Logical assignment of Prolog terms

1. Assignable terms and logical assignment
   assignable(Var) makes Var an assignable term. Initial state of it is an empty assignable term. An assignable terms have a value, and the value may be changed.
   • assignable(A) : A is an empty assignable term.
   • assignable(B,a) : B is an assignable term, and initial value is atom a.
   • assignable(C,p(1.2)) : initial value of C is p(1.2).
   • X <= V makes the value of assignable term X to V.
     (<=)/2 is deterministic, and on backtracking, the value of X
     backs up to the previous value of X at the choice point.
   • X => V unifies V to the current value of assignable term X.
     If X is empty, simply fails.
   • X <+ V, is similar to X <= V, makes the value of assignable term X to V,then commit it. On backtracking, the previous value of X never backs up. X will be empty.
     [Example]
     After assignable(X), X<=b, X<=c,X<=d, X => D :
     D == d
     
2. Unification between assignable terms.
   Undefined.
   Also, the results of decomposing an assignable term using arg/3,functor/3,=../2 are undefined.
   
3. History
   01/03/2005 Error checking and and <+compatibility with Prolog implementations.
   12/31/2004 First distribution.
   
4. Implementation
   http://www.kprolog.com/en/logical_assignment/assignable.pl
   is the implementation (using setarg/3).
   And the pure logical subset implementation is also available at
   http://www.kprolog.com/en/logical_assignment/assign_logial.pl
   
   Implementations and examples are checked against SWI-Prolog, GNU-Prolog and K-Prolog, distributed under Open Software License version 2.1, with NO WARRANTY.
   
5. Example programs
   • Finite state machine(download)
     A state machine holds its status in an assignable term.
     

         fsm(States,Initial, FSM), % FSM is state holder
         fsm_transition(FSM,0),    % transition with events
         fsm_transition(FSM,1),
         fsm_transition(FSM,1),

   • List enumerator(download)
     List enumeration done without backtracking.
     

         listEnumerator(E,[a,b,c,d,e,f]),
         listNextMember(E,L1),     % L1 == a
         listNextMember(E,L2),     % L2 == b
         listNextMember(E,L3),     % L3 == c

   • Listpermutor(download)
     List permutation example
     

         listPermutor(P,[a,b,c]),
         listPermute(P,L1),       % L1==[a,b,c]
         listPermute(P,L2),       % L2==[a,c,b]
         listPermute(P,L3),       % L3==[b,a,c]

   • Infinite loop(download)
     Memory consumed with assignable terms should be garbage collected.

Enjoy,

(C) Nobukuni Kino, KLS Research, Inc.
Comments and questions are welcome. kino@kprolog.com