Re: Falling through Perl 5.10 switch statement

The Imp of the Perverse still wonders if it is possible to directly "goto" (maybe that's a hint right there...) the next block, skipping the when check.

Well, goto itself won't work in my perl 5.9.4:

use feature qw(say switch);

my $data = 1;
given($data) {
    when (1) { say 'one'; goto two;  }
    when (2) { two: say 'two' ;      }
    default  { say 'something else'; }
}
[download]

This prints:

one
panic: goto at /tmp/test line 5.
[download]

Interesting error message though.

Comment on Re: Falling through Perl 5.10 switch statement Select or Download Code

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Re^2: Falling through Perl 5.10 switch statement by Trizor (Pilgrim) on Apr 07, 2007 at 20:33 UTC
Thats because your trying to goto a block that looks like it would be optimized out of existance. Since $data is a constant the optimizer components that look at switches are probably smart enough to notice that only the `when (1)` case will run and will replace the whole switch in the op tree with simply `say 'one'; goto two;`. Disclaimer: I don't claim to have intimate knowledge of how switches are implemented in the optree or how the optimizer treats them. Take this explanation with a grain of salt, it may sound good, but it may also be flat out wrong.	[reply] [d/l] [select]
Re^3: Falling through Perl 5.10 switch statement by ikegami (Patriarch) on Apr 07, 2007 at 21:37 UTC
I'd maybe buy your argument if the code was `given (1)` [download] but I've never known perl to optimize lexicals away, even if those who always hold the same constant value. The following two optrees should be identical if `perl` optimized constant lexicals away: >perl -MO=Concise -e"my $data = 1; print $data;" a <@> leave[1 ref] vKP/REFC ->(end) 1 <0> enter ->2 2 <;> nextstate(main 1 -e:1) v ->3 5 <2> sassign vKS/2 ->6 3 <$> const[IV 1] s ->4 4 <0> padsv[$data:1,2] sRM/LVINTRO ->5 6 <;> nextstate(main 2 -e:1) v ->7 9 <@> print vK ->a 7 <0> pushmark s ->8 8 <0> padsv[$data:1,2] l ->9 <-- It uses the lexical, -e syntax OK not a constant. >perl -MO=Concise -e"print 1;" 6 <@> leave[1 ref] vKP/REFC ->(end) 1 <0> enter ->2 2 <;> nextstate(main 1 -e:1) v ->3 5 <@> print vK ->6 3 <0> pushmark s ->4 4 <$> const[IV 1] s ->5 <-- This is a constant. -e syntax OK [download] Perl 5.8.8 Update: Here's another example using code more similar to the OP's: >perl -MO=Concise -e"my $data = 1; if ($data) { print 'moo' }" c <@> leave[1 ref] vKP/REFC ->(end) 1 <0> enter ->2 2 <;> nextstate(main 1 -e:1) v ->3 5 <2> sassign vKS/2 ->6 3 <$> const[IV 1] s ->4 4 <0> padsv[$data:1,4] sRM/LVINTRO ->5 6 <;> nextstate(main 4 -e:1) v ->7 - <1> null vK/1 ->c 8 <\|> and(other->9) vK/1 ->c <-- "if" not optimized away. 7 <0> padsv[$data:1,4] s ->8 <-- Lex not optimized away. - <@> scope vK ->- - <0> ex-nextstate v ->9 b <@> print vK ->c 9 <0> pushmark s ->a a <$> const[PV "moo"] s ->b -e syntax OK >perl -MO=Concise -e"if (1) { print 'moo' }" 6 <@> leave[1 ref] vKP/REFC ->(end) 1 <0> enter ->2 2 <;> nextstate(main 3 -e:1) v ->3 - <@> scope vK ->6 <-- "if" optimized away. - <0> ex-nextstate v ->3 5 <@> print vK ->6 3 <0> pushmark s ->4 4 <$> const[PV "moo"] s ->5 -e syntax OK [download]	[reply] [d/l] [select]
Re^3: Falling through Perl 5.10 switch statement by betterworld (Curate) on Apr 09, 2007 at 02:03 UTC
Thats because your trying to goto a block that looks like it would be optimized out of existance. That sounds like a nice explanation; however, the following script panics with the same message if there is a "1" in the command line: `use feature qw(say switch); for my $data (@ARGV) { given($data) { when (1) { say 'one'; goto two; } when (2) { two: say 'two' ; } default { say 'something else'; } } }` [download]	[reply] [d/l]
Re^2: Falling through Perl 5.10 switch statement by ysth (Canon) on Apr 08, 2007 at 06:09 UTC
Still a problem in bleadperl @ 30869.	[reply]
Re^3: Falling through Perl 5.10 switch statement by Anonymous Monk on Apr 16, 2007 at 08:01 UTC
Time to rollback :p	[reply]


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