I am currently taking a Prolog course.
I'm familiar with the [A|B] notation of lists in Prolog, but the teacher shows that [a,b,c|X]-X is also a valid way of having a list, where we have a reference to the tail of the list. When I try it out with Swi-Prolog however, I receive the following error: ERROR: Undefined procedure: (-)/2 (DWIM could not correct goal).
Is the (-)/2 operator just defined in standard Prolog but not in Swi-Prolog or am I missing something ?
This structure is called difference list. (https://en.wikibooks.org/wiki/Prolog/Difference_Lists)
Probably you used '-' wrong. Let's consider append predicate on difference lists:
app(X-Y, Y-Z, X-Z).
You can test it like this (after compiling file having this predicate):
?- app([1,2,3|A]-A, [4, 5|B]-B, C).
[1,2,3|A]-A can be considered as [1,2,3] and [4, 5|B]-B can be considered as [4,5], so having output:
A = [4, 5|B],
C = [1, 2, 3, 4, 5|B]-B.
You can see, that C became [1, 2, 3, 4, 5|B]-B that can be considered as [1,2,3,4,5].
Little visualization of what's going on there:
<-------------------X--------------------->
<-------------Y------------->
<-----Z------>
||............||.............||............|| <- List
<--- X-Y ----><---- Y-Z ---->
<---------- X-Z ------------>