no expected 'Useless use of a constant in void context' warning for expression in return statement

ed_hoch has asked for the wisdom of the Perl Monks concerning the following question:

Replies are listed 'Best First'.
Re: no expected 'Useless use of a constant in void context' warning for expression in return statement by LanX (Saint) on Dec 14, 2012 at 21:37 UTC
Calling a sub in scalar context means the last (returning) expression will be evaluated in scalar context. (see return and also Context propagation into subs and evals ) scalar of a list is defined as it's last element, here 20. To catch calls in scalar context you may wanna use wantarray. Cheers Rolf	[reply]
Re^2: no expected 'Useless use of a constant in void context' warning for expression in return statement by ed_hoch (Sexton) on Dec 15, 2012 at 04:25 UTC
right - isn't the 10 in the `return 10, 20;` statement being evaluated in a void context then? And so shouldn't it trigger a warning?	[reply] [d/l]
Re^3: no expected 'Useless use of a constant in void context' warning for expression in return statement by Anonymous Monk on Dec 15, 2012 at 05:03 UTC
No, the expression is `10, 20` and it as a whole is evaluated in scalar context Obvious mistakes (from Useless use of %s in void context ) $foo = 1, 2; Useless use of a constant (2) in void context at -e line 1 (#1) assignment(=) binds tighter than comma(,) so $foo is 1 `$ perl -MO=Deparse,-p -e " $foo = 1, 2; " (($foo = 1), '???'); -e syntax OK` [download] $one, $two = 1, 2; Useless use of a variable in void context at -e line 1 (#1) `$ perl -MO=Deparse,-p -e " $one, $two = 1, 2; " ($one, ($two = 1), '???'); -e syntax OK` [download] Another obvious one: $one, $two = foo(); Useless use of private variable in void context at -e line 1 (#1) `$ perl -MO=Deparse,-p -e " sub foo { 1, 2 } $one, $two = foo(); " sub foo { (1, 2); } ($one, ($two = foo())); -e syntax OK` [download] Since the behavior of comma operator in list/scalar context is well defined, since the behaviour of a list in scalar/list context is well defined (see If you believe in Lists in Scalar Context, Clap your Hands ), perl doesn't warn about : $two = foo(); because it is predictable/guaranteed/you're expected to know what it means `sub foo{1,2} ($f)= foo();` is like `($f) = (1,2);` # no warn, $f is 1 `sub foo{1,2} $f = foo();` is like `$f = (1,2);` # no warn, $f is 2 perl won't warn you, scalar context guarantees rightest most(last), list context guarantees leftest most (first)	[reply] [d/l] [select]
Re^3: no expected 'Useless use of a constant in void context' warning for expression in return statement by LanX (Saint) on Dec 15, 2012 at 05:35 UTC
I meant the "returned expression" not the "return statement"! it's effectively `return scalar(10,20);` [download] so no warning `DB<100> print scalar (10,20) 20` [download] UPDATE: maybe the following is clearer, `DB<113> sub tst { 2..10 } => 0 DB<114> ($a)=tst() => 2 DB<115> $a=tst() => "" DB<116> sub tst { /2/../10/ } => 0 DB<117> $_=2 => 2 DB<118> $a=tst() => 1 DB<119> $a=tst() => 2 DB<120> $_=10 => 10 DB<121> $a=tst() => "3E0"` [download] as you can see the range-op transforms to flip-flop-op in scalar context. So we are not returning a list but an expression which is evaluated according to the callers context. Cheers Rolf	[reply] [d/l] [select]
Re^4: no expected 'Useless use of a constant in void context' warning for expression in return statement by ed_hoch (Sexton) on Dec 15, 2012 at 23:27 UTC
Re: no expected 'Useless use of a constant in void context' warning for expression in return statement by tobyink (Canon) on Dec 15, 2012 at 07:51 UTC
This warning is a compile-time warning; not a run-time warning. At compile time, perl cannot tell what contexts the function `A()` will ever be called in. If `A()` is ever called in list context (perhaps even from within another module that hasn't been loaded yet) then `10` won't be useless. `perl -E'sub Monkey::do{say$_,for@_,do{($monkey=[caller(0)]->[3])=~s{::}{ }and$monkey}}"Monkey say"->Monkey::do'`	[reply] [d/l] [select]