How to tell apart numeric scalars and string scalars in Perl?

Question

Perl usually converts numeric to string values and vice versa transparently. Yet there must be something which allows e.g. Data::Dumper to discriminate between both, as in this example:

use Data::Dumper;
print Dumper('1', 1);

# output:
$VAR1 = '1';
$VAR2 = 1;

Is there a Perl function which allows me to discriminate in a similar way whether a scalar's value is stored as number or as string?

Answer 1

A scalar has a number of different fields. When using Perl 5.8 or higher, Data::Dumper inspects if there's anything in the IV (integer value) field. Specifically, it uses something similar to the following:

use B qw( svref_2object SVf_IOK );

sub create_data_dumper_literal {
    my ($x) = @_;  # This copying is important as it "resolves" magic.
    return "undef" if !defined($x);

    my $sv = svref_2object(\$x);
    my $iok = $sv->FLAGS & SVf_IOK;
    return "$x" if $iok;

    $x =~ s/(['\\])/\\$1/g;
    return "'$x'";
}

Checks:

• Signed integer (IV): ($sv->FLAGS & SVf_IOK) && !($sv->FLAGS & SVf_IVisUV)
• Unsigned integer (IV): ($sv->FLAGS & SVf_IOK) && ($sv->FLAGS & SVf_IVisUV)
• Floating-point number (NV): $sv->FLAGS & SVf_NOK
• Downgraded string (PV): ($sv->FLAGS & SVf_POK) && !($sv->FLAGS & SVf_UTF8)
• Upgraded string (PV): ($sv->FLAGS & SVf_POK) && ($sv->FLAGS & SVf_UTF8)

You could use similar tricks. But keep in mind,

• It'll be very hard to stringify floating point numbers without loss.

• You need to properly escape certain bytes (e.g. NUL) in string literals.

• A scalar can have more than one value stored in it. For example, !!0 contains a string (the empty string), a floating point number (0) and a signed integer (0). As you can see, the different values aren't even always equivalent. For a more dramatic example, check out the following:

    $ perl -E'open($fh, "non-existent"); say for 0+$!, "".$!;'
    2
    No such file or directory
  

Answer 2

It is more complicated. Perl changes the internal representation of a variable depending on the context the variable is used in:

perl -MDevel::Peek -e '
    $x = 1;    print Dump $x;
    $x eq "a"; print Dump $x;
    $x .= q(); print Dump $x;
'
SV = IV(0x794c68) at 0x794c78
  REFCNT = 1
  FLAGS = (IOK,pIOK)
  IV = 1
SV = PVIV(0x7800b8) at 0x794c78
  REFCNT = 1
  FLAGS = (IOK,POK,pIOK,pPOK)
  IV = 1
  PV = 0x785320 "1"\0
  CUR = 1
  LEN = 16
SV = PVIV(0x7800b8) at 0x794c78
  REFCNT = 1
  FLAGS = (POK,pPOK)
  IV = 1
  PV = 0x785320 "1"\0
  CUR = 1
  LEN = 16