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

According to perldoc http://perldoc.perl.org/functions/map.html

map should evaluate BLOCK in list context 

 Evaluates BLOCK or EXPR in list context

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but I am wondering why in my example, sometimes it's LIST context, sometimes SCALAR. 
print "A:";
print map { /(\S+)/g or die } qq{ab cd}; # prints "A:1" so evaluated i
+n scalar context
print "\n";

print "B:";
print map { /(\S+)/g  } qq{ab cd}; # prints "B:abcd", means evaluated 
+in list context
print "\n";

print "C:";
for (qq{ab cd}) {
    print /(\S+)/g or die; # prints "C:abcd", list context again
}
print "\n";

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Replies are listed 'Best First'.
Re: map BLOCK evaluation context
by tobyink (Canon) on Mar 08, 2013 at 14:20 UTC

    Nothing to do with map; it has more to do with precedence. The or operator always returns a scalar.

    Consider:

    print "D:";
for (qq{ab cd}) {
    print(/(\S+)/g or die);
}
print "\n";

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    package Cow { use Moo; has name => (is => 'lazy', default => sub { 'Mooington' }) } say Cow->new->name
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      Yes, but why in "A" example "or" enforces scalar, and in "C" it does not? Also: "or" does not always return a scalar.
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        or can supply list contexts to its arguments, but it always returns a scalar... or at least, I've never seen it return a list.

        use Data::Dumper;
my @A = qw(1 2 3);
my @B = qw(A B C D);
print Dumper(@A or @B);

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        In example C, or does return a scalar. You just don't do anything with the scalar. print @list or die is not parsed as print(@list or die); it's parsed as print(@list) or die. Therefore, print only sees the list; it doesn't see the result of the or operator.

        Update: meh... the right hand side of or can result in or returning a list:

        use Data::Dumper;
my @A = qw();
my @B = qw(A B C D);
print Dumper(@A or @B);

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        The left hand side cannot.

        package Cow { use Moo; has name => (is => 'lazy', default => sub { 'Mooington' }) } say Cow->new->name
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[d/l]
[select]
        Aha, you're right. The thing was that 
        print "C:";
for (qq{ab cd}) {
    print /(\S+)/g or die; # prints "C:abcd", list context again
}
print "\n";

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        was evaluated just like 
        print "C:";
for (qq{ab cd}) {
    print(/(\S+)/g) or die;
}
print "\n";

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        It can be fixed like this: 
        print "D:";
for (qq{ab cd}) {
    print /(\S+)/g || die;
}
print "\n";

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        However seems "or" not always enforces scalar context: 
        my @a = (1,2,3);
print "X:";
print (0 || @a);
print "\n";
print "Y:";
print (@a || 0);
print "\n";

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        prints 
        X:123
Y:3

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[d/l]
[select]

      The or operator always returns a scalar.

      No, only the LHS operand is evaluated in scalar context. The RHS inherits. 

      >perl -E"@a=qw( a b c ); say( 0 or @a );"
abc

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[select]
Re: map BLOCK evaluation context
by grondilu (Friar) on Mar 08, 2013 at 14:21 UTC

    Isn't it simply because invoking the boolean OR forces the scalar evaluation of /(\S+)/g ?

    Update: tobyink was quicker than me on this.

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