Declare my variable in sysread

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

Hi Monks,

I have a question about some code I saw in Mojo::File. In the slurp method, you see this:

sub slurp {
  my $self = shift;
 
  CORE::open my $file, '<', $$self or croak qq{Can't open file "$$self
+": $!};
  my $ret = my $content = '';
  <b>while ($ret = $file->sysread(my $buffer, 131072, 0)) { $content .
+= $buffer }</b>
  croak qq{Can't read from file "$$self": $!} unless defined $ret;
 
  return $content;
}
[download]

Inside the while, you see my $buffer used as the first argument to sysread, and then used in the body of the while loop. I've never seen this before, where you declare a variable as an argument and then access it later. Can anyone explain how this works? Is the body of the while loop able to access $buffer because the while loop's scope includes anything in the parentheses or the body of the loop?

Comment on Declare my variable in sysread - Mojo::File::slurp Select or Download Code

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Re: Declare my variable in sysread - Mojo::File::slurp by choroba (Cardinal) on Dec 21, 2018 at 16:59 UTC
It's the same trick as in `while (my $line = <$fh>) {` [download] which is a shorthand for `while (defined( my $line = readline($fh) )) {` [download] where, again, my $line appears as an ~~argument~~ operand of = which in turn is an ~~argument~~ operand of defined. my has two effects, one compile time and one run time. In compile time, you declare a lexical variable, and in runtime, you use it as an lvalue (i.e. assign a value to it). Variables declared in block conditions (if, while, unless, until) are accessible until the last conditional operator block. `if (my $x = func()) { print "$x accesible here"; } elsif ($x eq 0) { # even here, print "and also here: $x"; }` [download] Updated. `map{substr$_->[0],$_->[1]\|\|0,1}[\\|\|{},3],[[]],[ref qr-1,-,-1],[{}],[sub{}^ARGV,3]`	[reply] [d/l] [select]
Re^2: Declare my variable in sysread - Mojo::File::slurp by ikegami (Patriarch) on Dec 22, 2018 at 00:43 UTC
my $line appears as an argument of defined. No, it doesn't. It's an operand of `=`, not `defined`.	[reply] [d/l] [select]
Re^3: Declare my variable in sysread - Mojo::File::slurp by choroba (Cardinal) on Dec 22, 2018 at 00:46 UTC
But the = is, in turn, an ~~argument~~ operand to defined. `map{substr$_->[0],$_->[1]\|\|0,1}[\\|\|{},3],[[]],[ref qr-1,-,-1],[{}],[sub{}^ARGV,3]`	[reply] [d/l]
Re^4: Declare my variable in sysread - Mojo::File::slurp by ikegami (Patriarch) on Dec 22, 2018 at 00:56 UTC
Re^2: Declare my variable in sysread - Mojo::File::slurp by ikegami (Patriarch) on Dec 22, 2018 at 00:52 UTC
my $line appears as an argument of = which in turn is an argument of defined. You're still claiming `my $line` is an argument of `defined`, and it's not. Operators have operands (not arguments), and the variable `$line` is `defined`'s operand.	[reply] [d/l] [select]
Re: Declare my variable in sysread - Mojo::File::slurp by jwkrahn (Monsignor) on Dec 21, 2018 at 19:35 UTC
This is one of the idiosyncrasies of scoping that is rarely talked about. `while ( my $var = ... ) { # $var visible here } continue { # $var also visible here } if ( my $var = ... ) { # $var visible here } elsif ( ) { # $var also visible here } else { # $var also visible here }` [download]	[reply] [d/l]
Re: Declare my variable in sysread - Mojo::File::slurp by ikegami (Patriarch) on Dec 22, 2018 at 00:39 UTC
You say you've never seen this before, but you've done in a million times. Just look at the previous line of your code. `open my $file, ...` [download] `my $var` not only declares the variable at compile-time, it also returns the variable (as an lvalue) as if you had simply written `$var`.	[reply] [d/l] [select]
Re: Declare my variable in sysread - Mojo::File::slurp by Anonymous Monk on Dec 21, 2018 at 23:26 UTC
Hi How do you suppose `CORE::open my $file` works in sub slurp? What if it was written as `while( CORE::open my $file ... ){ ... }` ? The two arent much different	[reply] [d/l] [select]


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Declare my variable in sysread - Mojo::File::slurp