Re: statement vs. block?

If the block has a list in it, you can't use it as a "statement".

use Data::Dumper;

my @short = (0..2);
my @x = map { ($_, $_) } @short;
print Dumper \@x;
my @y = map ($_, $_), @short;
print Dumper \@y;
__END__
$VAR1 = [ 
          0,
          0,
          1,
          1,
          2,
          2
        ];
$VAR1 = [ 
          undef
        ];
[download]

With warnings, this says "Useless use of private array in void context".

Comment on Re: statement vs. block? Download Code

Replies are listed 'Best First'.
Re^2: statement vs. block? by Fletch (Bishop) on Apr 11, 2008 at 23:46 UTC
That's because your map line is being parsed as `map($_, $_)` which is assigned to `@y`, followed by the array `@short` in void context (i.e. B::Deparse with the appropriate flags says `((my(@y) = map($_, $_)), @short);`). If you disambiguate with a + (`my @z = map +($_, $_), @short;`) or explicitly parenthesize where map's arguments end (`my @w = map( ($_, $_), @short );`) it'll be parsed correctly (`(my(@z) = map(($_, $_), @short));`). The cake is a lie. The cake is a lie. The cake is a lie.	[reply] [d/l] [select]
Re^3: statement vs. block? by kyle (Abbot) on Apr 11, 2008 at 23:55 UTC
Maybe I'm not getting what the OP is asking for. I read "any piece of code that would run unchanged as a statement or a block, that would behave differently in one relative to the other" as some stuff such that "`map {`stuff `} @any`" doesn't do the same thing as "`map`stuff`, @any`" (but does run). I provided that.	[reply] [d/l] [select]
Re^4: statement vs. block? by Fletch (Bishop) on Apr 12, 2008 at 00:13 UTC
Right, but the difference in your example is because of the precedence affecting powers of parens. Your examples have fallen afoul of the "if it looks like a function call it is one" rule (see perlfunc). It's the difference between `print (42+23)2;` and `print +(42+23)2;`; the parens change where the LIST of arguments to the list operator begin. (Not that you're wrong; it is a case where the literal code is different. I just wanted to point out that the difference is because of what the actual parsed code works out to be rather than it being that stuff is behaving different as `map EXPR, LIST` versus how `map BLOCK LIST` would, which I think was what's being asked.) The cake is a lie. The cake is a lie. The cake is a lie.	[reply] [d/l] [select]