[jira] Commented: (VELOCITY-752) Evaluation of macro arguments

["Jarkko Viinamäki (JIRA)" <de...@velocity.apache.org>]

    [ https://issues.apache.org/jira/browse/VELOCITY-752?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12801256#action_12801256 ] 

Jarkko Viinamäki commented on VELOCITY-752:
-------------------------------------------

I'm not 100% sure I understand the problem correctly but I also noticed that the test case provided here didn't yield expected results. I don't have time to confirm this but commenting one line in ProxyVMContext might fix the issue:

org.apache.velocity.context.ProxyVMContext

    private boolean isConstant(Node node)
    {
        switch (node.getType())
        {
            case ParserTreeConstants.JJTINTEGERRANGE:
            case ParserTreeConstants.JJTREFERENCE:
            case ParserTreeConstants.JJTOBJECTARRAY:
            case ParserTreeConstants.JJTMAP:
        //    case ParserTreeConstants.JJTSTRINGLITERAL:  // commenting this line should fix it?
            case ParserTreeConstants.JJTTEXT:
                return (false);
            default:
                return (true);
        }
    }

However, this causes at least test Velocity615TestCase.testForIrrationallyFearedRelatedPossibleProblem to fail since the behaviour is now different. Nathan, what do you think?

> Evaluation of macro arguments
> -----------------------------
>
>                 Key: VELOCITY-752
>                 URL: https://issues.apache.org/jira/browse/VELOCITY-752
>             Project: Velocity
>          Issue Type: Bug
>          Components: Engine
>    Affects Versions: 1.6.2
>         Environment: Windows XP SP2, Java 1.5
>            Reporter: Steve O'Hara
>            Priority: Minor
>
> I've just come across a little gotcha that has taken a few hours to track down which might help other people.
> An example;
> #macro(myMacro $FieldName)
>     #foreach ($Name in $FieldList)
>         #if ($FieldName=="empty")
>             #set ($tmp=$FieldsFound.put($FieldName,$velocityCount))
>         #end
>     #end
> #end
> #foreach ($X in $MyFields)
>     #myMacro("Field$velocityCount")
> #end
> All looks OK doesn't it, but........
> The problem is one of when arguments are evaluated or in fact to be more precise, are not evaluated at all.
> Once you bear that in mind you can see the logical problem above - the use of $velocityCount (or indeed any variable) in the argument list of myMacro i.e. 
> #myMacro("Field$velocityCount")
> This sort of usage is dangerous because in effect, what is happening is that the macro argument literal is being placed within the macro body without being evaluated first.
> So, the body of the macro becomes this;
>     #foreach ($Name in $FieldList)
>         #if ("Field$velocityCount"=="empty")
>             #set ($tmp=$FieldsFound.put("Field$velocityCount",$velocityCount)
>         #end
>     #end
> Not what you intended at all and depending on the names of your variables, could cause all kinds of mayhem with values coming into scope that you hadn't intended.
> The workaround is to set a variable before the call to the macro and send that instead e.g.
> #foreach ($X in $MyFields)
>     #set ($tmp="Field$velocityCount")
>     #myMacro($tmp)
> #end

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