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{ CALC evaluates arithmetic expressions with store }
calc:- eval ([], RS), write (result store), write (RS), nl.
eval (SI, SO):-
read (CALC), nonvar (CALC), eval member (CALC, SI, SO).
eval member (CALC, SI, SO):-
member (CALC, [stop,end,bye,eof]), SO=SI;
eval (CALC,I,SI,ST), write (I), eval (ST,SO);
write (error in), write (CALC), nl, eval (SI, SO).
eval (I, I, S, S):- integer (I).
eval (N, I, S, S):- atom (N), eval atom (N, I, S).
eval atom (N, I, S):-
member (N=I, S);
write ("error: Cell"), write (N),
write("not found in store. 0 substituted."), nl, I=0.
eval ( L+R,I,SI,SO):- eval (L,J,SI,ST), eval (R,K,ST,SO), I IS J+K.
eval ( L-R,I,SI,SO):- eval (L,J,SI,ST), eval (R,K,ST,SO), I IS J-K.
eval ( L*R,I,SI,SO):- eval (L,J,SI,ST), eval (R,K,ST,SO), I IS J*K.
eval ( L/R,I,SI,SO):- eval (L,J,SI,ST), eval (R,K,ST,SO), I IS J/K.
eval (N=O, I, SI, SO):-
atom (N), eval (O,I,SI,ST), eval repl (N,I,ST,SO).
eval repl (N, I, [], [=(N,I)]).
eval repl (N, I, [=(N,_)|S], [=(N,I)|S]).
eval repl (N, I, [=(M,J)|SI], [=(M,J)|SO]):- eval repl (N, I, SI, SO).
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