Prolog: Why to define facts before rules

It is always good to define facts before rules.

For example,

listDemo.pl

isList([_|Tail]) :- isList(Tail).
isList([]).

Above snippet is used to check whether given element is a list or not. As you observe above snippet, I defined the rule first, followed by the fact.

1 ?- consult(listDemo).
true.

2 ?- isList([]).
true.

3 ?- isList([1, 2]).
true.

It is working pretty good for the above examples. What if, I pass an uninstantiated variable as an argument to isList rule.

4 ?- isList(X).
ERROR: Out of local stack
   Exception: (1,763,389) isList(_10580304) ? creep
   Exception: (1,763,388) isList(_10580298) ? exit

Oops, I end up in Out of local stack error.

We can get rid of these kind of problems, if we define the fact first followed by the rules.

listDemo.pl

isList([]).
isList([_|Tail]) :- isList(Tail).

1 ?- consult(listDemo).
true.

2 ?- isList([]).
true.

3 ?- isList([1, 2]).
true.

4 ?- isList(X).
X = [] .

As you see, uninstantiated variable X is unified with [].

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