Turbo Prolog

Tail recursion elimination is a technique for replacing such forms of recursion with
iteration, and in spite of its name, it is also useful in situations in which there is no direct
recursion or even no recursion at all-just a long chain of procedure calls.
To see how it works, suppose we had defined demopred, which uses member as
follows:
demopred(X,Y):-... ,member(A,B),test(A), ....
When member is first activated, the system must remember that once member has
been successfully evaluated, control must pass to the predicate test. This means that the
address of test must be saved on the stack.
Likewise, for each recursive call of member, the system must remember the address
the member predicate needs to return to after successful evaluation-namely, itself.
Since there are no backtracking points between
member(X,[_IY1)
and the recursive call
member(X,y)
there is no need to stack the address of member several times. It is enough to remember
that on successful termination of member. control should pass to test. This is tail
recursion elimination. Where the system itself cannot eliminate recursion, the programmer
can do a lot to limit its effect (and limit demand on the stack) by adopting a few
rules of thumb about programming style.
Rule I. Use more variables rather than more predicates. This rule is often in direct
conflict with program readability, so a careful matching of objectives is required to
achieve programs that are efficient both in their demands upon relatively cheap
machines and upon relatively expensive human resources.
Thus, if writing a predicate to reverse the elements of a list
reverse(X,y):- reverseL([),X,y).
reverseL(Y,[),Y).
reverseL(XL,[UIX21,Y):- reverseL([UIXL),X2,Y).
makes less demands upon the stack than
reverse ( [ ) , [ 1 ) .
reverse([UIX),Y):-reverse(X,YL),append(YL,[U),Y).
append([l,Y,Y).
append([UIX),Y,[UIZ)):- append(X,y,Z).
which uses the extra predicate append.
Rule 2. When ordering subgoals in a rule, those with the most bound variables should
come ftrst. Thus, if writing a Turbo Prolog predicate to solve the simultaneous equations
X + L = t;
X + Y = 5
and using a "generate and test" method,
solve(X,y):numb(X),plus(X,l,t;),
numb(y),plus(X,y,S).
146 Turbo Prolog Owner' s Handbook