Re: [pugs]weird thing with say ++$
On 21 Apr, fayland wrote: : It has been published at perl6.language, but have no reply. That was to be expected, as it's no language-design specific issue, and therefore, unsuitable for p6l. : In perl v5.8.6 built for MSWin32-x86-multi-thread: : : my $i = 1; : print $i++, ++$i; # 1 3 : my $i = 1; : print ++$i, $i++; # 3 2 You're misleaded here, by thinking, scalar context is being enforced, which is not the case; the results of the increment operator operations (ops) on the scalar are being passed in (implicit) array context to the print builtin. Every Perl program, that gets compiled and interpreted, will be converted to a syntax tree initially. The following is a selected excerpt from the output of the core module B::Terse, which displays, what syntax tree we actually got hold of. LISTOP (0x3c013760) print OP (0x3c013780) pushmark UNOP (0x3c013700) postinc [2] OP (0x3c0136e0) padsv [1] UNOP (0x3c013740) preinc OP (0x3c013720) padsv [1] So, LISTOP indicates, that a list context is being encountered, which is not, what we actually want. The syntax trees for the two examples, provided above, don't differ that much, except, as you'd assume, in case of the ordering of the UNOPs. I don't see any way to explain, what's going wrong *exactly* by examining the syntax tree; that'd require digging into the internals, I'd assume. my $i = 1; and either print $i++; print ++$i; or print ++$i; print $i++; does what you assumed, it was supposed to do, but without having the compiler struggling against implicit assumptions. : in pugs: : : my $i = 1; : say $i++, ++$i; # 1 3 : : my $i = 1; : say ++$i, $i++; # 2 2 This leads me to think, Pugs has implemented the behaviour, that has been outlayed in Apocalypse 03: Operators, RFC 082 - Arrays: Apply operators element-wise in a list context, http://dev.perl.org/perl6/doc/design/apo/A03.html : which is right?(I think perl5 is) or it's different between Perl5 and Perl6? I'm not inclined to think, that Perl 5 is wrong here, as letting it assume array context, is in this context, like assuming, filling your fridge with food, it consists of, by randomly throwing some of it in, will come out sorted again; Perl 6 does, what the specifications urge upon it to do. : /Fayland Lam/ Steven
Re: [pugs]weird thing with say ++$
Steven Philip Schubiger skribis 2005-04-25 5:41 (+0200): That was to be expected, as it's no language-design specific issue, and therefore, unsuitable for p6l. : print ++$i, $i++; # 3 2 You're misleaded here, by thinking, scalar context is being enforced, which is not the case; the results of the increment operator operations (ops) on the scalar are being passed in (implicit) array context to the print builtin. I don't know how it's called on other levels, but we're still calling that list|slurpy|plural context. array context looks too much like Array context, which is something else. In array context, @foo is passed as the array that it is. In list context, it is flattened. Big and very important difference. Juerd -- http://convolution.nl/maak_juerd_blij.html http://convolution.nl/make_juerd_happy.html http://convolution.nl/gajigu_juerd_n.html
Re: [pugs]weird thing with say ++$
On 25 Apr, Juerd wrote: : I don't know how it's called on other levels, but we're still calling : that list|slurpy|plural context. array context looks too much like : Array context, which is something else. I confess, it's likely a bad habit, to coin it array context on p6l, although it refers to Perl 5, where it I have seen it more than once - perhaps, overly abused too. : In array context, @foo is passed as the array that it is. In : list context, it is flattened. Big and very important difference. You're supposedly right. : Juerd Steven
Re: [pugs]weird thing with say ++$
Steven Philip Schubiger skribis 2005-04-25 18:44 (+0200): I confess, it's likely a bad habit, to coin it array context on p6l, although it refers to Perl 5, where it I have seen it more than once - perhaps, overly abused too. That's old Perl5-ese. There was a big jargon change when people started to realise and agree that having two things both called arrays or lists was confusing. There are still some occurrences of array context left, but in reality, even in Perl 5, that should only refer to the context in which an array is expected, as provided by a \@ prototype. Juerd -- http://convolution.nl/maak_juerd_blij.html http://convolution.nl/make_juerd_happy.html http://convolution.nl/gajigu_juerd_n.html
Re: [pugs]weird thing with say ++$
On Thu, Apr 21, 2005 at 04:32:41PM +0800, fayland wrote: It has been published at perl6.language, but have no reply. In perl v5.8.6 built for MSWin32-x86-multi-thread: my $i = 1; print $i++, ++$i; # 1 3 my $i = 1; print ++$i, $i++; # 3 2 in pugs: my $i = 1; say $i++, ++$i; # 1 3 my $i = 1; say ++$i, $i++; # 2 2 which is right?(I think perl5 is) or it's different between Perl5 and Perl6? I think I understand the implementation details leading to each behaviour, but rather than saying which was right, I think I'd be quite happy to see Perl6 copy (the ideas behind) C's rules regarding sequence points and undefined behaviour. I'm not so sure about implementation defined and unspecified behaviour. When I see code such as this, I usually encourage people to program Perl as if it had sequence points and undefined behaviour. This often results in explaining what they are, but maybe that's not such a great problem. See http://www.eskimo.com/~scs/C-faq/faq.html, especially sections 3.8 and 11.33 for details. -- Paul Johnson - [EMAIL PROTECTED] http://www.pjcj.net
Re: [pugs]weird thing with say ++$
Paul Johnson [EMAIL PROTECTED] writes: I think I understand the implementation details leading to each behaviour, but rather than saying which was right, I think I'd be quite happy to see Perl6 copy (the ideas behind) C's rules regarding sequence points and undefined behaviour. I'm not so sure about implementation defined and unspecified behaviour. Isn't this the old prefix-++ problem: @a = (++$i,++$i,++$i); print @a;# prints 3 3 3 -- Johan
Re: [pugs]weird thing with say ++$
On Thu, Apr 21, 2005 at 11:45:27AM +0200, Paul Johnson wrote: On Thu, Apr 21, 2005 at 04:32:41PM +0800, fayland wrote: It has been published at perl6.language, but have no reply. In perl v5.8.6 built for MSWin32-x86-multi-thread: my $i = 1; print $i++, ++$i; # 1 3 my $i = 1; print ++$i, $i++; # 3 2 in pugs: my $i = 1; say $i++, ++$i; # 1 3 my $i = 1; say ++$i, $i++; # 2 2 which is right?(I think perl5 is) or it's different between Perl5 and Perl6? I think I understand the implementation details leading to each behaviour, but rather than saying which was right, I think I'd be quite happy to see Perl6 copy (the ideas behind) C's rules regarding sequence points and undefined behaviour. I'm not so sure about implementation defined and unspecified behaviour. It certainly makes more sense to me that the answer would be 2 2. But however it ends up, so long as we know what the answer will be, we can utilize it effectively in our programs. -kolibrie
Re: [pugs]weird thing with say ++$
On Thu, Apr 21, 2005 at 11:45:27AM +0200, Paul Johnson wrote: It certainly makes more sense to me that the answer would be 2 2. But however it ends up, so long as we know what the answer will be, we can utilize it effectively in our programs. The trick with this construct usually in C is that the C standard doesn't specify the order of evaluation of function arguments, so you can never be sure if you'll get the same result if you compile it other than on your development system (different compilers may evaluate them in a different order). The Pugs example given in the original post seems to me to be fairly sane, as it shows left-to-right evaluation. The Perl 5 example, as far as I can tell, shows left-to-right for the first case and right-to-left for the second case, which is just odd... please correct me if I'm wrong, but that seems the only way those answers could have been arrived at. So really, what needs to be said is how Perl 6 is supposed to evaluate the arguments to function calls, then we can know if the current behaviour in Pugs is correct.
