| In the previous section we saw how it was possible to work through
a list, taking the head of a list and writing it, then performing
the same operation on the tail. The procedure used to implement this
included two rules: one recursive and the other the terminating condition.
Terminating conditions are essential in list processing because
they usually represent conditions that we are more interested in.
The three patterns that we are interested in are:
Terminate
when the list is empty
We've already seen an example of this in dissect_list/1. The
key point is that the computation ends when the empty list ([])
is found.
Terminate
when a specified element is found
Examples of this are searching for a given element in a list,
either to discover it is there, or to perform some operation on
it, such as replacement.
Terminate
when a specific point is reached
Here, we work through lists until we have reached a specific
point (irrespective of what that element is), and then perhaps
perform some operation on it.
Each of the three sections include examples designed to illustrate
the topic. However, the examples given aren't necessarily the ones
most frequently used by expert programmers, because there are other
versions that are more efficient. These are considerations that
have to take into account the way that Prolog is implemented on
the underlying hardware. In this sense, the previous versions are
not "wrong" but less appropriate.
The topic of efficiency in list processing is taken further in
discussion on the topic of accumulators.
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