RFC: parse a' as call expression

[simeonschaub]

It seems weird to me that the adjoint operator still gets its own expression, because except for its special syntax it is basically a regular function call, so I thought, I'd just give it a go. This should also simplify some meta programming, since ' doesn't have to be special-cased anymore. What do people think, is this a good idea?

[JeffBezanson]

Tough call. It does seem like this would have been better, but I'm not sure it's worth changing.

[StefanKarpinski]

This is technically breaking since macros may be relying on the current AST representation.

[simeonschaub]

I totally understand the concerns, and that this might be too breaking. On the other hand I wonder, how much code really relies on this, and if this wouldn't actually fix more potential bugs than it causes problems. 😁
(The section in the manual is quite outdated and doesn't even mention ', so one could make the argument, that it's an undocumented feature. )

[StefanKarpinski]

True, that's fair. We do make technically breaking changes if they don't actually break packages.

[MasonProtter]

Just bumping this thread. Has anyone formed any strong opinions on this in the past few months? I think it's a good idea. It's more consistent with how we treat prefix / infix operators and it makes it possible to do things like locally shadow ''s definition and make a new one without type piracy.

[jessebett]

The type piracy here is coming from AD packages wanting to use f' to represent, e.g., the Zygote.adjoint(f) for reverse-mode derivative function of f.

[simeonschaub]

Would it make sense to run PkgEval on this, to get a sense of how breaking this actually is?

[maleadt]

@nanosoldier runtests(ALL, vs = ":master")

[JeffBezanson]

Good news, everybody. It's not necessary to change parsing to do this. It can be done in lowering instead:

   '|'|  (lambda (e) (expand-forms `(call (top adjoint) ,(cadr e))))

(julia-syntax.scm)

Currently, all syntax without an identifier name (e.g. indexing, vcat, etc.) lowers to call some Base function. We have discussed before changing them all to call lexically-scoped names like __getindex__, __adjoint__, etc., where Base exports default definitions const __getindex__ = getindex etc. Then you can lexically shadow those strange operators. Maybe it's time to just do this?

[MasonProtter]

Is lowering to a :call expression necessary to get #34507 ?

[JeffBezanson]

If I understand correctly, #34507 is a pure parsing issue --- entirely about surface syntax. If we did it, we'd probably parse suffixed adjoint operators as call expressions. Yes, that would be a bit inconsistent with ' by itself being an expression head, but backwards compatibility is like that sometimes.

[nanosoldier]

Your package evaluation job has completed - possible new issues were detected. A full report can be found here. cc @maleadt

[simeonschaub]

I think this could be worth reconsidering for a 2.0 release. Would it make sense to add it to the milestone?