Re: [PATCH] Add 'switch' statement to match.pd language

[Michael Matz <matz at suse dot de>]

Hi,

On Thu, 16 Jul 2015, Richard Biener wrote:

> > 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).
> 
> Yes.  Though techincally the condition cannot be an atom because
> it has to be a c-expr and that has no notion of atom vs. no-atom.

"1" is a valid c-expr, and quite atomy :)  (Or "true")

> But the issue is to unambiguously parse the else clause, thus

Ah, yes, I remember, the c-expr vs expr case; the parser is too limited :)  
In that case I find the extra keyword without parens even better:

(switch
 when (bla) (foo)
 when (bar) (boo)
 (blob))

I.e. following 'when' it's an c-expr (when single token, parens optional), 
when not following 'when' its a result expr (atomic or not).  Think of it 
as an infix multi-part keyword (like smalltalk has multi-part method 
names), the keyword(s) being "switch(when)*".

I'm undecided if I'd allow function calls as atoms as well (because they 
contain parens), like so:

(switch
 when integer_zero(@0) @1
 when integer_zero(@1) @0
 (plus @0 @1))

This would mean that there would be no single-token conditions without 
parens where one could leave out outer parens, as otherwise you have a 
ambiguity between:

(switch
 when true (@0)    // (@0) is the result
 ...)

and

(switch
 when token(@0) @1    // (@0) belongs to the when-expr
 ...)

One has to chose between one or the other, and I think the latter (i.e. 
function calls as lone when-expr) occur more often.

(Limiting the number of parens is worthwhile IMHO, but you probably 
guessed that much already :))


Ciao,
Michael.