As seen by ?- you are at the top level and so the top level is parsing the input.
[ ] is a command used by the top level for loading source code and not a list.
Boris took the other side of the coin but the answer is the same.