Question

I expect the following code to print out [9 4], but this isn’t working

:- op(20,xfx,i).

i(X,Y, Z) :-
         Z=[X,Y].

main:-
       RESULT is 9 i 4, write(RESULT).  

Where am I getting wrong?

Answer 1

an operator is basically syntactic sugar; instead of writing +(1,2) we simply write 1+2.
therefore, 9 i 4 is equivalent to i(9,4)

now, +/2 is not only an operator but also an arithmetic function
note that the result should be a number so you cannot use it to return a list (and cannot use is/2 either)