Aggregate Filters can only take boolean attributes

[thoni56]

This is mostly to re-iterate what was talked about in alan-if/alan-i18n#18 and explained in the comment alan-if/alan-i18n#18 (reply in thread):

A few quick tests confirmed that adding a general expression as a filter is impossible because of the ambiguities it creates in the grammar. It is impossible for the parser to understand if a '= ' would be part of the filter or is the comparison part of the whole expression. So without extensive semantic analysis merged into the parsing pass to guide the parsing this will not be possible (in any parser generator).

I remember now that I was quite frustrated with this since I felt the aggregates and filters was a very handy addition. To make that possible I need to change the syntax for the filters, and I'm not sure even that would help. Fortunately you can do the same with explicit loop unrolling and If statements.

At least it is impossible given the current technology, tools and grammar for aggregate filters.