What is the explanation of this code (Prolog)?

Question

I am trying to learn Prolog. I have a problem and the solution for that in Prolog. Though I am unable to understand the code completely.

The call of the code goes like this-

conc(Before,[c|After],[a,b,c,d]) .

This code will return

 Before = [a,b]
    After = [d]

I have the solution -

conc([],L,L).
conc([X|L1],L2,[X|L3]) :- conc(L1,L2,L3) .

I am not able to understand the flow of the program completely. Let me start with a dry run. I added some write command in order understand the code flow. The code now looks like this -

conc([],L,L):-  write("Line1: "), write(L).
conc([X|L1],L2,[X|L3]) :-
     write("Line-2: "), write(L1), write(" "), 
     write(L2), write(" "), write(L3), conc(L1,L2,L3) .

Initial call is -

 conc(Before,[c|After],[a,b,c,d]) .

At this moment, the call goes to line 2 (because, Before is unknown term which is not empty)which is :

conc([X|L1],L2,[X|L3]) :-
    write("Line-2: "), write(L1), write(" "),
    write(L2), write(" "), write(L3), conc(L1,L2,L3) .

At this point, X=[a], l1= unknown, L2=[c|After] and L3=[b,c,d]. This prints -

Line2: _G481 [c|_G368] [b,c,d]

This will again call the recursion(code line 2) with the following value:

cons(L1, [c|After], [b,c,d]). (L1 is unknown still now)

And, prints -

Line2: _G494 [c|_G368] [c,d]

Now the next call will be :

cons(L1, [c|After], [c,d]).

But , I can see while printing the customized comments in the code, that, at this point the code control goes to #line 1 which I am not able to understand. Because, now,

L1= unknown(just as all the previous calls).
L(parameter 2)= [c|After]
L(parameter 3) = [c,d].

But, the control goes to the #line 1, and it prints :

Line1: [c,d]

I thought prolog executes the code from left to right. So, as per my understanding, while executing the value of L should be [c,d].

My question is:

1. After the second call, L1 is undefined as all the calls as before. And, second and third parameters , both are L. So, why, after second call the control goes to the #line1.?

2. Is my understanding correct in this matter ? "I thought prolog executes the code from left to right. So, as per my understanding, while executing the value of L should be [c,d]."

Thanks in advance... Please help me out!!

Answer 1

You cannot expect to understand everything that is happening here if you look at it procedurally, because there is too much going on at the same time. This is especially true if you are just beginning to learn the language.

A good approach for learning Prolog is to think declaratively, and ask: What are the conditions that make this hold?

In addition, you are currently writing atoms when you actually mean to write variables. Please correct your post to say for example:

cons(L1, [c|After], [c,d])

Note that After is a variable, and after is an atom.

Now, leaving everything else aside, just consider this single goal in isolation. Try it with:

?- cons(L1, [c|After], [c,d]).

In complete accordance with our expectation, we get the solution:

L1 = [],
After = [d]

Thus, it suffices to understand that this goal can be derived in isolation. Also notice the difference between X = [a] and X = a, which you are currently intermingling.