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.

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"?

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.

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

TC 2021-09-23: RESOLVED as proposed.