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RE: [ast-users] Arithmetic assignment side-effects


From: Janis Papanagnou
Subject: RE: [ast-users] Arithmetic assignment side-effects
Date: Tue, 6 Aug 2013 12:49:06 +0200

> > 
> > Go strictly from left to right.
> 
> How do you assign something that hasn't been evaluated yet? Evaluating the 
> expression on the RHS is an absolute prerequisite to evaluating the 
> assignment 
> itself. (x += x) = 1 is nonsense. It evaluates to 0 = 1.

It makes sense, depending on the language. With LVALUE=RVALUE and
LVALUE+=LVALUE, where the result is an LVALUE, consider it evaluated as

    ("ref"(x) = deref(x)+deref(x)) = 1

(Try your expression above in C++, for example. It works as expected.)

Nonetheless right to left parsing seems the only sensible semantics in
case of = and op= assignments.

Janis

> 
[...]
> 
> I've also already proven that at least in ksh it goes right to left, because 
> if you define a setter property to trigger a side-effect for each variable, 
> the RHS fires before the LHS of the +=.
> 
> There's just no amount of mind-bending I can think of that could make 
> evaluating the += first produce anything other than an error.
> -- 
> Dan Douglas
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