Peculiar incident: printing a variable changes its type

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

Hi Monks, I just scratched my head for a good chunk of my time resolving an issue which boils down to this:

use JSON;

$params->{timeout} = 2000;
$body->{"timeout"} = (defined $params->{timeout}?$params->{timeout} : 
+3000);
print "$body->{timeout}\n"; #<-----suspect line
my $body_string = to_json($body);
print "$body_string";
[download]

In the above code, the mere printing of the hash value adds double quotes to the timeout value in the json as shown in the output.

Output: 
2000
{"timeout":"2000"}
[download]

Now if you remove the "suspect line" from the above code:

use JSON;

$params->{timeout} = 2000;
$body->{"timeout"} = (defined $params->{timeout}?$params->{timeout} : 
+3000);
my $body_string = to_json($body);
print "$body_string";
[download]

Output: 
{"timeout":2000}
[download]

In the second output, the value 2000 wasn't wrapped in quotes. I find it really peculiar as to why the mere printing of the hash value changes the behavior of the output??

Comment on Peculiar incident: printing a variable changes its type Select or Download Code

Replies are listed 'Best First'.
Re: Peculiar incident: printing a variable changes its type by tobyink (Canon) on Nov 13, 2013 at 09:28 UTC
Strings and numbers in Perl are represented by the same data type: scalars. When the JSON module encounters something that looks like a number, it has to guess whether you wanted to output it as a number or a string. To do this it peeks at the scalar's internals (the SV structure - in particular, its flags) to see whether the scalar has ever been treated as a string. If it has, then it outputs it as a quoted string. Install Devel::Peek and note the difference here: `$ perl -MDevel::Peek -E'my $x = 1; print "$x\n"; print Dump($x)' 1 SV = PVIV(0x864e218) at 0x864d738 REFCNT = 1 FLAGS = (PADMY,IOK,POK,pIOK,pPOK) IV = 1 PV = 0x86483a8 "1"\0 CUR = 1 LEN = 12 $ perl -MDevel::Peek -E'my $x = 1; print $x,"\n"; print Dump($x)' 1 SV = IV(0x86b0724) at 0x86b0728 REFCNT = 1 FLAGS = (PADMY,IOK,pIOK) IV = 1` [download] In the first example, the variable `$x` has been interpolated into a string, and thus acquires a POK flag indicating that the SV structure has a pointer to a string. In the second example, although `$x` has still been printed out, with a new line afterwards, there was no concatenation; no string operator performed on `$x`; so it doesn't acquire a POK flag. The JSON module would output this as a number. You can make a variable's POK flag disappear by adding 0 to it: `$ perl -MDevel::Peek -E'my $x = 1; print "$x\n"; $x += 0 ;print Dump($ +x)' 1 SV = PVIV(0x9cba210) at 0x9cb9730 REFCNT = 1 FLAGS = (PADMY,IOK,pIOK) IV = 1 PV = 0x9cb43a0 "1"\0 CUR = 1 LEN = 12` [download] Notice that the actual string pointer has been kept (that's the PV line), but it is no longer "OK" - it might have been invalidated by the addition, so the POK flag has gone. `use Moops; class Cow :rw { has name => (default => 'Ermintrude') }; say Cow->new->name`	[reply] [d/l] [select]
Re^2: Peculiar incident: printing a variable changes its type by choroba (Cardinal) on Nov 13, 2013 at 14:23 UTC
Install Devel::Peek According to corelist, Devel::Peek is a core module since 5.6.0. لսႽ† ᥲᥒ⚪⟊Ⴙᘓᖇ Ꮅᘓᖇ⎱ Ⴙᥲ𝇋ƙᘓᖇ	[reply]
Re: Peculiar incident: printing a variable changes its type by mje (Curate) on Nov 13, 2013 at 09:23 UTC
Use Devel::Peek and Dump $body->{"timeout"} before and after print. Look at the part called pv. I'm afraid this is what a lot of JSON encoders do, they try to guess whether something is a number by looking to see if your scalar has a pv (has been used in string context) or not. printing your scalar causes the pv to be created. Some modules (e.g., DBD::Oracle and DBD::ODBC) have had code added to them to allow an attribute to be specified to remove the pv when fetching numbers from a database but this is XS code. I'm unsure how you do it in pure perl but there is bound to be a way.	[reply]
Re: Peculiar incident: printing a variable changes its type by KurtZ (Friar) on Nov 13, 2013 at 14:07 UTC
`print "$body->{timeout}\n"; #<-----suspect line` [download] avoid stringification! `print $body->{timeout}, "\n"; # better` [download] HTH!	[reply] [d/l] [select]


Problems? Is your data what you think it is?
	PerlMonks