package DefHash; # just to make PodWeaver happy

# VERSION

1;
# ABSTRACT: Define things according to a specification, using hashes

__END__

=pod

=encoding UTF-8

=head1 NAME

DefHash - Define things according to a specification, using hashes

=head1 SPECIFICATION VERSION

 1

=head1 VERSION

This document describes version 1.0.12 of DefHash (from Perl distribution DefHash), released on 2019-06-18.

=head1 SYNOPSIS

A function returning a list of books from database, where each book is a
hash (DefHash):

 sub list_books {
     my $search_title = shift;

     $search_title = "%$search_title%" unless $search_title =~ /[%?]/;
     my $sth = $dbh->prepare("SELECT * FROM books WHERE title=?");
     $sth->execute($search_title);

     my @books;
     while (my $row = $sth->fetchrow_hashref) {
         push @books, {
             title   => $row->{title},
             summary => $row->{abstract},
             tags    => [($row->{in_print} ? () : ("out-of-print"))],
         };
     }
     @books;
 }

=head1 ABSTRACT

This document describes DefHash, a specification for using hashes to define
things. DefHash was born out of several other projects/specifications like
L<Sah>, L<Rinci>, L<Riap>, L<Module::Patch>.

=head1 SPECIFICATION

In this document, hashes are written in JSON or pseudo-JSON (e.g. contains
ellipsis C<...> or JavaScript-style comments C<// ...>).

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL "NOT", "SHOULD",
"SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be
interpreted as described in RFC 2119.

=head2 Definitions

=over 4

=item * B<defhash>

A regular hash, or dictionary (as it is called in Python), or associative array
(as it is called in PHP), or object (as it is called in JavaScript). A defhash
has B<properties>, which translates to the hash key/value pairs. B<Property
names> translates to hash keys, while B<property values> translates to hash
values. The same hash key/value pairs are used to store B<property attributes>:

 {
     "v": 1,                         // set value for property 'v'
     "prop1": "value1",              // set value for property 'prop1'
     "prop2": ["value2", ...],       // set value for property 'prop2'
     "prop1.attr1": ...,             // set value for prop1's attribute
     "prop1.attr1.subattr1": ...,    // set value for prop1's attribute
     "_extra1": ...,                 // ignored property, starts with _
     "prop1._extra_attr": ...,       // ignored attribute, starts with _

     "prop(en)",                     // equivalent/normalizable to prop.alt.lang.en
 }

The above defhash defines two properties: C<prop1> and C<prop2>. C<prop1> has
two attributes, C<attr1> and C<attr1.subattr1>. Properties with names starting
with underscore (C<_>) are ignored; this can be used to put extra information.
Likewise for attribute names which start with underscore.

Property names must follow this regex '\A[a-z][a-z0-9_]*\z' (an
Lowercase-alphanumeric-only word). Property attributes must follow this regex:
'\A[a-z][a-z0-9_]*(\.[a-z][a-z0-9_]*)*\z' (a dotted lowercase-alphanumeric
word). An exception is some shortcuts which will be normalized during further
processing: "foo(LANG)" is a shortcut for "foo.alt.lang.LANG".

Property value can be anything. It can contain another defhash for defining
subentities, for example. In Rinci, a function metadata has a property called
C<args> to define function arguments; its value is a hash of argument names and
argument specification. Each argument specification is a defhash.

Property attributes can be used to store extra data into a property.

The hash itself can have attributes, stored in .<attr> keys:

 {
     ".attr1": ...,
     ".attr2.subattr": ...,
     "._ignored": ...
 }

=item * B<specification>

A set of recognized properties and property attributes, including whether the
properties are required, expected values (schema) for properties and attributes,
and default values.

For example, Rinci is a specification for function metadata (among others). One
writes a defhash (metadata) for a function, it contains properties to describe
the function. Rinci specifies what properties are available and the meaning and
expected values for each of those properties. An example of a Rinci function
metadata:

 // metadata for function 'sum'
 {
     "v": 2, // version of Rinci specification
     "summary": "Sum all the elements of array numerically",
     "description":
         "Non-numeric elements in array will be skipped. Empty array
          or no numeric elements in array will result in 0 for the
          sum.",
     "args": {
         "array": {
             "summary": "The array to sum",
             "schema": "array*",
         },
     },
 }

=back

=head2 Why write definitions in a defhash?

Hash is a basic data structure that is supported by all high-level languages,
including Perl, Python, PHP, Ruby, and JavaScript. It is particularly easy to
merge. It makes checking the existence of value of property very easy, by just
accessing the hash's key.

B<... instead of text (like POD)?> Putting definition in a data structure makes
it easier to manipulate the definition (merge, parse, normalize, convert, etc).

B<... instead of array?> Hash allows us to evolve more easily. If we deprecate a
property or add new ones, elements don't have to shift like in array.

B<... instead of a regular or nested hash?> Well, defhash is a regular hash. It
is just a convention to limit the range of valid keys (only alphanumeric
characters) in exchange for additional metadata for each key (which is stored as
regular keys in the same hash). Plus it establishes convention for some
predefined properties and attributes.

=head2 Common properties

These are the list of properties that all specifications must recognize:

=over 4

=item * B<v> => FLOAT (default: 1)

This specifies the version of specification that the defhash is following.

A specification can change over time. The C<v> property specifies the
specification version which the hash follows. Specification version is a
non-negative real number, but integer is recommended. If unspecified, it is
assumed to be 1. It can also be 0.

=item * B<defhash_v> => INT (default: 1)

This specifies the version of DefHash specification itself. It is hoped that
this should never change, so normally a defhash need not specify this.

=item * B<name> => TEXT

A short (usually single-word) name for the thing that is described. For example,
in Rinci function metadata, it is the function's name. In Sah, it is a name of
the schema that can be used by the human compiler.

 // metadata for function 'sum'
 {
     "name": "sum",
     ...
 }

 // schema for describing positive integer
 ["int", {
     "name": "pos_int",
     "min": 0,
 }]

=item * B<caption> => text

Like C<name>, but for display purposes.

=item * B<summary> => TEXT

A short (< 72 character), one-line summary about the thing that is described.
For example, in Rinci function metadata, the C<summary> describes what the
function does:

 // metadata for function 'sum'
 {
     "summary": "Sum all the elements of an array numerically",
     ...
 }

=item * B<description> => TEXT

A longer description. Normally a paragraph or longer of text. The text is
assumed to be marked up in Markdown.

=item * B<tags> => ARRAY[TEXT | DEFHASH]

A list of one or more tags, can be used to categorize the thing that is
described. Example:

 "tags": ["important", "category:filtering", "category:filtering-for-foo"],

Each tag can also be a defhash for more detailed specification:

 "tags": ["important",
          {"name":"category:filtering", "summary":"filtering field"},
          {"name":"category:filtering-for-foo", "summary":"filtering for field 'foo'"}]

=item * B<default_lang> => TEXT

Default language. Defaults to parent's value, or if parent does not exist, from
environment LANG, or if undefined or C<C>, C<en_US>.

=item * B<x> => ANY

This property is used to store extended (application-specific) attributes, much
like the C<X-> prefix in HTTP or email headers. This property can be used as an
alternative to using underscore prefix (e.g. C<_foo>). Some processing tools
strip properties/attributes that begin with underscores, so to pass extended
metadata around, it might be more convenient to use the C<x> property.

Example:

 {
     "x.myapp.foo" => 1,
     "x.myapp.bar" => "some value",
 }

=back

=head2 Property attributes

Below is the list of property attributes that must be supported.

=over

=item * alt

This attribute can be used to store alternate property values. The most common
is alternative language (C<alt.lang.LANG_CODE>), where you want to provide
translations. Example:

 {
     "summary": "An English summary",
     "summary.alt.lang.id_ID": "Ringkasan dalam bahasa Indonesia",
 }

There is a shortcut form for C<alt.lang>, which should be normalized to
C<alt.lang> when processing:

 {
     "summary": "An English summary",
     "summary(id_ID)": "Ringkasan dalam bahasa Indonesia",
 }

Another kind of alternate is environment (C<alt.env.NAME>), for example we might
want to use different summary when displayed in a CLI program vs web
application. Example (in a Rinci function argument's specification):

 {
     "summary.alt.env.cmdline": "Like --foo, do something foobar-ish",
     "summary.alt.env.web": "Like the `foo` field, do something foobar-ish",
 }

Another kind of alternate is boolean logic (C<alt.bool.WHICH>, where C<WHICH>
can be C<not>), when you want to specify the negative sense/sentence instead of
the regular positive one. Example (in a Rinci function argument's
specification):

 "tcp": {
     "schema": "bool",
     "default": true,
     "summary": "Parse TCP connections",
     "summary.alt.bool.not": "Do not parse TCP connections",
 }

Another kind of alternate is grammatical variance based on noun plurality
(C<alt.plurality.WHICH> where C<WHICH> can be C<singular> or C<plural>). Example
(in a Rinci function argument's specification, where an array argument like
C<files> translates in a CLI environment to multiple C<--file> option
specification):

 "files": {
     "schema": ["array*", {"of":"str*"}],
     "summary": "Specify one or more files to check",
     "summary.alt.plurality.singular": "Add a file to check (can be specified multiple times)",
 }

Combination of alternates. When two or more alternates is combined, they must be
ordered asciibetically and separated with an underscore C<_>. For example:

 {
     "default_lang": "en_US",
     "summary": "An English summary",
     "summary.alt.env_lang.web.id_ID":"(Summary in Indonesian, for web)",
     "summary.alt.env_lang.cmdline.id_ID":"(Summary in Indonesian, for cmdline)"
 }

=back

=head2 When should specification version be increased?

When a backward-incompatible change is introduced. This is defined to be removal
of a recognized property, or the semantic change of an existing property, or
other incompatible change. For example,

 XXX (modp 1->2, 2->3; ri ->2, not using defhash but that is not the real reason, removal of features property)
 XXX riap also bumped to 2 just because it uses hash

=head1 HOMEPAGE

Please visit the project's homepage at L<https://metacpan.org/release/DefHash>.

=head1 SOURCE

Source repository is at L<https://github.com/perlancar/perl-DefHash>.

=head1 BUGS

Please report any bugs or feature requests on the bugtracker website L<https://rt.cpan.org/Public/Dist/Display.html?Name=DefHash>

When submitting a bug or request, please include a test-file or a
patch to an existing test-file that illustrates the bug or desired
feature.

=head1 SEE ALSO

Semantic Versioning, L<http://semver.org>

Markdown specification

=head1 AUTHOR

perlancar <perlancar@cpan.org>

=head1 COPYRIGHT AND LICENSE

This software is copyright (c) 2019, 2015, 2014, 2013, 2012 by perlancar@cpan.org.

This is free software; you can redistribute it and/or modify it under
the same terms as the Perl 5 programming language system itself.

=cut
