Expressions in SADL

The SADL grammar has an expression language that is used in Rule, Ask, Graph, Update, Test, Explain, and Equation and External Equations, as well as Expr statements. This grammar combines the often-encountered junction, comparison, negation, and mathematical expressions with graph patterns and list operations. The result is a rich expression language described herein.

Expression Precedence

The SADL expression grammar can be thought of as a hierarchy of operations. The precedence of operations determines the order, in the absence of parentheses, in which multiple operations are carried out. For example, computing the value of this expression depends upon the precedence of the operations of addition and multiplication.

However, if addition had precedence over multiplication, the answer would be 66.

As with many expression grammars, the SADL grammar uses parentheses to override precedence defaults. So, one could write the expression

which would evaluate to 66. The use of parentheses shows up as the PrimaryExpression "embedded expression" in the precedence hierarchy below.

From lowest precedence to highest precedence, the operations supported by the SADL grammar include the following.

an embedded expression--an expression inside parentheses, meaning that what's inside the parentheses is evaluated first
a Declaration--one of the following

an article ("A", "a", "An", "an", "any", "some", "another") , optionally followed by an ordinal ("first", "second", "other", "third", ... "tenth"), followed by a class or data type.

If the declaration is of a list it can be followed by a "... length m-n..." where m is a number and n is a larger number or "*" and the expression specifies the minimum and maximum lengths of the list or no maximum if n is "*" (see Lists in SADL)

If the declaration is of an instance of a list it can be followed by a specification of the elements of the list (a ValueRow--a comma-separated list of expressions starting with "[" and ending with "]") (see Lists in SADL)

a StringLiteral (a quoted string, delimited with either single or double quotes)
a NumberLiteral
a BooleanLiteral ("true", "false")
a Constant ("PI", "known", "e", "--", "None", "a type", "type", <article> "type", "length", <article> "length", "count", <article> "count", "index", <article> "index", "first element", "last element", <article> "element", <article> <ordinal> "element", "value")
ValueTable (a ValueRow or a set of comma-separated ValueRows starting with "{" and ending with "}")

Notice that the PropOfSubject operation has a fairly high precedence. This allows one to write expressions such as the following without parentheses, since the PropOfSubject operation has higher precedence than math operations such as multiplication.

area of a Rectangle = height of the Rectangle * width of the Rectangle

Using Expr Statements to Validate Expressions

The Expr Statement in the SADL grammar is used only to provide validation of expressions using the SADL expression grammar in the development environment. The keyword Expr followed by a colon can be followed by any valid SADL expression followed by a period (end of statement). An information marker in the editor will contain the translation of the expression to the Intermediate Form, a reasoner/translator independent functional expression language that is ordinarily passed to the selected translator for translation to the target rule or query language. In the case of Expr Statements, the final result is the display of the Intermediate Form to the user in the information marker as a means of feedback to the user concerning the meaning of the expression.

Here are three examples of Expr Statements along with the Info Marker content for each.

Note that the SADL expression grammar is somewhat disjoint from the grammar for declaring classes, instances, properties, and property restrictions. Only expressions using the SADL expression grammar are valid in an Expr Statement.

Expressions in SADL

Last revised 4/27/2021. Contact us.

Introduction

Expression Precedence

Using Expr Statements to Validate Expressions