Page 210 - DCAP506_ARTIFICIAL_INTELLIGENCE

Artificial Intelligence

Notes 14.5.2 Cuts

The cut (!) functions in the conventional manner. When implemented, it succeeds and
throws away all backtrack points among itself and its CUTPARENT. Usually, the cutparent
is the query that caused execution to enter the existing clause. Though, if the cut is in
an environment that is OPAQUE TO CUTS, the cutparent is the commencement of that
environment.

Example: Examples of surroundings that are opaque to cuts are:
_ The argument of the negation operator (\+).
_ The argument of call, which can certainly be a compound goal, such as call((this,!,
that)).
_ The left–hand argument of ‘->’ (see below).
The objectives that are arguments of once, catch, findall, bagof, and setof (and, usually, any other
goals that are arguments of predicates).

14.5.3 If-then-else

The “if–then–else” build (Goal1 -> Goal2 ; Goal3) tries to implement Goal1, and, if
successful, proceeds to Goal2; or else, it proceeds to Goal3. The semicolon and Goal3 can be
omitted.
Observe that:
_ Only the first solution to Goal1 is established; any backtrack points produced while
executing Goal1 are thrown away.
_ If Goal1 succeeds, execution proceeds to Goal2, and then:
If Goal2 fails, the whole construct fails.
– If Goal2 succeeds, the whole construct succeeds.

– If Goal2 has multiple solutions, the whole construct has multiple solutions.
_ If Goal1 fails, execution proceeds to Goal3, and then:
If Goal3 fails, the whole construct fails.

– If Goal3 succeeds, the whole construct succeeds.
– If Goal3 has multiple solutions, the whole construct has multiple solutions.
_ If Goal1 fails and there is no Goal3, the whole construct fails.
_ Either Goal2 or Goal3 will be executed, but not both (not even upon backtracking).
_ If Goal1 contains a cut, that cut only has scope over Goal1, not the whole clause. That is,
Goal1 is opaque to cuts.
_ The whole if–then–else structure has numerous solutions if Goal1 succeeds and Goal2 has
multiple solutions, or if Goal1 fails and Goal3 has multiple solutions. That is, backtrack
points in Goal2 and Goal3 behave normally.

_ Cuts in Goal2 and Goal3 have scope over the entire clause (i.e., behave normally).

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