1 files changed, 89 insertions, 0 deletions
diff --git a/prolog/problems/algol/algol_for_3/common.py b/prolog/problems/algol/algol_for_3/common.py
index da7b520..b48610b 100644
--- a/prolog/problems/algol/algol_for_3/common.py
+++ b/prolog/problems/algol/algol_for_3/common.py
@@ -90,3 +90,92 @@ eval178( E1 / E2, State, Val) :- !,
   eval178( E2, State, V2),
   Val is V1 /  V2.
 '''
+
+initial = '''\
+% program
+algol_for(fun(S0, S, apply([printout=[]|S0], S, Minstructs))) -->
+    [begin], instructs(Minstructs), [end].
+
+% sequence of instructions
+instructs(Minstr) --> instr(Minstr).
+instructs(fun(S0,S,
+    (apply(S0,S1,Minstr),
+     apply(S1,S,Minstructs))))
+    -->
+    instr(Minstr), instructs(Minstructs).
+
+% statement (instruction)
+instr(Massign) --> assign(Massign).
+
+% put the current value of a variable into the output list
+instr(fun(S0, [printout = L1|S1],
+    (memb(X = V,S0),
+     del(printout = L0, S0, S1),
+     conc(L0, [V], L1))))
+    -->
+    [print(X)].
+
+% while-loop
+instr(fun(S0, S,
+    loop(S0, Mcond, Minstructs, S)))
+    -->
+    [while], cond(Mcond), [do, begin], instructs(Minstructs), [end].
+
+% assignment statement
+assign(fun(S0, [X = Value|S1],
+    (apply(S0, Value, Mexpr),
+     del(X = _, S0, S1))))
+    -->
+    var(X), [':='], expr(Mexpr).
+
+% conditional expression
+cond(fun(S, TruthVal,
+    (apply(S, Val1, ME1),
+     apply(S, Val2, ME2),
+     (Val1 < Val2, !, TruthVal = true ; TruthVal = false))))
+    -->
+    expr(ME1), ['<'], expr(ME2).
+
+% variable
+var(X) --> [X], { atom(X) }.
+
+% expression
+expr(fun(S, Value, eval(Expr, S, Value))) -->
+    [Expr].   % Expr is a Prolog arithmetic expression, e.g. x + 2*y + 3
+
+% helper predicates
+% function call
+apply(In, Out, fun(In, Out, Goals)):-
+    call(Goals).
+
+% loop
+loop(State0, Mcond, _, State0) :-
+    apply(State0, false, Mcond), !. % condition is false
+loop(S0, Mcond, MBody, S) :-
+    copy_term(MBody, MBodyCopy),    % copy goals
+    apply(S0, S1, MBody),           % run loop body
+    loop(S1, Mcond, MBodyCopy, S).  % next iteration
+
+% expression evaluation
+eval(N, _, N) :-
+    number(N), !.
+eval(X, State, Val) :-  % a program variable
+    atom(X), !,
+    memb(X = Val, State).
+eval(E1 + E2, State, Val) :- !,
+    eval(E1, State, V1),
+    eval(E2, State, V2),
+    Val is V1 + V2.
+eval(E1 - E2, State, Val) :- !,
+    eval(E1, State, V1),
+    eval(E2, State, V2),
+    Val is V1 - V2.
+eval(E1 * E2, State, Val) :- !,
+    eval(E1, State, V1),
+    eval(E2, State, V2),
+    Val is V1 * V2.
+eval(E1 / E2, State, Val) :- !,
+    eval(E1, State, V1),
+    eval(E2, State, V2),
+    Val is V1 /  V2.
+'''