Turbo Prolog

Definition 3
singlepeak([l,_).
singlepeak([_l,_).
singlepeak([U,VIW1,up):-
UV,singlepeak([VIW1,down).
Rule 4. Let Turbo Prolog's unification mechanism do as much of the work as possible. At
first thought, you might think to define a predicate equal to test two lists for equality as
equal ( [ 1 , [ 1 ) .
equal([UIX1,[UIY1):- equal(X,y).
but this is unnecessary. Using the definition
equal(X,X).
Turbo Prolog's unification mechanism does all the work!
Rule 5. Use backtracking instead of recursion to effect repetition. Backtracking
decreases stack requirements. The idea is to use the repeat . .. fail combination instead
of recursion. This is so important that the next section is dedicated to the technique.
Use of the Fail Predicate
To have a particular sequence of subgoals evaluated repeatedly, it is often necessary to
define a predicate like run with a clause of the form
run:- readln(X),
process(X,y),
write(Y) ,
run.
thus incurring unnecessary tail recursion overheads that cannot be automatically eliminated
by the system because process(X, Y) involves backtracking.
In this case, the repeat . .. fail combination avoids the need for the final recursive call.
Given
repeat.
repeat:-repeat.
we can redefine run without tail recursion as follows:
run:- repeat,
readln(X),
process(X,y),
write(Y) ,
fail.
fail causes Turbo Prolog to backtrack to process, and eventually to repeat, which always
succeeds.
148 Turbo Prolog Owner's Handbook