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Subject: [OASIS Issue Tracker] Commented: (ODATA-239) Allowing expressions to be passed as parameters to (super) functions

    [ http://tools.oasis-open.org/issues/browse/ODATA-239?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=32647#action_32647 ] 

Michael Pizzo commented on ODATA-239:

I don't think that leaving the scope of where the expression is evaluated up to the server works.

As Hubert says, generic query building tools, as well as generic OData URL parser libraries, can't rely on back-end specific secrets in order to understand how to build/parse the request. That's why, even for the built-in any() and all() functions, we use lambdas to have a well defined way to represent expressions that should be passed to the function as opposed to evaluated in order to pass the value in to the function.

If we wanted to implement such "super functions" we would need to do something like lambdas, including the ability to define a function signature that took a lambda of a particular type.

I'm concerned about the complexity of adding this type of functionality to this release, in a general way, and getting it right.

> Allowing expressions to be passed as parameters to (super) functions
> --------------------------------------------------------------------
>                 Key: ODATA-239
>                 URL: http://tools.oasis-open.org/issues/browse/ODATA-239
>             Project: OASIS Open Data Protocol (OData) TC
>          Issue Type: New Feature
>          Components: OData CSDL
>    Affects Versions: V4.0_WD01
>         Environment: [Proposed]
>            Reporter: Hubert Heijkers
>             Fix For: V4.0_WD01
> To be able to use/extend the set of functions that can be used in a $apply query option functions need parameters of type expression, which expression can then be applied to each and every instance in the set they are processing. 
> An very simple example of such a function would:
> $apply=GroupBy()/TopN(N=5, Value=Revenue)
> Where 5 is just a number indicating the top N we are interested in but were 'Revenue' is actually an expression referring to the Revenue property of the entity in the set the function is acting upon. A better example in which this becomes more apparent would be:
> $apply=GroupBy()/TopN(N=5, Value=Revenue sub Cost)
> For build in functions we can obviously simply define this but currently there is no way of defining a 'custom' function that could implement the same behavior because we can't pass in expressions.
> We then could annotate these functions to signal to tools that they can be used in $apply.

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