Hacking Thy Fearful Symmetry

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Say 'Hello' to JSON::Schema::AsType

December 6, 2015

In mathematics, a common approach to solve new problems is -- instead of trying to solve them head on -- to find a way to define them as a transformation of an already known, already solved problem. That way, you don't have to spend hours reinventing the wheel where you can spend days and weeks floating in the meta-sphere, getting your mind blown away by layers upon layers of abstractions until the sheer recursiveness of it all drive you to the nihilistic solace of alcohol and cave-dwelling seclusion.


What I'm trying to say there, is that there I was, working on a secret project (hint: it has to do with Dancer, with Swagger, and will be awesome). Project is doing amazing, and I reached the point where it'd be groovy if I could validate JSON schemas in Perl. There is JSON-Schema, but it's not quite passing all the draft4 test cases. And there is JSV, which I was going to use when I thought...

A JSON Schema, when you get down to it, is nothing but the definition of a structure, isn't? Or, as we programmers would say, a type.

Wouldn't it be nice, then, to be able to take a JSON Schema and turn it into a Type-Tiny type?

So say hello to JSON-Schema-AsType. It slices, it dices, and it passes all draft3 and draft4 test suites.

use JSON::Schema::AsType;

my $schema = JSON::Schema::AsType->new(
    specification => 'draft4',
    schema => {
        properties => {
            foo => {
                enum => [qw/ alpha beta gamma /],

my $type = $schema->type; # there ya go, a nice Type::Tiny type!

print "valid!" if $type->check( { foo => 'delta' } ); # won't print

And because we're piggy-backing on Type::Tiny's goodness, schemas that don't validate have explanations that are... well, okay, maybe not the most beautiful in the world, but at least decently informative.

For example,

say for @{ $schema->validate_explain({ foo => 'delta' }) };

would give you

"~JsonObject|Dict[foo=>Optional[Property],slurpy Any]" requires that the value pass "Dict[foo=>Optional[Property],slurpy Any]" or "~JsonObject"
Reference {"foo" => "delta"} did not pass type constraint "~JsonObject"
    Reference {"foo" => "delta"} did not pass type constraint "~JsonObject"
    "~JsonObject" is defined as: (not(((ref($_) eq 'HASH') and (not((Scalar::Util::blessed($_)))))))
Reference {"foo" => "delta"} did not pass type constraint "Dict[foo=>Optional[Property],slurpy Any]"
    Reference {"foo" => "delta"} did not pass type constraint "Dict[foo=>Optional[Property],slurpy Any]"
    "Dict[foo=>Optional[Property],slurpy Any]" constrains value at key "foo" of hash with "Optional[Property]"
    Value "delta" did not pass type constraint "Optional[Property]" (in $_->{"foo"})
    $_->{"foo"} exists
    "Optional[Property]" constrains $_->{"foo"} with "Property" if it exists
    "Property" is a subtype of "Enum"
    Value '"delta"' doesn't match any of the enum items:'"alpha"' '"beta"' '"gamma"' (in $_->{"foo"})
    "Enum" is defined as: sub { package JSON::Schema::AsType::Draft4; my $j = to_json($_, {'allow_nonref', 1, 'canonical', 1}); any sub { $_ eq $j; } , @enum; }

There is still room for improvement there, but then, we're still at version 0.0.1. In the next little while, I'd like to clean up the error messages, such that it's informative for all JSON schema keywords. I'd also add the sugar such that we can also declare a type as

use JSON::Schema::AsType qw/ JsonSchema /;

my $type = JsonSchema[{
    properties => { ... },

And, finally, to be able to create, and refine, schemas by providing examples.

my $schema = JSON::Schema::AsType->new;
  # would begin its live as ''{}'

$schema->refine({ foo => 'bar' });
  # would turn into 
  # '{ type => "object", properties => { foo => { type => "string" } }'

$schema->refine({ quux => 1 });
  # would turn into 
  # '{ type => "object", properties => {
  #     foo => { type => "string" },
  #     quux => { type => "integer" } }'

Bold plans. But then, there is the Holiday season, and lots of Egg Nog coming soon, so... who knows. It might actually happens. In the meantime... Enjoy!

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About the author

Yanick Champoux
Perl necrohacker , ACP writer, orchid lover. Slightly bonker all around. he/him/his