====== Interface 2 ======
{{tag>interactive}}
===== Description =====
Program ask user and show possible answer

**Source**:  Building Expert Systems in PROLOG Publisher
===== Download =====
Program source code: {{interface_2.pl}}
===== Listing =====
<code prolog>
size(X):- menuask(size, X, [large, plump, medium, small]).
flight(X):- menuask(flight, X, [ponderous, agile, flap_glide]).

menuask(A, V, MenuList) :-
write('What is the value for'), write(A), write('?'), nl,
write(MenuList), nl,
read(X),
check_val(X, A, V, MenuList),
asserta( known(yes, A, X) ),
X == V.

check_val(X, A, V, MenuList) :-
member(X, MenuList), !.

check_val(X, A, V, MenuList) :-
write(X), write(' is not a legal value, try again.'), nl,
menuask(A, V, MenuList).
</code>
===== Comments =====