Re^3: printing unitialized value of the 'do BLOCK'

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Re^4: printing unitialized value of the 'do BLOCK' by LanX (Saint) on Dec 17, 2019 at 20:01 UTC
> It's worth noting that if the two forms are equivalent, the latter is probably a lot less confusing when the output value is used. It's also worth noting that B::Deparse can't always be trusted. The clause of the if has the same scope like the inner block ( with `if (CLAUSE) {BLOCK}` ) `DB<3> use strict; my $x=3; if (my $x =1) { print $x } 1 DB<4> use strict; if (my $x =1) { print $x }; print $x Global symbol "$x" requires explicit package name (did you forget to d +eclare "my $x"?) at (eval 13)[C:/Perl_524/lib/perl5db.pl:737] line 2. DB<5>` [download] but Deparse fails to cover this with -x7 `>>perl -MO=Deparse,-x7 -e"if (my $x =1) { print $x }; print $x" my $x = 1 and do { print $x }; print $x; # <-- no error under strict! -e syntax OK` [download] Cheers Rolf _{(addicted to the Perl Programming Language :) Wikisyntax for the Monastery FootballPerl is like chess, only without the dice}	[reply] [d/l] [select]
Re^4: printing unitialized value of the 'do BLOCK' by haukex (Archbishop) on Dec 17, 2019 at 16:09 UTC
Very nice, thank you!	[reply]
Re^4: printing unitialized value of the 'do BLOCK' by rsFalse (Chaplain) on Dec 23, 2019 at 08:22 UTC
Hey! So it means that an 'if' statement in function/RHS context returns the last expression evaluated! And this isn't documented I guess. I guess newcomer would expect to get a 1 or ''/0 as a return value of 'if', but he can get also an 'undefined'. E.g. `print do { 3 if undef };` .= Interestingly, only one of these lines gives a warning: `print do { 3 if () }; print do { () };` [download]	[reply] [d/l] [select]
Re^5: printing unitialized value of the 'do BLOCK' by LanX (Saint) on Dec 23, 2019 at 12:54 UTC
> Interestingly, only one of these lines gives a warning: because the second one returns an empty list, which is a legal use of print. `C:\WINDOWS\system32>Perl -MData::Dump=pp -e "pp do{3 if ()} " undef C:\WINDOWS\system32>Perl -MData::Dump=pp -e "pp do{()} " ()` [download] Cheers Rolf _{(addicted to the Perl Programming Language :) Wikisyntax for the Monastery FootballPerl is like chess, only without the dice}	[reply] [d/l] [select]
Re^6: printing unitialized value of the 'do BLOCK' by rsFalse (Chaplain) on Dec 23, 2019 at 21:22 UTC
Thanks, now I understand the difference: in the first case '()' wanted in scalar context which was forced by 'if' (so it becomes an undefined value). Added: An 'if' forces scalar context as 'and' do, so it may be that these two are internally implemented the same.	[reply]


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