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<h1>A high-level type system for the Free Desktops</h1>
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<div id="preamble">
<div class="sectionbody">
<div class="paragraph"><p>Desktop environments are not just for starting applications anymore.
Data is flowing freely between well-integrated components, and the
easier the data flows, the better the integration of the components.</p></div>
<div class="paragraph"><p>Not all components are written in the same programming language, of
course, and when letting data flow between them, it needs to be
represented in many different ways. For example, GConf stores values
differently than they travel over D-Bus, which is different again from
how they are passed as GValues to signal handlers, which is different
from how Perl wants to store it.</p></div>
<div class="paragraph"><p>The desktop environment is heading towards a cooperative, dynamic
environment, and it needs a rich and strong type system to tie its
components together. Sending lines of text over pipes and matching
them against ad-hoc regular expressions just doesn’t cut it.</p></div>
<div class="paragraph"><p>In an attempt to define such a common type system, this document
collects many different systems for representing values, and unifies
them by mapping the common dynamic type system into them.</p></div>
<div class="paragraph"><p>The common type system defined here is rich enough to represent any
reasonable value; it’s roughly equivalent to what dynamic languages
like Perl and Python have.</p></div>
<div class="paragraph"><p>But it goes one crucial step further: it allows the definition of new
abstract, intentional types. Intentional types give additional
information about a value that is not available from the
representation alone.</p></div>
<div class="paragraph"><p>For example, a integer can be used to denote a point in time by saying
that it is the number of seconds since a certain epoch. All places
that interact with such a value need to agree on this intention.</p></div>
<div class="paragraph"><p>This agreement can happen informally, via documentation or just plain
common sense. Nothing wrong with that. It is, however, also helpful
to formalize this so that documentation can be precise without much
extra effort, up to a level where the machine itself is able to check
whether everybody agrees on the intentional types.</p></div>
<div class="paragraph"><p>The age old battle between static and dynamic types also matters here:
how much type information should be associated with the values
themselves? The boundary is exactly between intentional and
representational types. Intentional types are those that only the
programmer or compiler know about, representational types are those
that are only known at run-time.</p></div>
<div class="paragraph"><p>In a completely statically typed language, we only have raw bytes at
run-time without any representational type information. All parts of
the program need to agree that the intention is for those four bytes
over there to be used as a 32-bit integer. Statically typed programs
are littered with declarations of intentional types, and language
processors use them to (more or less) check program consistency and to
select the right division instruction based on whether the four bytes
over there are intended to be a signed number or an unsigned one.</p></div>
<div class="paragraph"><p>In a dynamically typed language, values carry a lot of
representational type information. Code can easily be polymorphic and
do different things depending on whether a value is an integer or a
string. It can also perform consistency checks at run-time, which is
more robust than doing it at compile time, but doesn’t go as far since
intentional types are not available.</p></div>
<div class="paragraph"><p>Dynamic languages often don’t have any means to declare intentional
types for the benefit of the compiler; they only exist in the head of
the programmer who expresses them by selecting the right operation
manually. For example, if a string is intended to be a URL, you need
to use <em>utils.net.web.url.get_scheme (url)</em> explicitly. If the
intentional type could have been declared in the language, it could
have selected the right function automatically from just <em>url.scheme()</em>.</p></div>
<div class="paragraph"><p>Thus, and coming back to the ground now, we define a concrete type
system here with a rich representational part and a lightweight and
flexible intentional part.</p></div>
<div class="paragraph"><p>For the representational part, we define how it is implemented for a
number of existing value systems. For the intentional part, we define
how it can be translated into a number of languages, both those with
static type declaration and those where intent is mainly expressed by
manually selecting the right operations.</p></div>
<div class="paragraph"><p>Intentional types are not optional; they are generally needed to make
sense of values. A programmer learns about them by reading
documentation; if a debugging tool needs to find out a intentional
type at run-time, there must be some way to find it.</p></div>
<div class="paragraph"><p>This means that declaration languages like D-Bus introspection
facilities and GConf schemas need to be extended to support our
intentional types. Thus, purely declarative languages like these are
also included in our list of supported languages.</p></div>
<div class="listingblock">
<div class="content"><!-- Generator: GNU source-highlight 3.1
by Lorenzo Bettini
http://www.lorenzobettini.it
http://www.gnu.org/software/src-highlite -->
<pre><tt><span style="font-style: italic"><span style="color: #9A1900">/* Witty example here. */</span></span></tt></pre></div></div>
<div class="paragraph"><p>We also give a list of common intentional types, of course.</p></div>
<div class="paragraph"><p>This document then has three dimensions of extensibility:</p></div>
<div class="ulist"><ul>
<li>
<p>
A new value system can be added by defining how the representational
part of the common type system maps to it.
</p>
</li>
<li>
<p>
A new language can be added by defining how intentional types are
implemented in it, and by implementing all common intentional types.
</p>
</li>
<li>
<p>
A new common intentional type can be added by defining it and
implementing it in all languages.
</p>
</li>
</ul></div>
<div class="paragraph"><p>The representational part of the common type system is not supposed to
change frequently, but adding a new intentional type should be
considered routine.</p></div>
<div class="paragraph"><p>The representation part of the common type system is restricted by the
lowest common denominator of all the value system implementations that
we want to include. We don’t want to distort the existing value
systems too much, and force people to write code that feels unnatural
for them.</p></div>
<div class="paragraph"><p>For example, not all value systems can directly represent complex
numbers or multiple precision integers, but any grown up type system
should include them. We solve this conflict by relying on the
intentional types: Instead of grafting complex numbers onto every
value system, we only define a intentional type for them.</p></div>
<div class="paragraph"><p>Currently supported value systems: QVariant, D-Bus messages, GValue,
GConfValue, GVariant, Python values, Perl values, JavaScript values,
GKeyFile, JSON, YAML, Nepomuk ontologies, SQL, SparQL, Common Lisp
values.</p></div>
<div class="paragraph"><p>Currently supported languages: Python, Perl, JavaScript, Java, C#, C<tt>
with QVariant, plain C</tt>, C with D-Bus/GValue/GConfValue/GVariant,
plain C, Vala, D-Bus introspection, D-Bus IDL (didl), GConf schema,
our own XML schema.</p></div>
</div>
</div>
<h2 id="_representational_types">Representational types</h2>
<div class="sectionbody">
<div class="paragraph"><p>Converting a value from one representation to another is not
guaranteed to be loss-less: if you convert the value back, it might be
different and even have a different type. Intentional types are used
to make sense of the value anyway. [ XXX - so maybe we shouldn’t
bother with representational types at all… ]</p></div>
<div class="paragraph"><p>Whenever there is a choice of representation in the following table,
it should be taken to mean: Represent the value with the first
alternative in the list that is possible, even if that loses
precision.</p></div>
<div class="paragraph"><p>For example, a 64 bit signed integer is represented in GConf as a
"int" if it fits, and as a "double" if not. It will always fit into a
double, but it might mean chopping off some low bits.</p></div>
<div class="ulist"><ul>
<li>
<p>
null
</p>
<div class="paragraph"><p>The null value.</p></div>
<div class="literalblock">
<div class="content">
<pre><tt>QVariant: QVariant::Null
D-Bus: '()'
GValue: G_TYPE_NONE
GConf: empty GCONF_VALUE_LIST with type GCONF_VALUE_BOOL
GVariant: '()'
Perl: undef
Python 2: None
CL: nil</tt></pre>
</div></div>
</li>
<li>
<p>
bool
</p>
<div class="paragraph"><p>A boolean</p></div>
<div class="literalblock">
<div class="content">
<pre><tt>QVariant: QVariant::Bool
D-Bus: 'b'
GValue: G_TYPE_BOOLEAN
GConf: GCONF_VALUE_BOOL
GVariant: 'b'
Perl: number, 0 or 1.
Python 2: number, 0 or 1.
CL: nil or t</tt></pre>
</div></div>
</li>
<li>
<p>
int32
</p>
<div class="paragraph"><p>Signed 32 bit integer</p></div>
<div class="literalblock">
<div class="content">
<pre><tt>QVariant: QVariant::Int
D-Bus: 'i'
GValue: G_TYPE_INT
GConf: GCONF_VALUE_INT
GVariant: 'i'
Perl: number
Python 2: int
CL: number</tt></pre>
</div></div>
</li>
<li>
<p>
int64
</p>
<div class="paragraph"><p>Signed 64 bit integer</p></div>
<div class="literalblock">
<div class="content">
<pre><tt>QVariant: QVariant::LongLong
D-Bus: 'x'
GValue: G_TYPE_INT64
GConf: GCONF_VALUE_INT or GCONF_VALUE_DOUBLE
GVariant: 'x'
Perl: number
Python 2: int or long
CL: number</tt></pre>
</div></div>
</li>
<li>
<p>
uint32
</p>
<div class="paragraph"><p>Unsigned 32 bit integer</p></div>
<div class="literalblock">
<div class="content">
<pre><tt>QVariant: QVariant::UInt
D-Bus: 'u'
GValue: G_TYPE_UINT
GConf: GCONF_VALUE_INT or GCONF_VALUE_DOUBLE
GVariant: 'u'
Perl: number
Python 2: int or long
CL: number</tt></pre>
</div></div>
</li>
<li>
<p>
uint64
</p>
<div class="paragraph"><p>Unsigned 64 bit integer</p></div>
<div class="literalblock">
<div class="content">
<pre><tt>QVariant: QVariant::ULongLong
D-Bus: 't'
GValue: G_TYPE_UINT64
GConf: GCONF_VALUE_INT or GCONF_VALUE_DOUBLE
GVariant: 't'
Perl: number
Python 2: int or long
CL: number</tt></pre>
</div></div>
</li>
<li>
<p>
double
</p>
<div class="paragraph"><p>Double precision floating point number</p></div>
<div class="literalblock">
<div class="content">
<pre><tt>QVariant: QVariant::Double
D-Bus: 'd'
GValue: G_TYPE_DOUBLE
GConf: GCONF_VALUE_DOUBLE
GVariant: 'd'
Perl: number
Python 2: double
CL: number</tt></pre>
</div></div>
</li>
<li>
<p>
string
</p>
<div class="paragraph"><p>String of Unicode code points</p></div>
<div class="literalblock">
<div class="content">
<pre><tt>QVariant: QVariant::QString
D-Bus: 's'
GValue: G_TYPE_STRING
GConf: GCONF_VALUE_STRING, UTF-8.
GVariant: 's'
Perl: string
Python 2: unicode
CL: string</tt></pre>
</div></div>
</li>
<li>
<p>
list
</p>
<div class="paragraph"><p>List of values</p></div>
<div class="literalblock">
<div class="content">
<pre><tt>QVariant: QVariant::List
D-Bus: 'av'
GValue: G_TYPE_POINTER pointing to a GSList of GValues.
(XXX - find something better, must be somewhere.)
GConf: GCONF_VALUE_LIST or chained GCONF_VALUE_PAIRs
GVariant: 'av'
Perl: array
Python 2: list
CL: list</tt></pre>
</div></div>
</li>
<li>
<p>
map
</p>
<div class="paragraph"><p>Mapping from strings to values, with no duplicated keys.</p></div>
<div class="literalblock">
<div class="content">
<pre><tt>QVariant: QVariant::Map
D-Bus: 'a{sv}'
GValue: G_TYPE_HASH_TABLE (?)
GConf: Chain of GCONF_VALUE_PAIRs,
with the cars being a pair of GCONF_VALUE_STRING and an
arbitrary value.
GVariant: 'a{sv}'
Perl: hash
Python: dict
CL: alist</tt></pre>
</div></div>
</li>
</ul></div>
</div>
<h2 id="_a_nano_dom">A Nano-DOM</h2>
<div class="sectionbody">
<div class="paragraph"><p>The representational types can be used as a Nano-DOM for a subset of
XML. This is useful when the small subset suffices but you still want
to be enterprise ready. Intentional type definitions use this subset,
for example, and are thus easily handled at run-time.</p></div>
<div class="paragraph"><p>Converting a piece of XML into its Nano-DOM representation proceeds
according to simple rules:</p></div>
<div class="ulist"><ul>
<li>
<p>
First, all attributes of elements are converted to child elements,
in order and at the beginning. Thus, the following XML fragments
are quivalent:
</p>
<div class="literalblock">
<div class="content">
<pre><tt><bar size="12">...</bar></tt></pre>
</div></div>
<div class="literalblock">
<div class="content">
<pre><tt><bar><size>12</size>...</bar></tt></pre>
</div></div>
</li>
<li>
<p>
Then, text is turned into strings, and elements are turned into
lists with the first element being a string with the name of the
element. For example, this XML
</p>
<div class="literalblock">
<div class="content">
<pre><tt><foo>hello</foo></tt></pre>
</div></div>
<div class="literalblock">
<div class="content">
<pre><tt>would be turned into this Python value</tt></pre>
</div></div>
<div class="literalblock">
<div class="content">
<pre><tt>['foo', 'hello']</tt></pre>
</div></div>
<div class="literalblock">
<div class="content">
<pre><tt>When creating the strings for text, surrounding whitespace is
removed.</tt></pre>
</div></div>
</li>
</ul></div>
<div class="paragraph"><p>More examples:</p></div>
<div class="literalblock">
<div class="content">
<pre><tt><key name="Example.Random"
type="string">
<doc>
A random property.
</doc>
</key></tt></pre>
</div></div>
<div class="literalblock">
<div class="content">
<pre><tt>=></tt></pre>
</div></div>
<div class="literalblock">
<div class="content">
<pre><tt>['key',
['name', 'Example.Random' ],
['type', 'string' ],
['doc', 'A random property.']
]</tt></pre>
</div></div>
<div class="literalblock">
<div class="content">
<pre><tt><key name="Example.Random">
<type>
<uniform-list type="number"/>
</type>
</key></tt></pre>
</div></div>
<div class="literalblock">
<div class="content">
<pre><tt>=></tt></pre>
</div></div>
<div class="literalblock">
<div class="content">
<pre><tt>['key',
['name', 'Example.Random' ],
['type',
['uniform-list', ['type', 'number' ] ]
]
]</tt></pre>
</div></div>
<div class="paragraph"><p>You can think of the Nano-DOM representation as a simple abstract
syntax tree for XML.</p></div>
</div>
<h2 id="_intentional_types">Intentional types</h2>
<div class="sectionbody">
<div class="paragraph"><p>The most important part by far of a intentional type definition is its
documentation. The documentation is the thing that explains the
intent to programmers, so that they can <em>reify</em> the abstract
intentional type into concrete code. For example, by reading the
documentation, they know how to write a C++ class for the intentional
type and add useful methods to it, or how to write a UI widget that
allows displaying and maybe high-level editing of values of that type.</p></div>
<div class="paragraph"><p>Intentional types are <em>not</em> a static type system. They are only a
tool for cross-referencing documentation. Sometimes, intentional
types are mapped into a static type system and the compiler will then
perform some additonal checks at compile time, and the code using the
types might look more natural, but that is not the main goal of the
intentional types.</p></div>
<div class="paragraph"><p>In essence, intentional types use English as the <em>formal</em> language to
express their definitions. Their documentation should basically be a
description of the set of values that are permissible for this type
(by referring to other already defined intentional types or the
representational types from above), and what they mean. For example,
the "calendar-time" type could say that only "uint64" values are
allowed, and that they are the number of nano-seconds since midnight
January 1, UTC.</p></div>
<div class="paragraph"><p>Another example are enumerations: the documentation of
"compass-direction" can say that the value is one of the four "int32"
values 0, 90, 180, 270 where 0 means North, 90 means East, etc.</p></div>
<div class="paragraph"><p>As shown in the examples, intentional types have names, so that you
can refer to them in the documentation of other types and in other
places that refer to intentional types, such as in D-Bus introspection
declarations.</p></div>
<div class="paragraph"><p>When other people refer to your type, they can provide a set of
parameters to specialize it. You should document which parameters are
meaningful and what they do, of course. You should also formally
declare which paramaters are valid. (See below for concrete
examples).</p></div>
<div class="paragraph"><p>Type parameters allow us to define a small set of fundamental and
general types, which can be instantiated to create a wide range of
useful types. For example, there is a generic "int-enum" type that
can be turned into a specific enumeration via its parameters. A
single UI widget could be written for "int-enum" that is then
(automatically) parameterized at run-time with the concrete set of
choices. The "int-enum" type is defined so that its parameters
include the text to use for each enumeration choice, and the UI widget
will get access to these parameters at run-time (as a map,
incidentally).</p></div>
<div class="paragraph"><p>A intentional type definition can specify a "base" type for itself, by
referring to another intentional type. This base can be used to make
the documentation a bit more formal, and of course to provide
parameters for the base type. For example, the documentation for the
"compass-direction" type would not need to explicitly say that the
numbers are "int32"s; it would just declare its base to be "int32".
Even better, it sould say that it’s actually a "int-enum" and specify
the concrete values.</p></div>
<div class="paragraph"><p>In a context where a type is expected:</p></div>
<div class="literalblock">
<div class="content">
<pre><tt>NAME - refers to type named NAME
<NAME>PARM...</NAME> - refers to type named NAME, specialized
with PARM...</tt></pre>
</div></div>
<div class="paragraph"><p>Attributes for type definitions:</p></div>
<div class="literalblock">
<div class="content">
<pre><tt>name - the name (string)
parms - valid paramaters (map from parm name to map of ...)
doc - documentation (either string, or a map language code -> string)
base - the base type (type)</tt></pre>
</div></div>
<div class="paragraph"><p>As an example, consider a hypothetical XML schema for describing
key-value pairs. Let’s also assume that this schema follows our
Nano-DOM rules. It has a "key" element which needs name, doc and type
attributes. The "type" attribute should refer to an intentional type
of course. We can describe a key for the current temperature,
expressed as one of "low", "medium", "high", in the following ways.</p></div>
<div class="paragraph"><p>First, we can refer to the predefined "three-level-enum" type, if
there would be such a type. Documentation of the possible values is
left to the definition of "three-level-enum", which presumably would
tell us that they are the strings "low", "medium", and "high".</p></div>
<div class="literalblock">
<div class="content">
<pre><tt><key>
<name>Temperature</name>
<doc>The current temperature.</doc>
<type>three-level-enum</type>
<key></tt></pre>
</div></div>
<div class="paragraph"><p>Using the Nano-DOM rules, this can be shortened to:</p></div>
<div class="literalblock">
<div class="content">
<pre><tt><key name="Temperature"
doc="The current temperature"
type="three-level-enum"/></tt></pre>
</div></div>
<div class="paragraph"><p>Instead of referring to the pre-defined "three-level-enum" type, we
can instantiate a "string-enum" explicitly, which is one of the
pre-defined generic types.</p></div>
<div class="literalblock">
<div class="content">
<pre><tt><key name="Temperature"
doc="The current temperature">
<type>
<string-enum>
<low doc="Brrrr"/>
<medium doc="Comfy."/>
<high doc="Siesta!"/>
</string-enum>
</type>
</key></tt></pre>
</div></div>
<div class="paragraph"><p>The common intentional types are defined in XML, as a list of "type"
elements that have "name", "parms", "doc", and "base" child elements,
as expected.</p></div>
<div class="paragraph"><p>In the following, we give the type definitions verbatim, as XML, as an
extended example (and because the real XML definition of the types
does not exist yet). In the future, this part of the document will be
generated from the type definitions and will look nicer.</p></div>
<h3 id="_fundamental_types">Fundamental types</h3><div style="clear:left"></div>
<div class="literalblock">
<div class="content">
<pre><tt><typedef name="int32">
<doc>
A int32 value.
</doc>
</type></tt></pre>
</div></div>
<div class="literalblock">
<div class="content">
<pre><tt><type name="int64">
<doc>
A int64 value.
</doc>
</type></tt></pre>
</div></div>
<div class="literalblock">
<div class="content">
<pre><tt><type name="uint32">
<doc>
A uint32 value within the given limits.
</doc>
</type></tt></pre>
</div></div>
<div class="literalblock">
<div class="content">
<pre><tt><type name="uint64">
<params>
<min doc="Minimum value"/>
<max doc="Maximum value"/>
<params>
<doc>
A uint64 value within the given limits.
</doc>
</type></tt></pre>
</div></div>
<div class="literalblock">
<div class="content">
<pre><tt><type name="double">
<params>
<min doc="Minimum value"/>
<max doc="Maximum value"/>
<params>
<doc>
A double value within the given limits.
</doc>
</type></tt></pre>
</div></div>
<div class="literalblock">
<div class="content">
<pre><tt><type name="string">
<parms>
<must-match doc="Regular expression that must match"/>
</parms>
<doc>
A string value that matches the given regular expression.
</doc>
</type></tt></pre>
</div></div>
<div class="literalblock">
<div class="content">
<pre><tt><type name="list">
<params>
<min doc="Minimum length"/>
<max doc="Maximum length"/>
<params>
<doc>
A list of arbitrary values, with the minimum and maximum
length given by the "min" and "max" parameters, respectively.
</doc>
</type></tt></pre>
</div></div>
<div class="literalblock">
<div class="content">
<pre><tt><type name="map">
<params>
<keys doc="Allowed keys"/>
</params>
<doc>
A map. If given, the "keys" parameter determines which keys are
allowed. The "keys" parameter should be a map itself, from key
names to a map with the attributes of the key. Attributes of a
key are "doc" and "type".
</doc>
</type></tt></pre>
</div></div>
<h3 id="_generic_types">Generic types</h3><div style="clear:left"></div>
<div class="literalblock">
<div class="content">
<pre><tt><type name="value">
<doc>
Any representable value.
</doc>
</type></tt></pre>
</div></div>
<div class="literalblock">
<div class="content">
<pre><tt><type name="number" base="value">
<doc>
A number, represented as either a "int32", "uint32", "int64", "uint64", or "double".
</doc>
</type></tt></pre>
</div></div>
<div class="literalblock">
<div class="content">
<pre><tt><type name="integer" base="number">
<params>
<min doc="Lower bound"/>
<max doc="Upper bound"/>
</params>
<doc>
A integer, represented as any of the numeric types. If the value
is a "double", it is rounded to an integer, but not necessarily to
the nearest. The "min" and "max" parameters, when given, constrain
the range of the integer.
</doc>
</type></tt></pre>
</div></div>
<div class="literalblock">
<div class="content">
<pre><tt><type name="uniform-list" base="list">
<params>
<min doc="Minimum length"/>
<max doc="Maximum length"/>
<type doc="Type of the elements"/>
</params>
<doc>
A list of values of the given type, with the
minimum and maximum length given by the "min" and "max" parameters.
The type of all elements is given by the "type" parameter.
</doc>
</type></tt></pre>
</div></div>
<div class="literalblock">
<div class="content">
<pre><tt><type name="string-enum" base="string">
<parms>
<rest doc="The possible values">
</parms>
<doc>
This is the base type for enumerations of fixed strings. The
parameters describe the possible values. Each parameter is one
of the choices: the name of the parameter is the string for the choice
itself and the value of the parameter is a map with further
information of that choice, such as a "doc" entry.
</doc>
</type></tt></pre>
</div></div>
<div class="literalblock">
<div class="content">
<pre><tt><type name="int-enum" base="int32">
<parms>
<rest doc="The possible values">
</parms>
<doc>
This is the base type for enumerations of fixed integers. The
parameters describe the possible values. Each parameter is one
of the choices: the name of the parameter is the symbolic name
for the choice itself and the value of the parameter is a map
with further information of that choice, such as a "val" entry
for the numerical value for that choice, and a "doc" entry.
</doc>
</type></tt></pre>
</div></div>
<h3 id="_specific_types">Specific types</h3><div style="clear:left"></div>
<div class="literalblock">
<div class="content">
<pre><tt><type name="type">
<doc>
A type, represented as a map. The map is the one you get as the Nano-DOM
for the type definiton. E.g., it will have "doc" mapped to a string,
and "base" mapped to either a string or another map, etc.
</doc>
<base>
<map>
<allowed-keys>
<name/>
<parms/>
<doc/>
<base/>
</allowed-keys>
</map>
</base>
</type></tt></pre>
</div></div>
<div class="literalblock">
<div class="content">
<pre><tt><type name="geoloc">
<doc>
A list of two or three doubles giving a geographical location.
The first number is latitude, the second longitude, both in degrees.
If a third number is present, it is the altitude in meters.
</doc>
<base>
<uniform-list min="2" max="3" type="double"/>
</base>
</type></tt></pre>
</div></div>
<div class="literalblock">
<div class="content">
<pre><tt><type name="temperature"
base="double"
doc="A temperature in Kelvin."/></tt></pre>
</div></div>
<div class="literalblock">
<div class="content">
<pre><tt><type name="screen-edge">
<doc>
Indicates an edge of a rectangular screen, relative to
the natural orientation of the video hardware driving it.
It can be one of the four strings "top", "left", "right",
and "bottom".
</doc>
<base>
<string-enum>
<top/> <left/> <right/> <bottom/>
</string-enum>
</base>
</type></tt></pre>
</div></div>
<div class="literalblock">
<div class="content">
<pre><tt><type name="screen-edge-ints">
<doc>
Indicates an edge of a rectangular screen, relative to
the natural orientation of the video hardware driving it.
It can be one of the four values "top", "left", "right",
and "bottom", encoded as an integer.
</doc>
<base>
<int-enum>
<top val="0"/> <left val="1"/> <right val="2"/> <bottom val="3"/>
</string-enum>
</base>
</type></tt></pre>
</div></div>
<div class="literalblock">
<div class="content">
<pre><tt><type name="energy" base="double">
<doc>
An amount of energy, in Joule.
</doc>
</type></tt></pre>
</div></div>
<div class="literalblock">
<div class="content">
<pre><tt><type name="power" base="double">
<doc>
A power, in Watt.
</doc>
</type></tt></pre>
</div></div>
<div class="literalblock">
<div class="content">
<pre><tt><type name="time" base="int64">
<doc>
A point in time, represented as the number of nano-seconds since
00:00 January 1, 1970, UTC.
</doc>
</type></tt></pre>
</div></div>
<div class="literalblock">
<div class="content">
<pre><tt><type name="duration" base="uint64">
<doc>
A time duration, in nano-seconds.
</doc>
</type></tt></pre>
</div></div>
<div class="literalblock">
<div class="content">
<pre><tt><type name="percentage" base="int32">
<doc>
A percentage.
</doc>
</type></tt></pre>
</div></div>
</div>
<h2 id="_intentional_types_and_static_languages">Intentional types and static languages</h2>
<div class="sectionbody">
<div class="paragraph"><p>The normal use of intentional types is as follows: while programming
in some language, you read some API documentation and find out that
some argument to a function is of type "geoloc"; you then go to the
documentation of "geoloc" and read how a "geoloc" works in your
language.</p></div>
<div class="paragraph"><p>For C++ with QVariants, say, a geoloc could simply remain a list of
two or three doubles, or it could be a QVariant::RectF with the height
being abused as the altitude, a new QMetaType could be invented, or a
completely new class could be defined that can be converted to and
from a QVariant (together with an appropriate QMetaType). In any
case, there will be a QVariant involved in there somewhere.</p></div>
<div class="paragraph"><p>In other words, we need C++ language bindings for the intentional
types. These language bindings are maintained together with the
intentional types.</p></div>
<div class="paragraph"><p>At the language boundaries, such as when marshalling and unmarshalling
values for a D-Bus message, a geoloc value needs to be converted to
and from a list of doubles. This conversion code is also maintained
together with the language bindings. In any case, the types are well
enough documented that the necessary code can be written manually if
needed.</p></div>
<h3 id="_c_with_gvariant">C with GVariant</h3><div style="clear:left"></div>
<div class="ulist"><ul>
<li>
<p>
geoloc
</p>
<div class="literalblock">
<div class="content">
<pre><tt>typedef struct {
double latitude;
double longitude;
double altitude; // NaN if unknown.
} DGeoLoc;</tt></pre>
</div></div>
</li>
</ul></div>
<div class="paragraph"><p>Rest is done with inheritance:</p></div>
<div class="ulist"><ul>
<li>
<p>
duration
</p>
<div class="literalblock">
<div class="content">
<pre><tt>typedef int64_t DDuration;</tt></pre>
</div></div>
</li>
<li>
<p>
screen-edge
</p>
<div class="literalblock">
<div class="content">
<pre><tt>typedef char *DScreenEdge;</tt></pre>
</div></div>
</li>
<li>
<p>
screen-edge-ints
</p>
<div class="literalblock">
<div class="content">
<pre><tt>typedef int DScreenEdgeInts;
#define D_SCREEN_EDGE_TOP 0
#define D_SCREEN_EDGE_LEFT 1
#define D_SCREEN_EDGE_RIGHT 2
#define D_SCREEN_EDGE_BOTTOM 3</tt></pre>
</div></div>
</li>
<li>
<p>
percentage
</p>
<div class="literalblock">
<div class="content">
<pre><tt>typedef int DPercentage;</tt></pre>
</div></div>
</li>
<li>
<p>
type
</p>
<div class="literalblock">
<div class="content">
<pre><tt>typedef GVariant DGType;</tt></pre>
</div></div>
</li>
</ul></div>
<div class="paragraph"><p>These definitions are in libdesktoptypes, built from the desktop-types
source package, which includes everything else as well, such as this
document, bindings for C++ with QVariant, etc.</p></div>
<div class="paragraph"><p>There is also g_variant_from_geoloc, etc.</p></div>
<div class="paragraph"><p>Then, libcontextprovider has context_provider_set_geoloc,
context_provider_set_duration, etc, probably as macros using
g_variant_from_geoloc. Maybe we can have support for doing this
automatically.</p></div>
<h3 id="_c_with_qvariant">C++ with QVariant</h3><div style="clear:left"></div>
<div class="paragraph"><p>Types should be compatible with C and have the same names.</p></div>
<div class="ulist"><ul>
<li>
<p>
geoloc
</p>
<div class="literalblock">
<div class="content">
<pre><tt>struct DGeoLoc {
double latitude;
double longitude;
double altitude; // NaN if unknown.</tt></pre>
</div></div>
<div class="literalblock">
<div class="content">
<pre><tt>DGeoLoc();
DGeoLoc(double, double);
DGeoLoc(double, double, double);</tt></pre>
</div></div>
<div class="literalblock">
<div class="content">
<pre><tt> DGeoLoc (QVariant);
operator QVariant ();
};</tt></pre>
</div></div>
<div class="paragraph"><p>Registered as "DGeoLoc" with QMetaType.</p></div>
</li>
<li>
<p>
rest identical with C, except "type", hmm…
</p>
</li>
</ul></div>
<div class="paragraph"><p>This is in libdesktoptypes-qt, built from the same desktop-types as
the C bindings.</p></div>
</div>
<h2 id="_run_time_introspection_for_intentional_types">Run-time introspection for intentional types</h2>
<div class="sectionbody">
<div class="paragraph"><p>libdesktoptypes contains an API for reading a type definition
repository, in /usr/share/desktop-types/. It can lookup DType values
given a name.</p></div>
<div class="paragraph"><p>A DType contains name, doc, parms, and base, of course.</p></div>
<div class="paragraph"><p>Existing introspection APIs need to be extended to return DTypes, as
well.</p></div>
</div>
<div id="footer">
<div id="footer-text">
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