= FDO RFC 22 - Enhance expression engine CONCAT function =

This page contains an change request (RFC) for the FDO Open Source project.  
More FDO RFCs can be found on the [wiki:FDORfcs RFCs] page.


== Status == 
 
||RFC Template Version||(1.0)|| 
||Submission Date|| June 25, 2008 ||
||Last Modified|| Dan Stoica [[Timestamp]]||
||Author||Dan Stoica, Orest Halustchak||
||RFC Status||adopted|| 
||Implementation Status||pending||
||Proposed Milestone||3.5.0.0||
||Assigned PSC guide(s)||Greg Boone||
||'''Voting History'''|| ||
||+1||Orest, Traian, Greg, Jackie ||
||+0|| ||
||-0|| ||
||-1|| ||
 
== Overview ==

This RFC is for adding an additional syntax to the CONCAT function of the expression engine.

== Motivation ==

The existing CONCAT function syntax is difficult to work with when multiple concatenations need to be done. It requires the CONCAT functions be nested whenever a new item needs to be concatenated.

== Proposed Solution ==

The proposed solution is to add support for an additional syntax to the CONCAT function.

Old syntax:
{{{
CONCAT ('Parcel ID: ', CONCAT ( PID , CONCAT ('\nOwner Name: ', CONCAT ( OWNER_NAME , CONCAT ('\nAddress: ',  SITE_ADDR )))))

}}}

New syntax:
{{{
CONCAT ('Parcel ID: ', PID , '\nOwner Name: ', OWNER_NAME, '\nAddress: ', SITE_ADDR )

}}}

The new syntax will consist of a comma delimited list of concatenation items.

In terms of API changes, the following addition to FdoFunctionDefinition class is proposed:

FdoFunctionDefinition.h
{{{
class FdoFunctionDefinition : public FdoIDisposable
{
...

public:
    /// \brief
    /// Indicates that this object allows its list of arguments to be variable so the 
    /// last argument may be repeated. 
    /// 
    /// \return
    /// Returns false
    /// 

FDO_API virtual bool FdoFunctionDefinition::SupportsVariableArgumentsList()
{
    return false;
}
}}}

FdoFunctionConcat will override SupportsVariableArgumentsList() to return true instead. It will relax the check for allowed number of literal values passed in for evaluation. 

FdoFunctionConcat.h
{{{
/// Indicates that this object allows its list of arguments to be variable so the 
/// last argument may be repeated. 
/// 
virtual bool SupportsVariableArgumentsList()
{
    return true;
}
}}}

== Implications ==

The existing CONCAT function syntax will continue to be supported in addition to the new syntax.

The client UI has the means to display "..." by checking the new method. 

== Test Plan ==

Test that the existing and new CONCAT function syntax work. Update the Expression Engine unit test to exercise CONCAT with extra arguments.

== Funding/Resources ==

Autodesk to provide resurces / funding.


== CORRECTION ==
FdoConcatFunction class doesn't extend FdoFunctionDefinition class. Therefore the new method FdoFunctionDefinition::SupportsVariableArgumentsList() is not virtual and the FdoFunctionDefinition constructors need to take an extra parameter instead.

Therefore the correct changes are:

FdoFunctionDefinition.h
{{{
class FdoFunctionDefinition : public FdoIDisposable
{
...

public:
    /// \brief
    /// Indicates that this object allows its list of arguments to be variable so the 
    /// last argument may be repeated. 
    /// 
    /// \return
    /// Returns true if the function allows a variable list of arguments and false otherwise.
    /// 

FDO_API bool FdoFunctionDefinition::SupportsVariableArgumentsList();

/// The existing 3 Create methods will take a new 'supportsVariableArgumentsList = false' input parameter as the last input parameter.

    /// Input a flag indicating whether or not this function supports a variable list of arguments
    /// \param supportsVariableArgumentsList

FDO_API static FdoFunctionDefinition *Create (..., bool supportsVariableArgumentsList = false);

}}}