CODEWITHSUNDEEP
prolog
user3423255
user3423255

Reputation: 71

how to trace a predicate in prolog?

This is a predicate that get permutations of a list. Can somebody explain to me how to trace this predicate? I am using SWI.

perm([H|T],L) :- perm(T,P) ,  insert(H,P,L).
perm([],[]).

insert(X,L,[X|L]).
insert(X,[H|T],[H|T1]) :- insert(X,T,T1).

Upvotes: 1

Views: 2055

Answers (1)

lurker
lurker

Reputation: 58244

Here's an example of using trace in SWI Prolog.

Entering the code:

?- [user].
|: perm([H|T],L) :- perm(T,P) ,  insert(H,P,L).
|: perm([],[]).
|:
|: insert(X,L,[X|L]).
|: insert(X,[H|T],[H|T1]) :- insert(X,T,T1).
|: % user://1 compiled 0.01 sec, 6 clauses
true.

Running a trace. Press "Enter" at the question marks ? to "creep" (take a step):

?- trace.
true.

[trace]  ?- perm([1,2,3], L).
   Call: (6) perm([1, 2, 3], _G366) ? creep
   Call: (7) perm([2, 3], _G445) ? creep
   Call: (8) perm([3], _G445) ? creep
   Call: (9) perm([], _G445) ? creep
   Exit: (9) perm([], []) ? creep
   Call: (9) insert(3, [], _G446) ? creep
   Exit: (9) insert(3, [], [3]) ? creep
   Exit: (8) perm([3], [3]) ? creep
   Call: (8) insert(2, [3], _G449) ? creep
   Exit: (8) insert(2, [3], [2, 3]) ? creep
   Exit: (7) perm([2, 3], [2, 3]) ? creep
   Call: (7) insert(1, [2, 3], _G366) ? creep
   Exit: (7) insert(1, [2, 3], [1, 2, 3]) ? creep
   Exit: (6) perm([1, 2, 3], [1, 2, 3]) ? creep
L = [1, 2, 3]

As you might expect, this trace shows that perm calls itself recursively until it gets to the empty tail ([]) of the input list [1,2,3]. It then shows calls to insert which follow those recursive calls, along with the arguments occurring in those calls. The _Gnnn variables are uninstantiated arguments on a Call that gets instantiated in the clause which you see on the Exit.

Upvotes: 2

Related Questions