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]