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<title>SWI-Prolog 7.3.6 Reference Manual: Section C.1</title><link rel="home" href="index.html">
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<h2 id="sec:portabilitystrategies"><a id="sec:C.1"><span class="sec-nr">C.1</span> <span class="sec-title">Some
considerations for writing portable code</span></a></h2>
<a id="sec:portabilitystrategies"></a>
<p>The traditional way to write portable code is to define custom
predicates for all potentially non-portable code and define these
separately for all Prolog dialects one wishes to support. Here are some
considerations.
<p>
<ul class="latex">
<li>Probably the best reason for this is that it allows to define
minimal semantics required by the application for the portability
predicates. Such functionality can often be mapped efficiently to the
target dialect. Contrary, if code was written for dialect <var>X</var>,
the defined semantics are those of dialect <var>X</var>. Emulating all
extreme cases and full error handling compatibility may be tedious and
result in a much slower implementation that needed. Take for example
<a id="idx:callcleanup2:2063"></a><a class="pred" href="metacall.html#call_cleanup/2">call_cleanup/2</a>.
The SICStus definition is fundamentally different from the SWI
definition, but 99% of the applications just want to make calls like
below to guarantee <var>StreamIn</var> is closed, even if <span class="pred-ext">process/1</span>
misbehaves.
<pre class="code">
call_cleanup(process(StreamIn), close(In))
</pre>
<p>
<li>As a drawback, the code becomes full of <i>my_call_cleanup</i>, etc. and
every potential portability conflict needs to be abstracted. It is hard
for people who have to maintain such code later to grasp the exact
semantics of the <i>my_*</i> predicates and applications that combine
multiple libraries using this compatibility approach are likely to
encounter conflicts between the portability layers. A good start is not
to use <i>my_*</i>, but a prefix derived from the library or application
name or names that explain the intended semantics more precisely.
<p>
<li>Another problem is that most code is initially not written with
portability in mind. Instead, ports are requested by users or arise from
the desire to switch Prolog dialect. Typically, we want to achieve
compatibility with the new Prolog dialect with minimal changes, often
keeping compatibility with the original dialect(s). This problem is well
known from the C/Unix world and we advise anyone to study the philosophy
of
<a class="url" href="http://www.gnu.org/software/autoconf/">GNU autoconf</a>,
from which we will illustrate some highlights below.
</ul>
<p>The GNU autoconf suite, known to most people as <b>configure</b>, was
an answer to the frustrating life of Unix/C programmers when Unix
dialects were about as abundant and poorly standardised as Prolog
dialects today. Writing a portable C program can only be achieved using
cpp, the C preprocessor. The C preprocessor performs two tasks: macro
expansion and conditional compilation. Prolog realises macro expansion
through <a id="idx:termexpansion2:2064"></a><a class="pred" href="consulting.html#term_expansion/2">term_expansion/2</a>
and <a id="idx:goalexpansion2:2065"></a><a class="pred" href="consulting.html#goal_expansion/2">goal_expansion/2</a>.
Conditional compilation is achieved using <code>:- if(Condition)</code>
as explained in
<a class="sec" href="consulting.html">section 4.3.1.2</a>. The situation
appears similar.
<p>The important lesson learned from GNU autoconf is that the <em>last</em>
resort for conditional compilation to achieve portability is to switch
on the platform or dialect. Instead, GNU autoconf allows you to write
tests for specific properties of the platform. Most of these are whether
or not some function or file is available. Then there are some standard
tests for difficult-to-write-portable situations and finally there is a
framework that allows you to write arbitrary C programs and check
whether they can be compiled and/or whether they show the intended
behaviour. Using a separate <b>configure</b> program is needed in C, as
you cannot perform C compilation step or run C programs from the C
preprocessor. In most Prolog environments we do not need this
distinction as the compiler is integrated into the runtime environment
and Prolog has excellent reflexion capabilities.
<p>We must learn from the distinction to test for features instead of
platform (dialect), as this makes the platform-specific code robust for
future changes of the dialect. Suppose we need <a id="idx:compare3:2066"></a><a class="pred" href="compare.html#compare/3">compare/3</a>
as defined in this manual. The <a id="idx:compare3:2067"></a><a class="pred" href="compare.html#compare/3">compare/3</a>
predicate is not part of the ISO standard, but many systems support it
and it is not unlikely it will become ISO standard or the intended
dialect will start supporting it. GNU autoconf strongly advises to test
for the availability:
<pre class="code">
:- if(\+current_predicate(_, compare(_,_,_))).
compare(<, Term1, Term2) :-
Term1 @< Term2, !.
compare(>, Term1, Term2) :-
Term1 @> Term2, !.
compare(=, Term1, Term2) :-
Term1 == Term2.
:- endif.
</pre>
<p>This code is <b>much</b> more robust against changes to the intended
dialect and, possibly at least as important, will provide compatibility
with dialects you didn't even consider porting to right now.
<p>In a more challenging case, the target Prolog has <a id="idx:compare3:2068"></a><a class="pred" href="compare.html#compare/3">compare/3</a>,
but the semantics are different. What to do? One option is to write a
<a id="idx:mycompare3:2069"></a><span class="pred-ext">my_compare/3</span>
and change all occurrences in the code. Alternatively you can rename
calls using <a id="idx:goalexpansion2:2070"></a><a class="pred" href="consulting.html#goal_expansion/2">goal_expansion/2</a>
like below. This construct will not only deal with Prolog dialects
lacking <a id="idx:compare3:2071"></a><a class="pred" href="compare.html#compare/3">compare/3</a>
as well as those that only implement it for numeric comparison or have
changed the argument order. Of course, writing rock-solid code would
require a complete test-suite, but this example will probably cover all
Prolog dialects that allow for conditional compilation, have core ISO
facilities and provide <a id="idx:goalexpansion2:2072"></a><a class="pred" href="consulting.html#goal_expansion/2">goal_expansion/2</a>,
the things we claim a Prolog dialect should have to start writing
portable code for it.
<pre class="code">
:- if(\+catch(compare(<,a,b), _, fail)).
compare_standard_order(<, Term1, Term2) :-
Term1 @< Term2, !.
compare_standard_order(>, Term1, Term2) :-
Term1 @> Term2, !.
compare_standard_order(=, Term1, Term2) :-
Term1 == Term2.
goal_expansion(compare(Order, Term1, Term2),
compare_standard_order(Order, Term1, Term2)).
:- endif.
</pre>
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