Re: undefined compound chaining behavior

[Daniel J Sebald]

On 06/06/2014 01:12 PM, Rik wrote:
> On 06/06/2014 08:06 AM, address@hidden wrote:
> > Message: 5
> > Date: Fri, 6 Jun 2014 17:20:08 +0330
> > From: Hossein Sajjadi<address@hidden>
> > To: address@hidden
> > Subject: chaining compound assignment results in undefined behavior
> > Message-ID:
> >         <address@hidden>
> > Content-Type: text/plain; charset=UTF-8
> >
> > Hi
> >
> > There is a problem with chaining compound assignment.
> > For example the expression a=1;a+=a+=4 will result 10.
> > I think such a behavior is borrowed from C language but this behavior
> > is known as "undefined behavior".
> >   This issue have been resolven in Java.
> >   Java language guarantees that the operands of operators appear
> > to be evaluated in a specific evaluation order , namely, from left to
> > right. ?15.7
> > Other specifications about evaluation of expressions are in
> > "The Java? Language Specification"
> >
> >   also this approach is followed by c# language
> >   so result of above expression in Java is 6
>
> I don't think we need to do anything here.  First, code should
> self-documenting and easy to understand.  When I first looked at the
> statement I couldn't figure out what the intent was, and so I'm not
> surprised that the compiler also has a hard time deciding what to do.  So,
> I would suggest that rather than introduce complexities in to the parser,
> the programmer should simply write what they mean.
>
> If the programmer wants 6 as an answer, then code
> a = 1; a += a; a+= 4
>
> If the programmer wants 10 as an answer, then code
> a = 1; a +=4; a += a
>
> I tried the example in C++ and the result with the gcc compiler is 10 so
> I'm perfectly happy to define the behavior that multiple in-place operators
> are evaluated in a specific evaluation order which happens to be "right to
> left".

Also, I'm not so sure the right-to-left evaluation makes sense because 
this is both an operation and assignment in one.  Explicitly applying 
associativity using parentheses, think about:

octave-cli:7> a=1;a+=(a+=4)
a =  10

which makes sense, I guess.  The parentheses are evaluated with an 
inherent assignment and serves as an input to the proceeding add/assign. 
 But the following makes no sense:

octave-cli:8> a=1;(a+=a)+=4
parse error:

  invalid left hand side of assignment

> > > a=1;(a+=a)+=4

and the parser gives up, appropriately.  The first parentheses is an 
evaluation (i.e., quantity) and the add/assign can only apply to a 
variable, not a quantity.

Dan