Re: [ast-users] Arithmetic assignment side-effects
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 ___ ast-users mailing list ast-users@lists.research.att.com http://lists.research.att.com/mailman/listinfo/ast-users ___ ast-users mailing list ast-users@lists.research.att.com http://lists.research.att.com/mailman/listinfo/ast-users
Re: [ast-users] Arithmetic assignment side-effects
Oops. Should have been... (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 LVALUE+=RVALUE (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 ___ ast-users mailing list ast-users@lists.research.att.com http://lists.research.att.com/mailman/listinfo/ast-users
Re: [ast-users] Arithmetic assignment side-effects
On 08/04/2013 12:41 AM, Dan Douglas wrote: On Sunday, August 04, 2013 12:30:48 AM Roland Mainz wrote: On Sun, Aug 4, 2013 at 12:04 AM, Dan Douglas orm...@gmail.com wrote: Is it specified what the value of x should be after this expression? x=0; : $((x+=x=1)) Bash, ksh93, mksh, posh say 1. zsh, dash, busybox say 2. Clang and gcc both throw warnings about it, but both plus icc agree on 2. Just curious: Is that x86-specific or is the result always the same on other architectures, too ? Maybe there is something in ISO C1X/C99 which actually defines or recommends a specific compiler behaviour. It is expressly forbidden by C99 (6.5.2: Between the previous and next sequence point an object shall have its stored value modified at most once by the evaluation of an expression. Furthermore, the prior value shall be read only to determine the value to be stored. As such, the expression in question here (x+=x=1) is undefined Anders ___ ast-users mailing list ast-users@lists.research.att.com http://lists.research.att.com/mailman/listinfo/ast-users
Re: [ast-users] Arithmetic assignment side-effects
On Mon, Aug 5, 2013 at 9:01 AM, Anders Johansson ajohans...@suse.com wrote: On 08/04/2013 12:41 AM, Dan Douglas wrote: On Sunday, August 04, 2013 12:30:48 AM Roland Mainz wrote: On Sun, Aug 4, 2013 at 12:04 AM, Dan Douglas orm...@gmail.com wrote: Is it specified what the value of x should be after this expression? x=0; : $((x+=x=1)) Bash, ksh93, mksh, posh say 1. zsh, dash, busybox say 2. Clang and gcc both throw warnings about it, but both plus icc agree on 2. Just curious: Is that x86-specific or is the result always the same on other architectures, too ? Maybe there is something in ISO C1X/C99 which actually defines or recommends a specific compiler behaviour. It is expressly forbidden by C99 (6.5.2: Between the previous and next sequence point an object shall have its stored value modified at most once by the evaluation of an expression. Furthermore, the prior value shall be read only to determine the value to be stored. As such, the expression in question here (x+=x=1) is undefined Thanks for the answer... :-) Bye, Roland -- __ . . __ (o.\ \/ /.o) roland.ma...@nrubsig.org \__\/\/__/ MPEG specialist, CJAVASunUnix programmer /O /==\ O\ TEL +49 641 3992797 (;O/ \/ \O;) ___ ast-users mailing list ast-users@lists.research.att.com http://lists.research.att.com/mailman/listinfo/ast-users
