# Make var_dump() use serialize_precision

- Thread: https://externals.io/message/108655
- Original archive: https://news-web.php.net/php.internals/108655
- Started by Nikita Popov on 2020-02-18T11:20:18+00:00
- Messages: 3
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## #108655 — Nikita Popov — 2020-02-18T11:20:18+00:00

URL: https://externals.io/message/108655#108655

Hi internals,

https://github.com/php/php-src/pull/5172 changes `var_dump()` to use
serialize_precision instead of precision to dump floating-point numbers.

To recap: serialize_precision defaults to -1, which will print exactly as
many floating-point digits as are needed to represent the number
accurately. precision defaults to 14, which will print a shorter but
potentially inaccurate float representation.

The motivation here is that `var_dump()` is debugging functionality and
should print values as accurately as possible. The single most common bug
report we receive is some kind of variation on:

    $sum = 0.1 + 0.2;
    var_dump($sum); // float(0.3)
    var_dump($sum == 0.3); // bool(false) WTF???

After this change, this would instead be:

    $sum = 0.1 + 0.2;
    var_dump($sum); // float(0.30000000000000004)
    var_dump($sum == 0.3); // bool(false) Makes sense...

I have little hope that developers will suddenly start understanding
floating-point numbers, but at least this should reduce the amount of
confusion.

Does anyone see an issue with doing this change?

Regards,
Nikita

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## #108656 — Andreas Heigl — 2020-02-18T12:03:09+00:00

URL: https://externals.io/message/108655#108656 · In reply to: #108655

Am 18.02.20 um 12:20 schrieb Nikita Popov:
> Hi internals,
> 
> https://github.com/php/php-src/pull/5172 changes `var_dump()` to use
> serialize_precision instead of precision to dump floating-point numbers.
> 
> To recap: serialize_precision defaults to -1, which will print exactly as
> many floating-point digits as are needed to represent the number
> accurately. precision defaults to 14, which will print a shorter but
> potentially inaccurate float representation.
> 
> The motivation here is that `var_dump()` is debugging functionality and
> should print values as accurately as possible. The single most common bug
> report we receive is some kind of variation on:
> 
>     $sum = 0.1 + 0.2;
>     var_dump($sum); // float(0.3)
>     var_dump($sum == 0.3); // bool(false) WTF???
> 
> After this change, this would instead be:
> 
>     $sum = 0.1 + 0.2;
>     var_dump($sum); // float(0.30000000000000004)
>     var_dump($sum == 0.3); // bool(false) Makes sense...
> 
> I have little hope that developers will suddenly start understanding
> floating-point numbers, but at least this should reduce the amount of
> confusion.
> 
> Does anyone see an issue with doing this change?

You mean apart from people now filing bugs how `var_dump()` can output
such a nonsensical number from such an easy equation? And that it again
shows that PHP is not a real programming language (unlike JavaScript)
and should never be used at all?

Nope ;-)

Cheers

Andreas

PS: I'd absolutely appreciate the change!!!
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| Andreas Heigl                                                       |
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## #108748 — Andrea Faulds — 2020-02-24T23:03:14+00:00

URL: https://externals.io/message/108655#108748 · In reply to: #108655

Nikita Popov wrote:
> Hi internals,
> 
> https://github.com/php/php-src/pull/5172 changes `var_dump()` to use
> serialize_precision instead of precision to dump floating-point numbers.

Hi Nikita,

Thank you so much for doing this! I had wanted to do this for a long 
time, and I actually implemented it and began updating tests a while 
ago, but I never got round to finishing it.

It looks in that PR like you also update `debug_zval_dump()`, maybe that 
should be mentioned?

The one that gave me pause before and now is `print_r()`. In principle, it 
is just `print` but recursive (I assume that's whence the name comes), 
but unfortunately some people use it for debugging, so there might be a 
case for changing it. With that said, it's already a very bad choice for 
debugging due to other things ("1", 1, 1.0 and `TRUE` look the same, "0", 
0.0, and 0 look the same, "", `FALSE` and `NULL` look the same…) that 
precision is the least of the developer's problems if they choose 
 `print_r()`, and it's also not in the spirit of "print but recursive". So, 
it is probably unreasonable to change `print_r()` here. Maybe we should 
put a massive red "DO NOT USE THIS FOR DEBUGGING" warning on its 
documentation page…

One more thing, does `var_export()` use serialize_precision already? I 
think it does but haven't checked.

Thanks!
Andrea