if ... then & if ... then ... else

Prolog can be made to obey control structures of this form.

The if ... then form makes use of the infix operator ->/2.

The extension to if ... then ... else is achieved with the help of the ;/2 predicate.

You may be comfortable with such constructs but it is usually better, if more cumbersome, to avoid them. Here is how one might define Prolog's ``if ...then ...else''.

 
(A -> B ; C) :-

		       call(A),

		      !,

		      call(B).

(A -> B ; C) :-

		      call(C).

Just to illustrate its application, we can rewrite the predicate analyse/1 used earlier.

 
analyse(Term):-

			type(Term,Type),

			( (Type=compound_term ; Type = list) ->

						(write(Term,Type),

					 	functor(Term,N,A),

		      		 	analyse_bit(0,A,Term))

			;

					 	write(Term,Type)).
 [-5pt]

It is almost always preferable to use auxillary predicates to tidy up the `mess'.

 
analyse(Term):-

			type(Term,Type),

			( non_simple(Type) ->

						analyse_non_simple(Term,Type)

			;

					 	write(Term,Type)).

non_simple(compound_term):-

			!.

non_simple(list).

analyse_non_simple(Term,Type):-

			write(Term,Type),

			functor(Term,N,A),

		 	analyse_bit(0,A,Term).
 [-5pt]