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Re: Pre-RFC: builtin:: functions for detecting numbers vs strings

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Tom Molesworth via perl5-porters
February 28, 2022 09:28
Re: Pre-RFC: builtin:: functions for detecting numbers vs strings
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On Mon, 28 Feb 2022 at 16:56, Dan Book <> wrote:

> On Mon, Feb 28, 2022 at 3:27 AM Philippe Bruhat (BooK) <>
> wrote:
>> When parsing `{ "a" : 2, "b": "c" }`, all that Perl reads is text. How
>> would a pure Perl JSON parser decode that `2` from the input source and
>> keep it in memory as if it was always the number 2?
> That's not the case - JSON parsers are aware whether they are parsing a
> number or a string because the JSON grammar is distinct, and can create the
> scalar differently depending. Not like e.g. certain databases where you
> can't always tell the type of the response values. Cpanel::JSON::XS::Type
> uses this information to return a type schema which you can already use to
> round-trip the output - these sort of changes enable more opportunities.

I think the point here is "how does pure perl say 'this string in $x was
always a number'". Extracting the number 2 from a string of JSON is easy
enough, presumably `substr` or regex were involved to get to that point -
but how do we then set the "this was originally a number" flag to make the
roundtrip work? Hopefully something in `builtin` or equivalent will provide
that, rather than requiring XS calls to internal APIs!

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