Select Redoing Until Failure, Why?

Question

I have the following function:

fillNonDrivers(Car, Pmd, Plo, ListOfPassengers) :-
  select(Passenger, Pmd, Plo1),
  Passenger = [_,n,_],
  /* etc */

I invoke it with trace on in the following way:

fillNonDrivers([hello, 2], [[david, n, punk]], PLO, LOP).
 1    1  Call: fillNonDrivers([hello,2],[[david,n,punk]],_29,_30) ? c
 2    2  Call: select(_111,[[david,n,punk]],_112) ? c
 2    2  Exit: select([david,n,punk],[[david,n,punk]],[]) ? c
 2    2  Redo: select([david,n,punk],[[david,n,punk]],[]) ? c
 2    2  Fail: select(_99,[[david,n,punk]],_100) ? c
 1    1  Fail: fillNonDrivers([hello,2],[[david,n,punk]],_29,_30) ? c
 no

I do not understand why Redo is called in the above trace. Shouldn't have the select "worked", and thus the next line invoked be

 Passenger = [_,n,_],

Could someone help explain the appearance of redo here? Thank you in advance.

Answer 1

GNU Prolog seems not to display unification goals (=) in the trace. See also this simplified example:

GNU Prolog 1.4.2
By Daniel Diaz
Copyright (C) 1999-2012 Daniel Diaz
| ?- [user].
compiling user for byte code...
f(X) :- X=3.

user compiled, 2 lines read - 182 bytes written, 12539 ms

(266 ms) yes
| ?- trace.
The debugger will first creep -- showing everything (trace)

yes
{trace}
| ?- f(N).
      1    1  Call: f(_17) ? 
      1    1  Exit: f(3) ? 

N = 3

yes
{trace}
| ?- 

Note that there is no step like X=3, in contrast to the SWI trace step 7 which CappeliC gave.

So it just means that the next goal after Passenger = ... fails.

Answer 2

it means that your rule is failing immediately before binding to Passenger, and this should never happen, given the test case. The trace should report what is failing, indeed in SWI-Prolog:

fillNonDrivers(Car, Pmd, Plo, ListOfPassengers) :-
  select(Passenger, Pmd, Plo1),
  Passenger = [_,n,_],
  /* etc */
  length(Plo1, 1). % expect a failure

4 ?- fillNonDrivers([hello, 2], [[david, n, punk]], PLO, LOP).
Call: (6) fillNonDrivers([hello, 2], [[david, n, punk]], _G995, _G996)
Call: (7) lists:select(_G1100, [[david, n, punk]], _G1102)
Exit: (7) lists:select([david, n, punk], [[david, n, punk]], [])
Call: (7) [david, n, punk]=[_G1093, n, _G1099]
Exit: (7) [david, n, punk]=[david, n, punk]
Call: (7) length([], 1)
Fail: (7) length([], 1)
Redo: (7) lists:select(_G1100, [[david, n, punk]], _G1102)
Fail: (7) lists:select(_G1100, [[david, n, punk]], _G1102)
Fail: (6) fillNonDrivers([hello, 2], [[david, n, punk]], _G995, _G996)
false.

Could be a bug of your Prolog debugger ?