[rules-dev] Misleading documentation? Eval-uate your understanding!

[Wolfgang Laun]

Compare these three quotes from the current Expert documentation:

(1) Return Value restriction (...) must return results that do not depend on
time.
(2) An inline eval constraint (...) expression (...) expression must be
constant over time.
(3) [CE] Evals (...) are (...) ideal (...) when functions return values that
change over time.

Note that this appears to mean that the behaviour of (1) == (2) != (3).

Many things can or must "change over time:" fact data, global data, results
of constructors such as new Date() or methods such as
System.currentTimeMillis().

So what can I use in an Eval CE that cannot be used in a Return Value or
Inline Eval?

Perhaps this is trying to convey some notion of caching for constraints and
the evaluation strategy for LHS, but then the wording is insufficient. Let's
make an experiment.

Example 1: Given these rules and one pair of facts A, B, which of the three
rules ab1, ab2, ab3 will fire after the last rule (a) fires? There are 8
possible answers. (You may notice that there is some redundancy in each
rule.)

rule ab1
when
    A( $va:va, $b: b )       eval( $va + $b.getVb() > 100 )
    B( this == $b, $vb: vb ) eval( $va + $vb > 100 )
then
    System.out.println( "ab1: a+b > 100" );
end

rule ab2
when
    A( $va:va, $b: b ,      eval( $va + $b.getVb() > 100 ) )
    B( this == $b, $vb: vb, eval( $va + $vb > 100 ) )
then
    System.out.println( "ab2: a+b > 100" );
end

rule ab3
when
    $a: A( $b: b ,     $va: va > ( 100 - $b.getVb() ) )
        B( this == $b, $vb: vb > ( 100 - $a.getVa() ) )
then
    System.out.println( "ab3: a+b > 100" );
end

rule a
salience -10
no-loop true
when
    $a: A( $va: va )
    $b: B( $vb: vb )
then
    modify( $b ){ setVb( 200 ) }
end

Example 2: Now change the first line in the when parts according to:
ab1:
    A( $va:va, $b: b )
ab2:
   A( $va:va, $b: b )
ab3:
   $a: A( $b: b )

Which of the three rules ab1, ab2, ab3 will fire now after the last rule (a)
fires?

Wolfgang

PS: Scroll down for the answers.

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Example 1: None of the rules fires.
Example 2: All three rules fire.
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