SE-0415: Function Body Macros

[kabiroberai]

I'm a big fan of this concept, but I want to reiterate my feedback from the pitch:

Function body macros

This sounds great, especially in the form suggested in the alternatives section: that is, if preamble macros were replaced with “wrapper” macros (maybe we could borrow from Python and call them “decorator” macros?) Besides what others have said, it sounds like implementing the decorator style could remove the need for preamble macros entirely — it’s very possible I’m missing something but aren’t the latter effectively a subset of the former? Seems to me that if users wanted to add a preamble th…

I would be strongly in favour of this proposal if preamble macros were replaced with wrapper macros, given that the latter are a strict superset of the former (unless I'm missing something?) To reiterate, AFAICT if a macro author wishes to write a preamble-style macro they can create a wrapper macro that returns something like

{ preamble; return h-impl }()

I think there's a lot of patterns that fit the wrapper form but not the preamble form. For example, there's currently an open PR on the Swift repo to implement support for the preamble form in task locals:

[PoC] Offer unsafe push/pop APIs for task-locals, for easier macro use

Seems to me that Wrapper macros would preclude the need for new APIs like this, offer a safer interface, and work in far more situations. Reiterating my feedback from the pitch again, this also makes it a far simpler decision when choosing which style of function body macro you want:

• wrapper: can compose, can't rewrite the body. improved local reasoning.
• body: can't compose, but can fully rewrite the body. reduced local reasoning.