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Document :exists #33

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Document :exists #33

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Document :exists
eraserhd Dec 6, 2018
cd73bee
Tweak language for :exists and mention unbound variables
eraserhd Dec 10, 2018
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12 changes: 12 additions & 0 deletions _docs/expressions.md
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Expand Up @@ -104,6 +104,18 @@ This will generate a rule that fires if the Customer fact has a vip status, **or

For further information, see [Boolean Expressions](/docs/booleans).

## Exists Expressions

_:exists_ tests the existence or absence of facts without triggering for every fact instance. For example:

{% highlight clojure %}
[:exists [Person (= name "Bob")]]
{% endhighlight %}

This will trigger the _first_ time a Person with the name "Bob" is inserted, but not the second or third. It will only retract after all supporting facts are removed.

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Unbound variables are treated the same as in [accumulators](/docs/accumulators/); the :exists rule will trigger once for each distinct set of unbound values.

## Tests
![test expression](/img/diagram/TEST_EXPR.png)

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