NAME¶
DefHash - Define things according to a specification, using hashes
VERSION¶
version 1.0.5
RELEASE DATE¶
2014-04-27
SPECIFICATION VERSION¶
1
ABSTRACT¶
This document describes DefHash, a specification for using hashes to define
things. DefHash was born out of several other projects/specifications like
Sah, Rinci, Riap, Module::Patch.
SPECIFICATION¶
In this document, hashes are written in JSON or pseudo-JSON (e.g. contains
ellipsis "..." or JavaScript-style comments "// ...").
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.
Definitions¶
- •
- 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 properties, which translates to the hash key/value
pairs. Property names translates to hash keys, while
property values translates to hash values. The same hash key/value
pairs are used to store 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 _
}
The above defhash defines two properties: "prop1" and
"prop2". "prop1" has two attributes, "attr1"
and "attr1.subattr1". Properties with names starting with
underscore ("_") 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).
Property value can be anything. It can contain another defhash for defining
subentities, for example. In Rinci, a function metadata has a property
called "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": ...
}
- •
- 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*",
},
},
}
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.
... instead of text (like POD)? Putting definition in a data structure
makes it easier to manipulate the definition (merge, parse, normalize,
convert, etc).
... 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.
... 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.
Common properties¶
These are the list of properties that all specifications must recognize:
- •
- v => FLOAT (default: 1)
Must not have attributes. This specifies the version of specification that
the defhash is following.
A specification can change over time. The "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.
- •
- defhash_v => INT (default: 1)
Must not have attributes. This specifies the version of DefHash
specification itself. It is hoped that this should never change, so
normally a defhash need not specify this.
- •
- 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,
}]
- •
- summary => TEXT
A short (< 72 character), one-line summary about the thing that is
described. For example, in Rinci function metadata, the
"summary" describes what the function does:
// metadata for function 'sum'
{
"summary": "Sum all the elements of an array numerically",
...
}
- •
- description => TEXT
A longer description. Normally a paragraph or longer of text. The text is
assumed to be marked up in Markdown.
- •
- 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'"}]
- •
- default_lang => TEXT
Default language. Defaults to parent's value, or if parent does not exist,
from environment LANG, or if undefined or "C",
"en_US".
- •
- x => ANY
This property is used to store extended (application-specific) attributes,
much like the "X-" prefix in HTTP or email headers. This
property can be used as an alternative to using underscore prefix (e.g.
"_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 "x" property.
Example:
{
"x.myapp.foo" => 1,
"x.myapp.bar" => "some value",
}
Property attributes¶
Below is the list of property attributes that must be supported.
- •
- alt
This attribute can be used to store alternate property values. The most
common are:
alt.lang.<LANG_CODE>
Example:
{
"summary": "An English summary",
"summary.alt.lang.id_ID": "Ringkasan dalam bahasa Indonesia",
}
Another example (alternate value for different language, "lang",
and different environment, "env"; the ordering should be
asciibetical and a care should be taken to not be ambiguous, since
attribute names can only be words):
{
"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)"
}
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
SEE ALSO¶
Semantic Versioning, <
http://semver.org>
Markdown specification
HOMEPAGE¶
Please visit the project's homepage at
<
https://metacpan.org/release/DefHash>.
SOURCE¶
Source repository is at <
https://github.com/sharyanto/perl-DefHash>.
BUGS¶
Please report any bugs or feature requests on the bugtracker website
<
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.
AUTHOR¶
Steven Haryanto <stevenharyanto@gmail.com>
COPYRIGHT AND LICENSE¶
This software is copyright (c) 2014 by Steven Haryanto.
This is free software; you can redistribute it and/or modify it under the same
terms as the Perl 5 programming language system itself.