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.
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 as test
. 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 a number
; if it were an uppercase alphabetic character such as Test
, the parser will treat it as a variable.