context-free grammar

[Jonathan M Davis]

On Friday 04 March 2011 19:10:35 uri wrote:
> Jonathan M Davis Wrote:
> > On Friday 04 March 2011 17:05:57 Simon Buerger wrote:
> > > It is often said that D's grammar is easier to parse than C++, i.e. it
> > > should be possible to seperate syntactic and semantic analysis, which
> > > is not possible in C++ with the template-"< >" and so on. But I found
> > > following example:
> > > 
> > > The Line "a * b = c;" can be interpreted in two ways:
> > > -> Declaration of variable b of type a*
> > > -> (a*b) is itself a lvalue which is assigned to.
> > > 
> > > Current D (gdc 2.051) interprets it always in the first way and yields
> > > an error if the second is meant. The Workaround is simply to use
> > > parens like "(a*b)=c", so it's not a real issue. But at the same time,
> > > C++ (gcc 4.5) has no problem to distinguish it even without parens.
> > > 
> > > So, is the advertising as "context-free grammar" wrong?
> > 
> > Umm. How could a * b be assigned to? It's definitely not an lvalue. Do
> > you mean that an overloaded opBinary!"*" is used which returns a ref? It
> > certainly can't be done normally.
> 
> Explain why (a*b) is lvalue in bearophile's second example. This is one of
> the weird things in D. The language is too complex. It takes years to find
> out about the corner cases. I wouldn't use it for anything reliable

I'd argue that it's a bug related to operator overloading. = is converted to 
opAssign, which is then essentially a normal function. And because the result of 
* is a Foo which has an overloaded opAssign, the call to opAssign succeeds like 
any other function call would, because = was lowered to opAssign. I don't see 
what else could possibly be happening here. And since this violates the fact 
that opAssign is supposed to operate on lvalues, and the result of a * b is an 
rvalue, not an lvalue, this is a bug.

- Jonathan M Davis