Labels and computations

Prolog code

Finite mappings for environments and stores

override(M1,M2,M) when M represents M2[M1]

override([X=Y|M1],M2,[X=Y|M]) :- delete(M2,X=_,M3), override(M1,M3,M).

fresh(ST, L) when L is not in dom(ST)

fresh(ST, loc(N1)) :- length(ST, N), N1 is N+1.

loc(loc(N)) :- integer(N).

Transition relations

Labelled transition S ---X---> S1

Unlabelled transition S ------> S1


Records are represented by lists of equations:

  • unprimed index: I=A (initial value of label component)
  • “primed” index: I+=A (final value of label component)


Unobservable steps S ---U---> S1

S ---U---> S1 :-
        S ------> S1, unobs(U).

Unbounded computation S ---X--->* F

F ---U--->* F :- final(F), unobs(U).

S ---X--->* F :-
        pre_comp(X, X1),
        S ---X1---> S1,
        mid_comp(X1, X2),
        S1 ---X2--->* F,
        post_comp(X1, X2, X).

Bounded computation (up to N steps) S ---X---N>* SN

F ---U---_N>* F :- final(F), unobs(U).

S ---X---N>* SN :-
        N =:= 0 -> SN = S ;
        pre_comp(X, X1),
        S ---X1---> S1,
        mid_comp(X1, X2),
        N1 is N - 1,
        S1 ---X2---N1>* SN,
        post_comp(X1, X2, X).

pre_comp(X, X1) sets readable components of X1

pre_comp([I=C|L], X1) :-
        select(I=C, X1, L1), !,
        pre_comp(L, L1).

pre_comp([I+=_C|L], X1) :-
        select(I+=_, X1, L1), !,
        pre_comp(L, L1).

pre_comp([], []).

mid_comp(X1, X2) sets readable components of X2

mid_comp([I=C1|L1], X2) :-
        ( changeable(I) ->
            select(I+=_, X2, L2) ;
            select(I=C1, X2, L2) ), !,
        mid_comp(L1, L2).

mid_comp([I+=C1|L1], X2) :-
        ( changeable(I) ->
            select(I=C1, X2, L2) ;
            select(I+=_, X2, L2) ), !,
        mid_comp(L1, L2).

mid_comp([], []).

post_comp(X1, X2, X) sets writable components of X

post_comp([_=_|L1], X2, X) :-
        post_comp(L1, X2, X).

post_comp([I+=C1|L1], X2, X) :-
        member(I+=C2, X2),
        ( changeable(I) ->
            member(I+=C2, X) ;
          append(C1, C2, C),
          member(I+=C, X) ), !,
        post_comp(L1, X2, X).

post_comp([], _X2, _X).

unobs(L) when L has no observable effects

unobs([I=C|L]) :-
        \+ writable(I) -> unobs(L) ;
        select(I+=C, L, L1), unobs(L1).

unobs([I+=C|L]) :-
        \+ readable(I) -> C = [], unobs(L) ;
        select(I=C, L, L1), unobs(L1).


readable(dummy). writable(dummy).

changeable(I) :- readable(I), writable(I).