Logical Look-Ahead

The operators, Logical And (&&) and Logical Or (||) are sometimes used to create Boolean rule sets as part of an expert system application. In this type of application, you combine rules with nodes that prompt the user for input information. For example, the following set of node definitions create part of an expert system that determines if a patient has a cold.

Cold:= Runny Nose&&Slight Fever

Runny Nose:=askyesno("Does the patient have a runny nose?")

Slight Fever:=Temperature>=99&&Temperature<=102

Temperature:=asknumber("What is the patient's temperature?")

The first definition contains a rule that is true if the patient both has a runny nose AND has a slight fever. The next definition is a rule that defines slight fever. The remaining two definitions present questions to the user to ask for a numeric input (asknumber) or ask for the response to a Yes/No question (askyesno).

Now suppose these definitions are part of a larger medical diagnostic system. Also, assume the user had previously answered the Temperature question and indicated that the patient's temperature is 104. Then, when evaluating the node Cold, you would like the application simply to recognize that Cold can never be true and don't even bother asking if the patient has a runny nose. This effect can be accomplished using Logical Look-Ahead.

Logical Look-Ahead is a feature of DScript that controls how the Logical And and Logical Or operators behave. Specifically, this feature prevents applications from prompting the user for information that, ultimately, will not be needed.

In evaluating a Boolean expression, DScript scans the entire logic tree to determine if there is any way to calculate a result without asking the user for input. If it is not possible to calculate a solution, DScript will ask one question and then repeat the scan.

Logical Look-Ahead allows you to build rule-based applications without worrying about the order in which questions are posed to the user. In addition, you don't have to worry about redundant logic. DScript will only ask questions that are absolutely necessary.

You can choose whether to use Logical Look-Ahead in your rule-based applications by clicking System Options in the Tools menu and choosing the appropriate option in the Calculation page.

Gate Primitive

The gate primitive is designed to force a branch in a Logical Look-Ahead to fail while not interfering with calculations at other times. If one of the primitives And/Or is active and the system is attempting to evaluate a Boolean expression without prompting the user, gate causes the current branch being explored to fail. In all other cases, gate is benign.

All input primitives have the functionality of the gate primitive built in. The purpose of the gate primitive is to tell DScript which calculations are expensive. Asking the user to respond to a question is expensive in terms of user effort and time. If you are getting information from a source other than the user and the source is slow, you might want to gate access to the source.

For example, if you build an application that uses stock prices pulled from the Internet as an input, you may want your application to retrieve the price only if absolutely necessary. This is accomplished by changing a node from

Price:=StockPrice("IBM")

to

Price:=gate(StockPrice("IBM"))

With the latter expression, DScript will avoid pulling data from the Internet if there is any other way to draw a complete conclusion.