TC 2021-09-23: RESOLVED as proposed.

Merged PR #37 into PR #35 in oasis-tcs/odata-abnf.

Adding aggregate(...) as a common expression extends the core spec, so adding $these/aggregate(...) instead does not make a difference.

Regarding the fourth bullet: when would we need to aggregate over a single instance instead of just using a normal common expression or $compute?

If we don't need it, I'd rephrase the fourth bullet point to:

• When evaluating a query option like $filter and $compute on a collection-valued resource, the context collection is the resource addressed by the resource path [OData-URL, section 4]. If the addressed resource is a single instance, the context collection is not defined.

That means replacing all the bullet points from the description with:

• The aggregate function must be prepended with a navigation path that identifies a collection. Inside it, common expressions are evaluated like in lambda operators. The lambda variable evaluates to the current instance within that collection.

  Products?$filter=Sales/aggregate(p:p/Amount) gt 100
  

• $these is a new kind of collectionPathExpr that evaluates to a "current collection" (defined below), in analogy with the current instance $this [OData-URL, section 5.1.1.14.6].
• $these can be prepended to collection path expressions like any, all and aggregate. (But we allow only $these/aggregate for now, so as not to touch the core spec.) See also ODATA-1456.
• In a system query option, when evaluating the boolCommonExpr in a filter or the commonExpr in a computeExpr, the current collection is the resource addressed by the resource path [OData-URL, section 4]. If the addressed resource is a single instance, the current collection is undefined.
• In an $apply transformation, when evaluating the boolCommonExpr in a filterTrafo or the commonExpr in a computeExpr, the current collection is the input set of the transformation.
• When evaluating a commonExpr in its nearest collectionPathExpr, the current collection is the collection addressed by the path that prepends the collectionPathExpr. This allows things like

  Products?$filter=Sales/any(p:p/Amount gt $these/aggregate(q:q/Amount))
  

But can we introduce $these in OData V4.01 without changing the core spec? Perhaps by restricting its use to within $apply?

Wording question: "current collection" or "context collection"?

Context instance is an excellent name for the existing symbol $this.

How about defining an explicit symbol $these for the context collection and requiring the aggregate(...) common expression to be prefixed with a collection path, which can be $these?

So we could use pathToCollection/aggregate(...) to aggregate a collection within the context instance, and $these/aggregate(...) to aggregate the context collection itself.

This would avoid to make aggregate working implicitly on the not yet defined context collection, in contrast to all other expressions.