exp(x) taylor

fact(0,1).
fact(N,R):- N1 is N-1,
            fact(N1,R1),
            R is N*R1,!.

% Calculating the exponential (complex version)
exp(_,0,1).
exp(X,N,Res):- (N > 0,
               fact(N,Fact),
               Frac is X**N/Fact,
               N1 is N-1,
               exp(X,N1,Res1),
               Res is Res1 + Frac);
               (N < 0, write("N must be positive")).

%% Simple version:
exp2(_,0,1).
exp2(X,N,Res):- fact(N,Fact),
               Frac is X**N/Fact,
               N1 is N-1,
               exp2(X,N1,Res1),
               Res is Res1 + Frac.