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	<?xml version="1.0" encoding="UTF-8"?>
	<chapter version="5.1" xmlns="http://docbook.org/ns/docbook"
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	<?xxe-sn 26n42fz1reo 1?>

	<title><?xxe-sn 26n42fz1reo 2?>Polymorphism</title>

	<para><?xxe-sn 2be091dx6gw 1?>See <link
	xlink:href="/d/syntax/#function-specializations"><?xxe-sn 2be091dx6gw 2?>function
	specializations</link> for syntax.</para>

	<para><?xxe-sn 26n42fz1reo 3?><emphasis><?xxe-sn 26n42fz1reo 4?>Polymorphism</emphasis>
	is an umbrella term to denote a number of techniques across different
	programing paradigms. They all share the same intention to provide the
	ability to easily recombine software components in a different way with as
	little manual work on developer's side as possible. It serves two major
	goals: <emphasis><?xxe-sn 26n42fz1reo 5?>specialization</emphasis>, which
	purports that software initially designed to support a wide range of use
	cases is being configured for a specific case, and <emphasis><?xxe-sn 26n42fz1reo 6?>extension</emphasis>
	– adapting software to an environment and conditions it was not specifically
	designed for.</para>

	<para><?xxe-sn 26n42fz1reo 7?>In course of software engineering evolution, a
	number of polymorphism techniques was proposed and experimented with, all
	suited for different use-cases. Xreate presents a generalized and elegant
	approach that exhaustively covers the wide landscape of polymorphism
	variations.</para>

	<para><?xxe-sn 26n42fz1reo 8?>Polymorphism in Xreate can be applied on two
	levels:</para>

	<itemizedlist>
	<?xxe-sn 26n42fz1reo 9?>

	<listitem>
	<?xxe-sn 26n42fz1reo a?>

	<para><?xxe-sn 26n42fz1reo b?><emphasis><?xxe-sn 26n42fz1reo c?>Functions
	level</emphasis>. Function in Xreate can have multiple
	<emphasis><?xxe-sn 26n42fz1reo d?>specializations</emphasis>, and
	polymorphism is compiler's ability to decide which exactly
	specialization to use depending on various factors</para>
	</listitem>

	<listitem>
	<?xxe-sn 26n42fz1reo e?>

	<para><?xxe-sn 26n42fz1reo f?><emphasis><?xxe-sn 26n42fz1reo g?>Modules
	level</emphasis>. Multiple modules can provide the same service for
	users. <link
	xlink:href="/d/syntax/modules/#modules-resolution"><?xxe-sn 26n42fz1reo h?>Modules
	Resolution</link> is a process to decide which exactly module to
	use.</para>

	<remark><?xxe-sn 26vapv2jrow pu35zrt1aby9?>По поводу Modules Resolution:
	кажется в предыдущем файле я внес несколько исправлений, добавив
	апостроф после Modules. По грамматическим правилам это верно, но если
	Modules Resolution это устоявшийся термин в твоей документации, то
	возможно лучше без апострофа.</remark>
	</listitem>
	</itemizedlist>

	<section>
	<?xxe-sn 26n42fz1reo i?>

	<title><?xxe-sn 26n42fz1reo j?>Function Level Polymorphism</title>

	<para><?xxe-sn 26n42fz1reo k?>Basic idea is to allow developer to define
	several functions with the same name or, in other words, several
	<emphasis><?xxe-sn 26n42fz1reo l?>specializations</emphasis>. Caller code
	then invokes the necessary function by its shared name, but cannot specify
	a particular specialization directly. The exact specialization to be
	invoked is decided later by a decision process called <emphasis><?xxe-sn 26n42fz1reo m?>polymorphism
	resolution</emphasis> carried out by Transcend. This indirect invocation
	approach gives enough flexibility to use or replace different
	specializations depending on various conditions during compile time as
	well as at runtime.</para>

	<para><?xxe-sn 26n42fz1reo n?>Each specialization must have unique
	<emphasis><?xxe-sn 26n42fz1reo p?>guard</emphasis> (among all
	specializations with the same name) to be discernible from others. To
	summarize, function invocation is a two-layered process, where the client
	code specifies the callee function's shared name, while polymorphism
	resolution specifies specialization guard, if needed.</para>

	<para><?xxe-sn 26n42fz1reo q?>For example, let us assume that we have been
	developing a software program to operate under specified time constraints.
	To model an implementation suitable for real time environment, one
	specialization of <code><?xxe-sn 26n42fz1reo r?>crucialOperation</code> is
	defined with <code><?xxe-sn 26n42fz1reo s?>env(realtime)</code> guard,
	i.e. it satisfies some fixed execution time constraints. Caller
	<code><?xxe-sn 26n42fz1reo t?>main</code> specifies just a function name
	<code><?xxe-sn 26n42fz1reo u?>crucialOperation</code> thus delegating the
	decision in respect of guard to polymorphism resolution done elsewhere,
	based on the environment's constraints the code is executed in.</para>

	<programlisting xml:id="FnLvlPoly_1"><?xxe-sn 26n42fz1reo v?>name="tests/polymorph.cpp: Polymorphs.Doc_FnLvlPoly_1"
	guard:: env(realtime)
	{
	crucialOperation = function:: int
	{ 0 }
	}

	main = function:: int; entry
	{
	crucialOperation()
	} </programlisting>
	</section>

	<section>
	<?xxe-sn 26n42fz1reo w?>

	<title><?xxe-sn 26n42fz1reo x?>Polymorphism Resolution</title>

	<synopsis><?xxe-sn 26n42fz1reo y?>SYNTAX:
	dfa_callguard(//call-site-ref//, //guard//)</synopsis>

	<itemizedlist>
	<?xxe-sn 26n42fz1reo z?>

	<listitem>
	<?xxe-sn 26n42fz1reo 10?>

	<para><?xxe-sn 26n42fz1reo 11?><emphasis><?xxe-sn 26n42fz1reo 12?>call-site-ref</emphasis>
	reference to a call site in AST</para>
	</listitem>

	<listitem>
	<?xxe-sn 26n42fz1reo 13?>

	<para><?xxe-sn 26n42fz1reo 14?><emphasis><?xxe-sn 26n42fz1reo 15?>guard</emphasis>
	resolved function specialization guard</para>
	</listitem>
	</itemizedlist>

	<para><?xxe-sn 26n42fz1reo 16?>When compiler encounters function
	invocation that has several specializations it refers to the table
	<code><?xxe-sn 26n42fz1reo 17?>dfa_callguard</code> to find out which
	specialization to call. It must have an entry with appropriate
	<code><?xxe-sn 26n42fz1reo 18?>guard</code> for every invocation site
	<code><?xxe-sn 26n42fz1reo 19?>call-site-ref</code> of a polymorphic
	function. Polymorphism resolution is a process of filling out
	<code><?xxe-sn 26n42fz1reo 1a?>dfa_callguard</code> for the compiler based
	on custom Transcend rules reflecting one or another polymorphism
	strategy.</para>
	</section>

	<section>
	<?xxe-sn 26n42fz1reo 1b?>

	<title><?xxe-sn 26n42fz1reo 1c?>Late Polymorphism</title>

	<para><?xxe-sn 26n42fz1reo 1d?>Late Polymorphism is an extension to allow
	polymorphism resolution to be based on data known only at runtime, i.e.
	resolve function specializations dynamically. The Idea is to use Late
	Transcend to access runtime data. See <link
	xlink:href="/d/transcend/latetranscend/"><?xxe-sn 26n42fz1reo 1e?>Late
	Transcend</link> for details.</para>

	<para><?xxe-sn 26n42fz1reo 1f?>Example below demonstrates <code><?xxe-sn 26n42fz1reo 1g?>test</code>
	invoking polymorphic function <code><?xxe-sn 26n42fz1reo 1h?>compute</code>:</para>

	<programlisting xml:id="LatePoly_1"><?xxe-sn 26n42fz1reo 1i?>name="tests/polymorph.cpp: Polymorphs.Doc_LatePoly_1"
	Strategy = type variant {fast, precise}.

	guard:: fast
	{
	compute = function:: int
	{0}
	}

	guard:: precise
	{
	compute = function:: int
	{1}
	}

	test = function(s:: Strategy; alias(strategy)):: int; entry
	{
	switch late (s):: int
	{
	compute():: int; guardalias(strategy)
	}
	}</programlisting>

	<para><?xxe-sn 26n42fz1reo 1j?>Function <code><?xxe-sn 26n42fz1reo 1k?>compute</code>
	has two specializations, <code><?xxe-sn 26n42fz1reo 1l?>fast</code> and
	<code><?xxe-sn 26n42fz1reo 1m?>precise</code>. We see that <code><?xxe-sn 26n42fz1reo 1n?>test</code>
	gets parameter <code><?xxe-sn 26n42fz1reo 1o?>s</code> that dictates exact
	strategy to use. Clearly, resolution should work dynamically to deal with
	cases like this, for not only the value of the parameter <code><?xxe-sn 26n42fz1reo 1p?>s</code>
	is unknown at the compile time, but also it can change with each
	<code><?xxe-sn 26n42fz1reo 1q?>test</code> execution.</para>

	<para><?xxe-sn 26n42fz1reo 1r?>Operation <code><?xxe-sn 2be091dx6gw 3?>switch late</code>
	is compiled into several branches, two in this case, each branch executing
	appropriate <code><?xxe-sn 26n42fz1reo 1s?>compute</code> specialization.
	The correct branch is executed depending on the current <code><?xxe-sn 26n42fz1reo 1t?>s</code>
	value. Custom annotations <code><?xxe-sn 26n42fz1reo 1u?>alias(Alias)</code>
	and <code><?xxe-sn 26n42fz1reo 1v?>guardalias(Alias)</code> are used to
	assign an alias in order to specify which parameter to use as basis for
	resolution.</para>
	</section>

	<section>
	<?xxe-sn 26n42fz1reo 1w?>

	<title><?xxe-sn 26n42fz1reo 1x?>Auto Expansion of Late Parameters</title>

	<para><?xxe-sn 26n42fz1reo 1y?>In the previous example, operation
	<code><?xxe-sn 2be091dx6gw 4?>switch late</code> was used to facilitate
	calling of a polymorphic function with late polymorphism resolution. It is
	not that convenient to wrap each invocation using <code><?xxe-sn 2be3dypnsao 1?>switch
	late</code> whenever we need to call a late polymorphic function. Compiler
	uses the late parameter auto expansion technique in order to specifically
	handle cases like this.</para>

	<para><?xxe-sn 26n42fz1reo 1z?>If compiler discovers that <code><?xxe-sn 26n42fz1reo 20?>late(dfa_callguard())</code>
	entry exists for current invocation, and it does not have enclosing
	<code><?xxe-sn 2be091dx6gw 5?>switch late</code> already, compiler
	automatically generates different branches that invoke relevant
	specializations and transfers control to a branch depending on the late
	parameter's value. In other words, invocation is implicitly wrapped into
	<code><?xxe-sn 2be091dx6gw 6?>switch late</code> if needed.</para>
	</section>
	</chapter>
	<?xxe-revisions
	#5 2019-03-07T17:53:18Z pgess
	#4 2019-03-07T17:09:57Z pgess
	1sPEAAABzxMAJM5rAAEQDVNzwgojBRMMU4Zt1VMFY4VXAMcao16ePpdskCeHIA==

	#3 2019-01-04T15:38:33Z Acer2
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	#2 2018-12-16T15:30:15Z Acer2
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	#1 2018-12-12T21:49:13Z pgess
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	?>

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