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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=32760#action_32760 ] 

Hubert Heijkers commented on ODATA-239:

The implementer of the function knows the scope of the expression so the producer/server doesn't have an issue. Relaxing the definition of a Function Parameter to allow common expressions gives us all we need in that respect and the type of the parameter would even tell the consumer what type the expression would have to produce. The only thing the consumer, especially those generic once, wouldn't know is 'what' the expression is going to be evaluated against. Typically this would the type of whatever the collection it acts on is holding but it could even be something else for some very fancy functions. 

We'd propose add annotations to give those generic query tools just enough information about these. Even if we introduced a special type, say expression, for those parameters the consumer would need additional information to understand the context in which this expression would be executed. Therefore we feel we wouldn't need an additional type, just he appropriate annotation(s).

> 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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