[Purdue-pm] TIL

[Mark Senn]

Dave Jacoby <jacoby.david at gmail.com> wrote on 2019-12-18 at 18:53+00:
> use JSON;
> use PerlX::Maybe qw{ maybe provided };
> 
> my $json = JSON->new->canonical;
> 
> for my $i ( 1 .. 5 ) {
>     my $x;
>     $x = 1 if int rand 2;
>     my $data = { maybe x1 => $x, x2 => $x, provided $i > 3, i => $i, };
>     say $json->encode($data);
> }
> 
> $ ./maybe.pl
> {"x1":1,"x2":1}
>  {"x1":1,"x2":1}
> {"x2":null}
> {"i":4,"x1":1,"x2":1}
> {"i":5,"x2":null}
> 
> x1 will always be a thing, but the value will be undefined half the time.
> 
> x2 will not be assigned unless the value it it defined exists. we get this
> from `maybe`.
> 
> This is roughly
> 
> if ( defined $x ) { $data->{x2} = $x }
> 
> but so much more concise.
> 
> `provided` gives us conditionals.
> 
> if ( $i > 3 ) { $data->{$i} = $i }
> 
> becomes
> 
> provided $i > $3 , i => $i
> 
> it's small, among the smallest things thrown at me recently, but if nothing
> else, it makes a smaller JSON string when used.



Here is how I would do this in Raku.    -mark

#
# From "The JSON Data Interchange Syntax", second edition, December 2017,
# retrieved from
#     http://www.ecma-international.org/publications/files/ECMA-ST/ECMA-404.pdf
# on 2019-12-20 at 06:31+00:
#     A JSON value can be an object, array,
#     number, string, true, false, or null.
# There is no "undefined" in JSON so it is my understanding is that a
# name that is not set (i.e., is not in a JSON file) is the same as
# setting that name to "null".
#
# I'd do the above using the following code in Raku (formerly known
# as Perl 6).
#

# Use version 6.d of the Raku language specification.
# This will cause Perl 5 to complain if you try to use
# Perl 5 to run this program.
use v6.d;

use JSON::Class;

class Triple does JSON::Class
{
    has Int $.i  is rw;
    has Int $.x1 is rw;
    has Int $.x2 is rw;
}

for 1..5 -> $i
{
    $_ = Triple.new();

    ($i > 3)  and  $_.i = $i;

    (Bool.pick)  and  $_.x1 = $_.x2 = 1;

    # This program doesn't set names to "null" but if it
    # did the ":skip-null" below would not print them out.
    say $_.to-json():skip-null;
}