List processing: computing different return values
Define a predicate all_as(+List) which succeeds if
List is a list containing only a's as elements (if
it contains any elements at all).
Hint
+ in
all_as(+List) is all about. It's a notation that is
commonly used in specifications of what particular Prolog
predicates do (or should do). The plus expresses that the
argument that it is attached to should be instantiated when
calling the predicate. Similarly, arguments can be prefixed by a
minus to express that the argument should not be instantiated or
by a question mark to express that the argument may or may not
be instantiated.
Define a predicate replace_a_b_c(+InList,?OutList)
where OutList is obtained from InList
by replacing all a's in InList with b's, all b's
with c's, and all c's with a's.
Hint
Define a predicate list_length(+List,?Length) where
Length is the number of elements in list
List.
Hint
Compare the structure of the predicates that you had to define
for the above exercises. In all cases, you have to recurse
through the list until you reach the empty list. The predicates
differ, however, in their return values. Predicate
all_as/1 just performs a test on the list which
will succeed or fail. No additional return value is
computed. Predicate replace_a_b_c/2, on the other hand, maps
the input list to an output list of equal length. To obtain the
output list, something is done to each element of the input
list. Predicate list_length/2, finally, computes one single
return value.