MEL logical and acting strange, operand order?

Question

Can someone explain why the following behavior occurs? Why does the order of the operands matter when using logical AND? Running on mule-ce 3.3.1.

The output is as follows:

true  
true  
false  
true


<?xml version="1.0" encoding="UTF-8"?>

<mule xmlns:http="http://www.mulesoft.org/schema/mule/http" xmlns="http://www.mulesoft.org/schema/mule/core" xmlns:doc="http://www.mulesoft.org/schema/mule/documentation" xmlns:spring="http://www.springframework.org/schema/beans" version="EE-3.4.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans-current.xsd
http://www.mulesoft.org/schema/mule/core http://www.mulesoft.org/schema/mule/core/current/mule.xsd
http://www.mulesoft.org/schema/mule/http http://www.mulesoft.org/schema/mule/http/current/mule-http.xsd">

    <flow name="testFlow1" doc:name="testFlow1">
        <http:inbound-endpoint exchange-pattern="request-response" host="localhost" port="20005" path="test" doc:name="HTTP"/>

        <message-properties-transformer overwrite="true" scope="session" doc:name="Message Properties">
              <add-message-property key="productNumber" value="1" />
        </message-properties-transformer>

        <logger level="INFO" message="#[(sessionVars['productNumber'] == '1' || sessionVars['productNumber'] == '2') and  (sessionVars['quantity'] == null)]" doc:name="Logger"/>
        <!-- true -->

        <logger level="INFO" message="#[(sessionVars['quantity'] == null) and (sessionVars['productNumber'] == '1' || sessionVars['productNumber'] == '2')]" doc:name="Logger"/>
        <!-- true -->

        <message-properties-transformer overwrite="true" scope="session" doc:name="Message Properties">
              <add-message-property key="productNumber" value="3" />
        </message-properties-transformer>

        <logger level="INFO" message="#[(sessionVars['productNumber'] == '1' || sessionVars['productNumber'] == '2') and  (sessionVars['quantity'] == null)]" doc:name="Logger"/>
        <!-- false -->

        <logger level="INFO" message="#[(sessionVars['quantity'] == null) and (sessionVars['productNumber'] == '1' || sessionVars['productNumber'] == '2')]" doc:name="Logger"/>
        <!-- true? it should not be true! -->        
    </flow>
</mule>

Answer 1

use & ampamp; not and. it calculates the correct value. updated stream

<logger level="INFO" message="#[(sessionVars['productNumber'] == '1' || sessionVars['productNumber'] == '2') &amp;&amp;  (sessionVars['quantity'] == null)]" doc:name="Logger"/>
    <!-- false -->


<logger level="INFO" message="#[(sessionVars['quantity'] == null) &amp;&amp; (sessionVars['productNumber'] == '1' || sessionVars['productNumber'] == '1')]" doc:name="Logger"/>

Answer 2

I have no idea why this is happening, but I have the same experience in 3.4.0 CE. One of the workarounds to achieve the correct result is to use "& &", as in

#[(sessionVars['quantity'] == null) && (sessionVars['productNumber'] == '1' || sessionVars['productNumber'] == '2')]