How does recursion in Prolog works from inside. One example

Question

I\'ve got here a small script, that converts list of elements into a set. For example list [1,1,2,3] -> set [1,2,3]. Can somebody explain to me, step by step, whats happenin

Answer 1

In certain implementations of Prolog, e.g. GNU Prolog, you can trace the execution of your code. Using this feature you can step through the evaluation process, as demonstrated below.

$ gprolog
GNU Prolog 1.4.2
By Daniel Diaz
Copyright (C) 1999-2012 Daniel Diaz
| ?- consult('list_to_set.pro').
yes
| ?- trace.
yes
{trace}
| ?- list_to_set([1,1,2,3], X).
      1    1  Call: list_to_set([1,1,2,3],_25) ?
      2    2  Call: list_to_set([1,2,3],_57) ?
      3    3  Call: list_to_set([2,3],_83) ?
      4    4  Call: list_to_set([3],_109) ?
      5    5  Call: list_to_set([],_135) ?
      5    5  Exit: list_to_set([],[]) ?
      6    5  Call: \+member(3,[]) ?
      7    6  Call: member(3,[]) ?
      7    6  Fail: member(3,[]) ?
      6    5  Exit: \+member(3,[]) ?
      4    4  Exit: list_to_set([3],[3]) ?
      7    4  Call: \+member(2,[3]) ?
      8    5  Call: member(2,[3]) ?
      ...

An explanation of how to interpret Prolog's traces can be found at http://remus.rutgers.edu/cs314/f2007/ryder/projects/prolog/prologTrace.html, section READING A TRACE.