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Date: Wed, 15 Jul 2015 15:21:00 -0000
From: Michael Matz <matz@suse.de>
To: Richard Biener <richard.guenther@gmail.com>
cc: Richard Biener <rguenther@suse.de>, gcc-patches@gcc.gnu.org
Subject: Re: [PATCH] Add 'switch' statement to match.pd language
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Hi,

On Wed, 15 Jul 2015, Richard Biener wrote:

> >>  (switch
> >>   (A) B
> >>   (B) C
> >>   (C) D
> >>   E)
> >
> >The lispy way would have been
> >
> >   (switch
> >    (A) (B)
> >    (C) (D)
> >    (E) (F)
> >    G)
> >
> >i.e. parenthesize the result as well, which then would be unambiguously
> 
> That's just atoms vs. Expressions.

But if the result is no atom, you'd want parentheses.  Similar if the 
condition is no expression but an atom, you'd want to leave out 
parentheses as well.  My point is, that both condition and result are at 
the same level, and hence should be subject to the same parenthesis rules, 
namely: surrounding parens by default, optional for atoms.

> Like (Plus @0 @1) vs. Plain @1.  So you suggest to require ((plus @0 
> @1)) here to make it unambiguous?

No :)  Just look at your example again:

 (switch
  (A) B
 )

Both A and B are at the same level, and are both expressions, but still 
you parenthesize them differently; that can't be right.  You also don't 
write

 (switch
  ((plus @0 @1))  (@0)
 )

You write

 (switch
  (plus @0 @1) (@0)
 )

And as syntactic sugar you are allowed to leave out the parens around @0 
as it's an atom:

 (switch
  (plus @0 @1) @0
 )

Similar, if the condition is an atom you should be able to leave the 
parens away:

 (switch
  cond (minus @0 @1)
 )

(given a predicate 'cond' defined appropriately).


Ciao,
Michael.