[cxx-abi-dev] mangling for fold-expressions

[John McCall]

On Nov 10, 2014, at 10:56 AM, Richard Smith <richardsmith at google.com> wrote:
> On 10 November 2014 10:30, John McCall <rjmccall at apple.com> wrote:
> On Nov 8, 2014, at 9:16 AM, Richard Smith <richardsmith at google.com> wrote:
> > Hi,
> >
> > WG21 is voting on a proposal for "fold-expressions" today. These are syntactically of the form:
> >
> >   ( .... + pack )
> >   ( pack + ... )
> >   ( p0 + ... + pack )
> >   ( pack + ... + pn )
> >
> > (where + can be any binary operator). These expand to
> >
> >   (((p0 + p1) + ...) + pn)
> >
> > for the first and third cases and
> >
> >   (p0 + (p1 + (... + pn)))
> >
> > for the other two cases.
> 
> I should add:
>  - in the first and second cases, the pack is p0 ... pn
>  - in the third case, the pack is p1 ... pn
>  - in the fourth case, the pack is p0 ... p{n-1}

Your notation is terrible, Richard. :)

Okay, so the idea is that:
  E op … op P => (((E op P1) op P2) op …) op Pn
  P op … op E => P1 op (P2 op (… op (Pn op E)))

How are E and P determined?  Everything preceding/following the … term, or does this actually follow the grammar’s associativity rules if you had e.g.
  1+2+…+packref+3+4
or does it only work within parentheses?

> > These need a mangling; I suggest (and have implemented):
> >
> > <expression> ::=
> >   fl <binary operator-name> <expression>   # ( ... op pack )
> >   fr <binary operator-name> <expression>   # ( pack op ... )
> >   fx <binary operator-name> <expression> <expression>   # ( expr op ... op expr )
> 
> This doesn’t seem to correspond to one of your examples.
> 
> You get this for the third and fourth cases.

Okay, and you’re saying that left/right folding are disambiguated because one of the expressions contains a pack and the other doesn’t?  I think I would prefer this to be explicit in the mangling.

John.
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