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Using Generic Reference Expressions

Certain expressions allow you to use generic references to a set of items or values, depending on the expression's context in an action or procedure statement. These are called generic reference expressions.

Generic reference expressions can refer to one or more items, attributes, variable or parameter values, or list or array elements in certain contexts. For instance, the generic reference expression any custom-object connected to my-valve in the antecedent of the following rule refers to a set of items:

whenever the authors of any custom-object connected to my-valve receives 
a value then 
	inform the operator that 
	"The object [the public-name of the custom-object] has been edited.";

The generic reference expression any symbol in any symbol-list in the antecedent of the following rule refers to a set of values:

if any symbol in any symbol-list is not ok 
	then inform the operator that 
	"Validation of [the superior item of the symbol-list] found discrepancies." ;

To create a generic reference expression:

generic-reference-expression  :=  {class-name | type}  [local-name] 
[generic-reference-qualifier]

Including a Generic Reference Qualifier Expression

In a generic reference expression that identifies a set of items, use a generic reference qualifier expression to refer to those items with respect to their system-defined relationships with other items.

To create a generic reference qualifier expression:

generic-reference-qualifier  := 
{upon kb-workspace}  |  {connected connected-expression}  | 
{at at-expression}  |  {nearest to item}  | 
{superior to kb-workspace  |  object-attribute}  | 
{that is relation-name    item}  |  {named by symbolic-expression}  | 
{in {g2-list  |  g2-array}  }  |  {name of item}

For example, the antecedent of this rule specifies a generic reference expression that includes a generic reference qualifier expression connected to my-valve:

whenever the authors of any custom-object connected to my-valve receives 
a value then 
	inform the operator that 
	"The object [the public-name of the custom-object] has been edited." ;

Using Quantifiers

You must prefix each generic reference expression with a G2 reserved symbol, called a quantifier. Each quantifier indicates whether the expression produces one, one and only one, at least one, or any number of items or values.

You use particular quantifiers in certain contexts, as summarized in this table:

Quantifier Meaning Context and Example
a an
At least one
Use only in there exists expressions:
there exists a tank T in tank-list-1 such that ( T is full )

any
Zero or more
Use only in generic rules:
if any tank T in tank-list-1 is full then remove T from tank-list-1

each
Zero or more
In expressions over a set of values, items, or attributes, use to compute a value based on the set:
the count of each tank T in tank-list-1 such that ( T is full )
In expressions in procedures, use to iterate over a set of items:
for T = each tank in tank-list-1 do ...

every
Zero or more
In a for every expression, use to iterate over a set of items or attributes:
for every tank T in tank-list-1 ( T is full )
In the consequent of a rule, causes an action to execute on every item that matches the reference:
if the time-of-day of floor-clock > 600 then show the subworkspace of every die-cutting-machine
In this example, G2 iterates over all tanks in tank-list-1, and produces the truth-value true if all tanks are full.

the
One and only one
In any expression: the tank in tank-list-1 exists This expression produces the truth-value true only when there is one tank in tank-list-1.

Quantifier	Meaning	Context and Example
`a an`	At least one	Use only in `there exists` expressions: `there exists a tank T in tank-list-1 such that ( T is full )`
`any`	Zero or more	Use only in generic rules: `if any tank T in tank-list-1 is full then remove T from tank-list-1`
`each`	Zero or more	In expressions over a set of values, items, or attributes, use to compute a value based on the set: `the count of each tank T in tank-list-1 such that ( T is full )` In expressions in procedures, use to iterate over a set of items: `for T = each tank in tank-list-1 do ...`
`every`	Zero or more	In a `for every` expression, use to iterate over a set of items or attributes: `for every tank T in tank-list-1 ( T is full )` In the consequent of a rule, causes an action to execute on every item that matches the reference: `if the time-of-day of floor-clock > 600 then show the subworkspace of every die-cutting-machine` In this example, G2 iterates over all tanks in `tank-list-1`, and produces the truth-value `true` if all tanks are `full`.
`the`	One and only one	In any expression: `the` tank in tank-list-1 exists This expression produces the truth-value `true` only when there is one tank in `tank-list-1`.

It is possible to use the every quantifier in outer and inner statements:

for every tank T in tank-list1 conclude that the background-color of 
	the workspace of every T is the symbol blue

When doing so, however, the second every statement cannot refer to a local variable declared in the first, and a statement such as the example here would not compile correctly.

Embedded Generic Reference Expressions

In some G2 expressions you must specify an embedded generic reference expression. In these cases, you must use a particular quantifier, not any quantifier.

For example, the there exists expression requires only the a or an quantifier for its embedded generic reference expression. This rule's antecedent tests whether any items exist that have a value other than OK in the Notes attribute:

initially 
	if there exists an action-button such that 
	(the text of the notes of the action-button /= "ok") 
		then inform the operator that 
		"At least one action-button in this KB has errors.";

Note: In this chapter the syntax descriptions for each expression show the required quantifier for the embedded generic reference expression, if any.

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Using Generic Reference Expressions

Including a Generic Reference Qualifier Expression

Using Quantifiers

Embedded Generic Reference Expressions

| Prev | Next | Start of Chapter | End of Chapter | Contents | Glossary | Index | Comments | (7 out of 16)

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