Hi,My only concern is to produce unexpected and/or counterintuitive effects. "view_as<some geometry>" should always be an "exact" operation. That is, it should always return a geometry which is the exact equivalent of the input geometry (i.e. calling any algorithm on it should return the same result). For instance a 2D box can be exactly expressed in terms of a 2D polygon, a point can be exactly expressed in terms of a box (with null dimensions) so these are good uses. However, saying view_as<point>(sphere) and expecting it to return the center of the sphere is an abuse, the returned geometry is not geometrically equivalent to what we had in the first place. So that should be achieved by a more explicit call, for instance view_as_center<point>(sphere). I want to avoid obscure behaviors, basically.
I basically agree with the propositions discussed here. Regarding the pipe-style syntax - it's just an extra layer on top of the classic syntax we've already started (view_as / make_view_as etc...) so let's first finalize it and document it, then we can discuss more in detail what a higher level syntax would look like.