ansaurus

Question

Why does File::Slurp return a scalar when it should return a list?

Answer 1

+6 A:

The || operator puts its left-hand operand in boolean (scalar) context:

C-style Logical Or

Binary || performs a short-circuit logical OR operation. That is, if the left operand is true, the right operand is not even evaluated. Scalar or list context propagates down to the right operand if it is evaluated.

You could also discover this empirically:

#! /usr/bin/perl

sub lhs {
  my $ctx = wantarray;
  if (defined $ctx) {
    if ($ctx) { "list" }
    else      { "scalar" }
  }
  else { "void" }
}

print lhs || 0, "\n";

Output:

$ ./or-ctx.pl
scalar

To fix your program, you could modify it slightly:

my @array = eval { read_file("/etc/issue") };
die unless @array;

This is necessary only if you want to add to the error message (stored in the special variable $@ after the eval) because read_file will die if something goes wrong.

Greg Bacon 2010-06-02 13:49:48

Thanks so much for the clear explanation and example!

BrianH 2010-06-02 14:14:16

@BrianH You're welcome! I'm glad it helps.

Greg Bacon 2010-06-02 14:16:06

Answer 2

+9 A:

The boolean or operator || binds tighter than the assignment operator, and puts the function call into a scalar context.

my @array = read_file( $file ) || die "Cannot read $file\n";

That says: try to read the file, and return either the concatenation of the file or the "return" from die as the first element in @array. You don't die because it read the file and returned a true value, albeit a scalar one.

The standard usage is the statement or operator (or ), like so:

my @array = read_file( $file ) or die "Cannot read $file\n";

This tries to assign @array in a list context, and then evaluates the assigned list in a scalar context, yielding the number of items in array. Thus, you assign the array first, and then you do not die because the array has entries.

The second form does not try to assign the "return" from die to @array because the assignment is done first, so @array either holds the lines of the file or is an empty array.

Notice that the documentation for File::Slurp read_file says:

In list context it will return a list of lines (using the current value of $/ as the separator including support for paragraph mode when it is set to ''). In scalar context it returns the entire file as a single scalar. [italics mine]

And gives the following examples:

    my $text = read_file( 'filename' ) ;
    my @lines = read_file( 'filename' ) ;

But these are the simplest cases, and the most basic expressions of context. Assigning to a variable of a certain type does not ensure that @lines would be assigned to in a list context regardless of surrounding code.

Axeman 2010-06-02 14:02:42

This explanation helps me as well - Thanks!

BrianH 2010-06-02 14:15:33

@BrianH: Some people *love* context once they get the hang of it and some never warm to it. Some people think it's a needless source of confusion in Perl. I like it because it increases expressiveness, at the cost of causing some learning bumps and occasional surprises.

Axeman 2010-06-02 14:26:54

`File::Slurp::read_file` croaks by default on error. So the check is unnecessary.

Sinan Ünür 2010-06-02 15:42:08

@Sinan Ünür: So the OP just wants to make sure that the process is good and dead.

Axeman 2010-06-02 16:03:45

Answer 3

+4 A:

Your question has been answered already. As a side note, first, keep in mind that or and and are better suited for flow control statements. However, more importantly, the check below

 my @array = read_file( $file ) || die "Cannot read $file\n";

is completely unnecessary. By default, File::Slurp::read_file croaks on error:

err_mode

You can use this option to control how read_file behaves when an error occurs. This option defaults to 'croak'. You can set it to 'carp' or to 'quiet to have no error handling.

Sinan Ünür 2010-06-02 15:41:14

+1 Indeed. That's one reason why `File::Slurp` is so handy.

FM 2010-06-02 17:15:43

ansaurus

tags:

views:

answers:

Why does File::Slurp return a scalar when it should return a list?

C-style Logical Or

err_mode

related questions