There will come a point when we hear about the so-called cut. It is spoken of with words of caution, as if it were a forbidden power capable of great good and great evil.
We'll take our time learning about it, starting with a minimal example.
% Example 12 - The Cut
% who is happy
happy(john).
happy(jane) :- !.
happy(jill).
Setting aside the exclamation mark ! for now, this short prolog program creates three facts. John is happy, Jane is happy, and Jill is happy.
Who Is Happy?
Let's start by running some very basic queries to establish a firm and familiar foundation.
?- happy(john).
true
?- happy(jane).
true
?- happy(jill).
true
Everything is familiar, and as expected. The queries confirm John, Jane and Jill are happy.
Let's now ask “who is happy?” in the usual way using a variable.
?- happy(X).
X = john
X = jane
This isn't quite what we expected. Prolog didn't provide X=jill as an answer. It is as if prolog stopped trying to find additional answers after that ! symbol.
The Cut
The exclamation mark ! is called a cut. The following solution search tree illustrates the effect of the cut on our query.
Let's break it down. To find solutions to the query happy(X), prolog tries unifying with rules in its database.
The first rule is happy(john) and this results in the first solution X=john. Prolog backtracks to the point where X is again unbound and tries to find another solution.
This time prolog finds the rule happy(jane):-! and successfully unifies with its head. Before it can settle on X=jane as the next solution, the body of the rule needs to be true. That body is simply the cut !. The cut always succeeds, but also has the side-effect of telling prolog to stop any further backtracking. This stops prolog searching for more solutions. This is why prolog doesn't attempt to unify with the rule happy(jill), and why X=jill is not returned as a third solution.
So the effect of a cut is to stop prolog backtracking over it, and commit to whatever parts of a solution it has found up to the point the cut appears in a rule. It is called the cut because it cuts off part of the solution search tree, as illustrated above.
Looking back to our direct querying of facts, prolog did confirm that happy(jill) is true. This tells us the cut only takes effect when prolog is searching for solutions using the backtracking mechanism.
Cut With Caution
Even with this small example, we can see the cut introduces a logical inconsistency.
One the one hand, asking happy(jill) tells us Jill is happy, but on the other hand, asking happy(X) excludes Jill from the list of happy people.
This breakdown of logical consistency is not ideal, and can be dangerous in higher risk applications. The larger and more complex our programs become, the harder it is to find such logical inconsistencies. Even worse, we may not even be aware inconsistencies are happening.
It is natural to ask why such something as dangerous as the cut even exists at all. We'll see later how the cut can be useful, if used carefully.
Key Points
- The exclamation mark symbol
!is called a cut. - A cut always succeeds but stops prolog from backtracking over it, preventing prolog from searching for more possible solutions to a query.
- The cut only effects backtracking, which is why a specific query can return a solution, whereas a more general query using a variable might not.
- The cut can cause different, but logically consistent, queries to give logically inconsistent solutions.