in reply to Re^7: Unhappy returns
in thread Unhappy returns

Note that the very fact that I am speaking of the "context" of a statement demonstrates that statements can be said to have return values.

Heh... uh... no. My daughter talks about Santa Claus. That doesn't demonstrate his existence.

All you've demonstrated is that you think every statement has a return value. But, you've already stated that explicitly, so a demonstration was not necessary.

It is true that expressions have values and it is true that expressions are statements, but, it doesn't follow from that that all statements have values.

-sauoq
"My two cents aren't worth a dime.";

Replies are listed 'Best First'.
Re^9: Unhappy returns
by Perl Mouse (Chaplain) on Oct 11, 2005 at 09:38 UTC
    The return value of a for statement is the empty string, as the following program shows: 
    #!/usr/bin/perl

use strict;
use warnings;
               
my $x = do {for (1 .. 4) {}};
my @x = do {for (1 .. 4) {}};

printf "Scalar return value of for is %s\n",
               defined $x ? "defined" : "undefined";
printf "Scalar return value of for is %s\n",
               $x ? "true" : "false";
printf "Scalar return value of for is has length %d\n",
               length $x;
printf "Scalar return value of for is '%s'\n",
               $x;

printf "List return value of for returns %d element\n",
               scalar @x;
printf "List return value of for contains %d undefined elements\n",
               scalar grep {!defined} @x;
printf "List return value of for contains %d true elements\n",
               scalar grep {$_} @x;
printf "List return value of for contains %d elements of non-zero leng
+th\n",
               scalar grep {length} @x;
printf "List return value of for equals [%s]\n",
               join ", ", map {"'$_'"} @x;

__END__
Scalar return value of for is defined
Scalar return value of for is false
Scalar return value of for is has length 0
Scalar return value of for is ''
List return value of for returns 1 element
List return value of for contains 0 undefined elements
List return value of for contains 0 true elements
List return value of for contains 0 elements of non-zero length
List return value of for equals ['']

    [download]
    Note also that while Perl makes a difference between functions and statements (mostly for the benefit of mortals), perl doesn't. Operators, functions, statements, they're all opcodes. Here's the handling of the leaveloop opcode: 
    PP(pp_leaveloop)
{
    dSP;
    register PERL_CONTEXT *cx;
    I32 gimme;
    SV **newsp;
    PMOP *newpm;
    SV **mark;

    POPBLOCK(cx,newpm);
    mark = newsp;
    newsp = PL_stack_base + cx->blk_loop.resetsp;

    TAINT_NOT;
    if (gimme == G_VOID)
        ; /* do nothing */
    else if (gimme == G_SCALAR) {
        if (mark < SP)
            *++newsp = sv_mortalcopy(*SP);
        else
            *++newsp = &PL_sv_undef;
    }
    else {
        while (mark < SP) {
            *++newsp = sv_mortalcopy(*++mark);
            TAINT_NOT;          /* Each item is independent */
        }
    }
    SP = newsp;
    PUTBACK;

    POPLOOP(cx);        /* Stack values are safe: release loop vars ..
+. */
    PL_curpm = newpm;   /* ... and pop $1 et al */
 
    LEAVE;
    LEAVE;
    
    return NORMAL;
}

    [download]
    You don't have to understand what it does, but do note that it's looking at its context to determine what to leave behind on the stack.

    And here is a trace: 

    perl -Dt -e '$x = do {for (1) {}}'

EXECUTING...

(-e:0)  enter
(-e:0)  nextstate
(-e:1)  pushmark
(-e:1)  const(IV(1))
(-e:1)  gv(main::_)
(-e:1)  enteriter
(-e:1)  iter
(-e:1)  and
(-e:1)  stub
(-e:1)  unstack
(-e:1)  iter
(-e:1)  and
(-e:1)  leaveloop
(-e:1)  gvsv(main::x)
(-e:1)  sassign
(-e:1)  leave

    [download]
    Perl --((8:>*
[reply]
[d/l]
[select]

In Section Meditations