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.