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<H1><a name="Modula3"></a>23 SWIG and Modula-3</H1>
<!-- INDEX -->
<div class="sectiontoc">
<ul>
<li><a href="#modula3_overview">Overview</a>
<ul>
<li><a href="#whyscripting">Why not scripting ?</a>
<li><a href="#whymodula3">Why Modula-3 ?</a>
<li><a href="#whycpp">Why C / C++ ?</a>
<li><a href="#whyswig">Why SWIG ?</a>
</ul>
<li><a href="#conception">Conception</a>
<ul>
<li><a href="#cinterface">Interfaces to C libraries</a>
<li><a href="#cppinterface">Interfaces to C++ libraries</a>
</ul>
<li><a href="#preliminaries">Preliminaries</a>
<ul>
<li><a href="#compilers">Compilers</a>
<li><a href="#commandline">Additional Commandline Options</a>
</ul>
<li><a href="#modula3_typemaps">Modula-3 typemaps</a>
<ul>
<li><a href="#inoutparam">Inputs and outputs</a>
<li><a href="#ordinals">Subranges, Enumerations, Sets</a>
<li><a href="#class">Objects</a>
<li><a href="#imports">Imports</a>
<li><a href="#exceptions">Exceptions</a>
<li><a href="#typemap_example">Example</a>
</ul>
<li><a href="#hints">More hints to the generator</a>
<ul>
<li><a href="#features">Features</a>
<li><a href="#pragmas">Pragmas</a>
</ul>
<li><a href="#remarks">Remarks</a>
</ul>
</div>
<!-- INDEX -->
<p>
This chapter describes SWIG's support of
<a href="http://www.m3.org/">Modula-3</a>.
You should be familiar with the
<a href="SWIG.html#SWIG">basics</a>
of SWIG,
especially
<a href="Typemaps.html">typemaps</a>.
</p>
<H2><a name="modula3_overview"></a>23.1 Overview</H2>
<p>
The Modula-3 support is very basic and highly experimental!
Many features are still not designed satisfyingly
and I need more discussion about the odds and ends.
Don't rely on any feature, incompatible changes are likely in the future!
The Modula-3 generator was already useful for interfacing
to the libraries
</p>
<ol>
<li>
<a href="http://www.elegosoft.com/cgi-bin/cvsweb.cgi/cm3/m3-libs/plplot/">
PLPlot
</a>
</li>
<li>
<a href="http://www.elegosoft.com/cgi-bin/cvsweb.cgi/cm3/m3-libs/fftw/">
FFTW
</a> .
</li>
</ol>
<p>
I took some more time to explain
why I think it's right what I'm doing.
So the introduction got a bit longer than it should ... ;-)
</p>
<H3><a name="whyscripting"></a>23.1.1 Why not scripting ?</H3>
<p>
SWIG started as wrapper from the fast compiled languages C and C++
to high level scripting languages like Python.
Although scripting languages are designed
to make programming life easier
by hiding machine internals from the programmer
there are several aspects of today's scripting languages
that are unfavourable in my opinion.
</p>
<p>
Besides C, C++, Cluster (a Modula derivate for Amiga computers)
I evaluated several scripting like languages in the past:
Different dialects of BASIC,
Perl, ARexx (a variant of Rexx for Amiga computers),
shell scripts.
I found them too inconsistent,
too weak in distinguishing types,
too weak in encapsulating pieces of code.
Eventually I have started several projects in Python
because of the fine syntax.
But when projects became larger
I lost the track.
I got convinced that one can not have
maintainable code in a language
that is not statically typed.
In fact the main advantages of scripting languages
e.g. matching regular expressions,
complex built-in datatypes like lists, dictionaries,
are not advantages of the language itself
but can be provided by function libraries.
</p>
<H3><a name="whymodula3"></a>23.1.2 Why Modula-3 ?</H3>
<p>
Modula-3 is a compiler language
in the tradition of Niklaus Wirth's Modula 2,
which is in turn a successor of the popular Pascal.
I have chosen Modula-3
because of its
logical syntax,
strong modularization,
the type system which is very detailed
for machine types compared to other languages.
Of course it supports all of the modern games
like exceptions, objects, garbage collection, threads.
While C++ programmers must
control three languages,
namely the preprocessor, C and ++,
Modula-3 is made in one go
and the language definition is really compact.
</p>
<p>
On the one hand Modula-3 can be safe
(but probably less efficient) in normal modules
while providing much static and dynamic safety.
On the other hand you can write efficient
but less safe code in the style of C
within <tt>UNSAFE</tt> modules.
</p>
<p>
Unfortunately Modula's safety and strength
requires more writing than scripting languages do.
Today if I want to safe characters
I prefer Haskell (similar to OCAML) -
it's statically typed, too.
</p>
<H3><a name="whycpp"></a>23.1.3 Why C / C++ ?</H3>
<p>
Although it is no problem to write Modula-3 programs
that performs as fast as C
most libraries are not written in Modula-3 but in C.
Fortunately the binary interface of most function libraries
can be addressed by Modula-3.
Even more fortunately even non-C libraries may provide C header files.
This is where SWIG becomes helpful.
</p>
<H3><a name="whyswig"></a>23.1.4 Why SWIG ?</H3>
<p>
The C headers and the possibility to interface to C libraries
still leaves the work for you
to write Modula-3 interfaces to them.
To make things comfortable you will also need
wrappers that convert between high-level features of Modula-3
(garbage collecting, exceptions)
and the low level of the C libraries.
</p>
<p>
SWIG converts C headers to Modula-3 interfaces for you.
You could call the C functions without loss
of efficiency but it won't be joy
because you could not pass <tt>TEXT</tt>s
or open arrays and
you would have to process error return codes
rather then exceptions.
But using some typemaps SWIG will also generate
wrappers that bring the whole Modula-3 comfort to you.
If the library API is ill designed
writing appropriate typemaps can be still time-consuming.
E.g. C programmers are very creative to work-around
missing data types like (real) enumerations and sets.
You should turn such work-arounds back to the Modula-3 way
otherwise you lose static safety and consistency.
</p>
<p>
But you have still a problem:
C library interfaces are often ill.
They lack for certain information
because C compilers wouldn't care about.
You should integrate detailed type information
by adding <tt>typedef</tt>s and <tt>const</tt>s
and you should persuade the C library programmer
to add this information to his interface.
Only this way other language users can benefit from your work
and only this way you can easily update your interfaces
when a new library version is released.
You will realise that writing <b>good</b> SWIG interfaces
is very costly and it will only amortise
when considering evolving libraries.
</p>
<p>
Without SWIG you would probably never consider
to call C++ libraries from Modula-3.
But with SWIG this is worth a consideration.
SWIG can write C wrappers to C++ functions and object methods
that may throw exceptions.
In fact it breaks down C++ libraries to C interfaces
which can be in turn called from Modula-3.
To make it complete you can hide the C interface
with Modula-3 classes and exceptions.
</p>
<p>
Although SWIG does the best it can do
it can only serve as a one-way strategy.
That means you can use C++ libraries
with Modula-3 (even with call back functions),
but it's certainly not possible to smoothly
integrate Modula-3 code into a C / C++ project.
</p>
<H2><a name="conception"></a>23.2 Conception</H2>
<H3><a name="cinterface"></a>23.2.1 Interfaces to C libraries</H3>
<p>
Modula-3 has an integrated support for calling C functions.
This is also extensively used by the standard Modula-3 libraries
to call OS functions.
The Modula-3 part of SWIG and the corresponding SWIG library
<a href="../../Lib/modula3/modula3.swg"><tt>modula3.swg</tt></a>
contain code that uses these features.
Because of the built-in support there is no need
for calling the SWIG kernel to generate wrappers written in C.
All conversion and argument checking can be done in Modula-3
and the interfacing is quite efficient.
All you have to do is to write pieces of Modula-3 code
that SWIG puts together.
</p>
<table border summary="Modula-3 C library support">
<tr><th colspan=2>C library support integrated in Modula-3<th></tr>
<tr>
<td>Pragma <tt>&lt;* EXTERNAL *&gt;</tt></td>
<td>Precedes a declaration of a PROCEDURE that is implemented
in an external library instead of a Modula-3 module.</td>
</tr>
<tr>
<td>Pragma <tt>&lt;* CALLBACK *&gt;</tt></td>
<td>Precedes a declaration of a PROCEDURE that should be called
by external library code.</td>
</tr>
<tr>
<td>Module <tt>Ctypes</tt></td>
<td>Contains Modula-3 types that match some basic C types.</td>
</tr>
<tr>
<td>Module <tt>M3toC</tt></td>
<td>Contains routines that convert between Modula-3's <tt>TEXT</tt> type
and C's <tt>char *</tt> type.</td>
</tr>
</table>
<p>
In each run of SWIG the Modula-3 part
generates several files:
</p>
<table border summary="Modula-3 generated files">
<tr>
<th>Module name scheme</th>
<th>Identifier for <tt>%insert</tt></th>
<th>Description</th>
</tr>
<tr>
<td>Module<tt>Raw.i3</tt></td>
<td><tt>m3rawintf</tt></td>
<td>Declaration of types that are equivalent to those of the C library,
<tt>EXTERNAL</tt> procedures as interface to the C library functions</td>
</tr>
<tr>
<td>Module<tt>Raw.m3</tt></td>
<td><tt>m3rawimpl</tt></td>
<td>Almost empty.</td>
</tr>
<tr>
<td>Module<tt>.i3</tt></td>
<td><tt>m3wrapintf</tt></td>
<td>Declaration of comfortable wrappers to the C library functions.</td>
</tr>
<tr>
<td>Module<tt>.m3</tt></td>
<td><tt>m3wrapimpl</tt></td>
<td>Implementation of the wrappers that
convert between Modula-3 and C types,
check for validity of values,
hand-over resource management to the garbage collector using <tt>WeakRef</tt>s
and raises exceptions.</td>
</tr>
<tr>
<td><tt>m3makefile</tt></td>
<td><tt>m3makefile</tt></td>
<td>Add the modules above to the Modula-3 project and
specify the name of the Modula-3 wrapper library
to be generated.
Today I'm not sure if it is a good idea
to create a <tt>m3makefile</tt> in each run,
because SWIG must be started for each Modula-3 module it creates.
Thus the m3makefile is overwritten each time. :-(
</td>
</tr>
</table>
<p>
Here's a scheme of how the function calls to Modula-3 wrappers
are redirected to C library functions:
</p>
<table summary="Modula-3 C library">
<tr>
<td align=center>
Modula-3 wrapper<br>
Module<tt>.i3</tt><br>
generated by Modula-3 part of SWIG
</td>
<td></td>
<td align=center></td>
</tr>
<tr>
<td align=center>
<!-- pre tag overrides centering -->
|<br>
v
</td>
<td></td>
<td align=center></td>
</tr>
<tr>
<td align=center>
Modula-3 interface to C<br>
Module<tt>Raw.i3</tt><br>
generated by Modula-3 part of SWIG
</td>
<td>--&gt;</td>
<td align=center>
C library
</td>
</tr>
</table>
<p>
I have still no good conception how one can split C library interfaces
into type oriented interfaces.
A Module in Modula-3 represents an Abstract DataType
(or call it a static classes, i.e. a class without virtual methods).
E.g. if you have a principal type, say <tt>Database</tt>,
it is good Modula-3 style to set up one Module with the name <tt>Database</tt>
where the database type is declared with the name <tt>T</tt>
and where all functions are declared that operates on it.
</p>
<p>
The normal operation of SWIG is to generate a fixed set of files per call.
To generate multiple modules one has to write one SWIG interface
(different SWIG interfaces can share common data) per module.
Identifiers belonging to a different module may ignored (<tt>%ignore</tt>)
and the principal type must be renamed (<tt>%typemap</tt>).
</p>
<H3><a name="cppinterface"></a>23.2.2 Interfaces to C++ libraries</H3>
<p>
Interfaces to C++ files are much more complicated and
there are some more design decisions that are not made, yet.
Modula-3 has no support for C++ functions
but C++ compilers should support generating C++ functions
with a C interface.
</p>
<p>
Here's a scheme of how the function calls to Modula-3 wrappers
a redirected to C library functions:
</p>
<table summary="Modula-3 C++ library">
<tr>
<td align=center>
Modula-3 wrapper<br>
Module<tt>.i3</tt><br>
generated by Modula-3 part of SWIG
</td>
<td></td>
<td align=center>C++ library</td>
</tr>
<tr>
<td align=center>
<!-- pre tag overrides centering -->
|<br>
v
</td>
<td></td>
<td align=center>
^<br>
|
</td>
</tr>
<tr>
<td align=center>
Modula-3 interface to C<br>
Module<tt>Raw.i3</tt><br>
generated by Modula-3 part of SWIG
</td>
<td>--&gt;</td>
<td align=center>
C interface to C++<br>
module<tt>_wrap.cxx</tt><br>
generated by the SWIG core
</td>
</tr>
</table>
<p>
Wrapping C++ libraries arises additional problems:
</p>
<ul>
<li>
Is it sensible to wrap C++ classes with Modula-3 classes?
</li>
<li>
How to find the wrapping Modula-3 class
for a class pointer that is returned by a C++ routine?
</li>
<li>
How to deal with multiple inheritance
which was neglected for Modula-3 for good reasons?
</li>
<li>
Is it possible to sub-class C++ classes with Modula-3 code?
This issue is addressed by directors,
a feature that was experimentally added to some Language modules
like
<a href="Java.html#java_directors">Java</a> and
<a href="Python.html#directors">Python</a>.
</li>
<li>
How to manage storage with the garbage collector of Modula-3?
Support for
<a href="Customization.html#ownership">
<tt>%newobject</tt> and <tt>%typemap(newfree)</tt></a>
isn't implemented, yet.
What's about resources that are managed by the garbage collector
but shall be passed back to the storage management of the C++ library?
This is a general issue which is not solved in a satisfying fashion
as far as I know.
</li>
<li>
How to turn C++ exceptions into Modula-3 exceptions?
There's also no support for
<a href="Customization.html#exception">
<tt>%exception</tt></a>, yet.
</li>
</ul>
<p>
Be warned:
There is no C++ library I wrote a SWIG interface for,
so I'm not sure if this is possible or sensible, yet.
</p>
<H2><a name="preliminaries"></a>23.3 Preliminaries</H2>
<H3><a name="compilers"></a>23.3.1 Compilers</H3>
<p>
There are different Modula-3 compilers around:
cm3, pm3, ezm3, Klagenfurth Modula-3, Cambridge Modula-3.
SWIG itself does not contain compiler specific code
but the library file
<a href="../../Lib/modula3/modula3.swg"><tt>modula3.swg</tt></a>
may do so.
For testing examples I use Critical Mass cm3.
</p>
<H3><a name="commandline"></a>23.3.2 Additional Commandline Options</H3>
<p>
There are some experimental command line options
that prevent SWIG from generating interface files.
Instead files are emitted that may assist you
when writing SWIG interface files.
</p>
<table border summary="Modula-3 specific options">
<tr>
<th>Modula-3 specific options</th>
<th>Description</th>
</tr>
<tr>
<td valign=top>-generateconst &lt;file&gt;</td>
<td>
Disable generation of interfaces and wrappers.
Instead write code for computing numeric values of constants
to the specified file.
<br>
C code may contain several constant definitions
written as preprocessor macros.
Other language modules of SWIG use
compute-once-use-readonly variables or
functions to wrap such definitions.
All of them can invoke C code dynamically
for computing the macro values.
But if one wants to turn them into Modula-3
integer constants, enumerations or set types,
the values of these expressions has to be known statically.
Although definitions like <tt>(1 &lt;&lt; FLAG_MAXIMIZEWINDOW)</tt>
must be considered as good C style
they are hard to convert to Modula-3
since the value computation can use every feature of C.
<br>
Thus I implemented these switch
to extract all constant definitions
and write a C program that output the values of them.
It works for numeric constants only
and treats all of them as <tt>double</tt>.
Future versions may generate a C++ program
that can detect the type of the macros
by overloaded output functions.
Then strings can also be processed.
</td>
</tr>
<tr>
<td valign=top>-generaterename &lt;file&gt;</td>
<td>
Disable generation of interfaces and wrappers.
Instead generate suggestions for <tt>%rename</tt>.
<br>
C libraries use a naming style
that is neither homogeneous nor similar to that of Modula-3.
C function names often contain a prefix denoting the library
and some name components separated by underscores
or capitalization changes.
To get library interfaces that are really Modula-3 like
you should rename the function names with the <tt>%rename</tt> directive.
This switch outputs a list of such directives
with a name suggestion generated by a simple heuristic.
</td>
</tr>
<tr>
<td valign=top>-generatetypemap &lt;file&gt;</td>
<td>
Disable generation of interfaces and wrappers.
Instead generate templates for some basic typemaps.
</td>
</tr>
</table>
<H2><a name="modula3_typemaps"></a>23.4 Modula-3 typemaps</H2>
<H3><a name="inoutparam"></a>23.4.1 Inputs and outputs</H3>
<p>
Each C procedure has a bunch of inputs and outputs.
Inputs are passed as function arguments,
outputs are updated referential arguments and
the function value.
</p>
<p>
Each C type can have several typemaps
that apply only in case if a type is used
for an input argument, for an output argument,
or for a return value.
A further typemap may specify
the direction that is used for certain parameters.
I have chosen this separation
in order to be able to write general typemaps for the typemap library
<a href="../../Lib/modula3/modula3.swg"><tt>modula3.swg</tt></a>
.
In the library code the final usage of the type is not known.
Using separate typemaps for each possible use
allows appropriate definitions for each case.
If these pre-definitions are fine
then the direction of the function parameter
is the only hint the user must give.
</p>
<p>
The typemaps specific to Modula-3 have a common name scheme:
A typemap name starts with "m3",
followed by "raw" or "wrap"
depending on whether it controls the generation
of the Module<tt>Raw.i3</tt> or the Module<tt>.i3</tt>, respectively.
It follows an "in" for typemaps applied to input argument,
"out" for output arguments, "arg" for all kind of arguments,
"ret" for returned values.
</p>
<p>
The main task of SWIG is to build wrapper function,
i.e. functions that convert values between C and Modula-3
and call the corresponding C function.
Modula-3 wrapper functions generated by SWIG
consist of the following parts:
</p>
<ul>
<li>Generate <tt>PROCEDURE</tt> signature.</li>
<li>Declare local variables.</li>
<li>Convert input values from Modula-3 to C.</li>
<li>Check for input value integrity.</li>
<li>Call the C function.</li>
<li>Check returned values, e.g. error codes.</li>
<li>Convert and write back values into Modula-3 records.</li>
<li>Free temporary storage.</li>
<li>Return values.</li>
</ul>
<table border summary="Modula-3 typemaps">
<tr>
<th>Typemap</th>
<th>Example</th>
<th>Description</th>
</tr>
<tr>
<td>m3wrapargvar</td>
<td><tt>$1: INTEGER := $1_name;</tt></td>
<td>
Declaration of some variables needed for temporary results.
</td>
</tr>
<tr>
<td>m3wrapargconst</td>
<td><tt>$1 = "$1_name";</tt></td>
<td>
Declaration of some constant, maybe for debug purposes.
</td>
</tr>
<tr>
<td>m3wrapargraw</td>
<td><tt>ORD($1_name)</tt></td>
<td>
The expression that should be passed as argument to the raw Modula-3 interface function.
</td>
</tr>
<tr>
<td>m3wrapargdir</td>
<td><tt>out</tt></td>
<td>
Referential arguments can be used for input, output, update.
???
</td>
</tr>
<tr>
<td>m3wrapinmode</td>
<td><tt>READONLY</tt></td>
<td>
One of Modula-3 parameter modes
<tt>VALUE</tt> (or empty),
<tt>VAR</tt>,
<tt>READONLY</tt>
</td>
</tr>
<tr>
<td>m3wrapinname</td>
<td></td>
<td>
New name of the input argument.
</td>
</tr>
<tr>
<td>m3wrapintype</td>
<td></td>
<td>
Modula-3 type of the input argument.
</td>
</tr>
<tr>
<td>m3wrapindefault</td>
<td></td>
<td>
Default value of the input argument
</td>
</tr>
<tr>
<td>m3wrapinconv</td>
<td><tt>$1 := M3toC.SharedTtoS($1_name);</tt></td>
<td>
Statement for converting the Modula-3 input value to C compliant value.
</td>
</tr>
<tr>
<td>m3wrapincheck</td>
<td><tt>IF Text.Length($1_name) &gt; 10 THEN RAISE E("str too long"); END;</tt></td>
<td>
Check the integrity of the input value.
</td>
</tr>
<tr>
<td>m3wrapoutname</td>
<td></td>
<td>
Name of the <tt>RECORD</tt> field to be used for returning multiple values.
This applies to referential output arguments that shall be turned
into return values.
</td>
</tr>
<tr>
<td>m3wrapouttype</td>
<td></td>
<td>
Type of the value that is returned instead of a referential output argument.
</td>
</tr>
<tr>
<td>m3wrapoutconv</td>
<td></td>
<td>
</td>
</tr>
<tr>
<td>m3wrapoutcheck</td>
<td></td>
<td>
</td>
</tr>
<tr>
<td>m3wrapretraw</td>
<td></td>
<td>
</td>
</tr>
<tr>
<td>m3wrapretname</td>
<td></td>
<td>
</td>
</tr>
<tr>
<td>m3wraprettype</td>
<td></td>
<td>
</td>
</tr>
<tr>
<td>m3wrapretvar</td>
<td></td>
<td>
</td>
</tr>
<tr>
<td>m3wrapretconv</td>
<td></td>
<td>
</td>
</tr>
<tr>
<td>m3wrapretcheck</td>
<td></td>
<td>
</td>
</tr>
<tr>
<td>m3wrapfreearg</td>
<td><tt>M3toC.FreeSharedS(str,arg1);</tt></td>
<td>
Free resources that were temporarily used in the wrapper.
Since this step should never be skipped,
SWIG will put it in the <tt>FINALLY</tt> branch
of a <tt>TRY .. FINALLY</tt> structure.
</td>
</tr>
</table>
<H3><a name="ordinals"></a>23.4.2 Subranges, Enumerations, Sets</H3>
<p>
Subranges, enumerations, and sets are machine oriented types
that make Modula very strong and expressive compared
with the type systems of many other languages.
</p>
<ul>
<li>
Subranges are used for statically restricted choices of integers.
</li>
<li>
Enumerations are used for named choices.
</li>
<li>
Sets are commonly used for flag (option) sets.
</li>
</ul>
<p>
Using them extensively makes Modula code very safe and readable.
</p>
<p>
C supports enumerations, too, but they are not as safe as the ones of Modula.
Thus they are abused for many things:
For named choices, for integer constant definitions, for sets.
To make it complete every way of defining a value in C
(<tt>#define</tt>, <tt>const int</tt>, <tt>enum</tt>)
is somewhere used for defining something
that must be handled completely different in Modula-3
(<tt>INTEGER</tt>, enumeration, <tt>SET</tt>).
</p>
<p>
I played around with several <tt>%feature</tt>s and <tt>%pragma</tt>s
that split the task up into converting
the C bit patterns (integer or bit set)
into Modula-3 bit patterns (integer or bit set)
and change the type as requested.
See the corresponding
<a href="../../Examples/modula3/enum/example.i">example</a>.
This is quite messy and not satisfying.
So the best what you can currently do is
to rewrite constant definitions manually.
Though this is a tedious work
that I'd like to automate.
</p>
<H3><a name="class"></a>23.4.3 Objects</H3>
<p>
Declarations of C++ classes are mapped to <tt>OBJECT</tt> types
while it is tried to retain the access hierarchy
"public - protected - private" using partial revelation.
Though the
<a href="../../Examples/modula3/class/example.i">implementation</a>
is not really useful, yet.
</p>
<H3><a name="imports"></a>23.4.4 Imports</H3>
<p>
Pieces of Modula-3 code provided by typemaps
may contain identifiers from foreign modules.
If the typemap <tt>m3wrapinconv</tt> for <tt>blah *</tt>
contains code using the function <tt>M3toC.SharedTtoS</tt>
you may declare <tt>%typemap("m3wrapinconv:import") blah * %{M3toC%}</tt>.
Then the module <tt>M3toC</tt> is imported
if the <tt>m3wrapinconv</tt> typemap for <tt>blah *</tt>
is used at least once.
Use <tt>%typemap("m3wrapinconv:import") blah * %{MyConversions AS M3toC%}</tt>
if you need module renaming.
Unqualified import is not supported.
</p>
<p>
It is cumbersome to add this typemap to each piece of Modula-3 code.
It is especially useful when writing general typemaps
for the typemap library
<a href="../../Lib/modula3/modula3.swg"><tt>modula3.swg</tt></a>
.
For a monolithic module you might be better off
if you add the imports directly:
</p>
<div class="code">
<pre>
%insert(m3rawintf) %{
IMPORT M3toC;
%}
</pre></div>
<H3><a name="exceptions"></a>23.4.5 Exceptions</H3>
<p>
Modula-3 provides another possibility
of an output of a function: exceptions.
</p>
<p>
Any piece of Modula-3 code that SWIG inserts
due to a typemap can raise an exception.
This way you can also convert an error code
from a C function into a Modula-3 exception.
</p>
<p>
The <tt>RAISES</tt> clause is controlled
by typemaps with the <tt>throws</tt> extension.
If the typemap <tt>m3wrapinconv</tt> for <tt>blah *</tt>
contains code that may raise the exceptions <tt>OSError.E</tt>
you should declare
<tt>%typemap("m3wrapinconv:throws") blah * %{OSError.E%}</tt>.
</p>
<H3><a name="typemap_example"></a>23.4.6 Example</H3>
<p>
The generation of wrappers in Modula-3 needs very fine control
to take advantage of the language features.
Here is an example of a generated wrapper
where almost everything is generated by a typemap:
</p>
<div class="code"><pre>
<I> (* %relabel m3wrapinmode m3wrapinname m3wrapintype m3wrapindefault *)</I>
PROCEDURE Name (READONLY str : TEXT := "" )
<I> (* m3wrapoutcheck:throws *)</I>
: NameResult RAISES {E} =
CONST
arg1name = "str"; <I>(* m3wrapargconst *)</I>
VAR
arg0 : C.char_star; <I>(* m3wrapretvar *)</I>
arg1 : C.char_star; <I>(* m3wrapargvar *)</I>
arg2 : C.int;
result : RECORD
<I> (*m3wrapretname m3wraprettype*)</I>
unixPath : TEXT;
<I> (*m3wrapoutname m3wrapouttype*)</I>
checksum : CARDINAL;
END;
BEGIN
TRY
arg1 := M3toC.SharedTtoS(str); <I>(* m3wrapinconv *)</I>
IF Text.Length(arg1) &gt; 10 THEN <I>(* m3wrapincheck *)</I>
RAISE E("str too long");
END;
<I> (* m3wrapretraw m3wrapargraw *)</I>
arg0 := MessyToUnix (arg1, arg2);
result.unixPath := M3toC.CopyStoT(arg0); <I>(* m3wrapretconv *)</I>
result.checksum := arg2; <I>(* m3wrapoutconv *)</I>
IF result.checksum = 0 THEN <I>(* m3wrapoutcheck *)</I>
RAISE E("invalid checksum");
END;
FINALLY
M3toC.FreeSharedS(str,arg1); <I>(* m3wrapfreearg *)</I>
END;
END Name;
</pre></div>
<H2><a name="hints"></a>23.5 More hints to the generator</H2>
<H3><a name="features"></a>23.5.1 Features</H3>
<table border summary="Modula-3 features">
<tr>
<th>Feature</th>
<th>Example</th>
<th>Description</th>
</tr>
<tr>
<td>multiretval</td>
<td><tt>%m3multiretval get_box;</tt> or
<tt>%feature("modula3:multiretval") get_box;</tt></td>
<td>Let the denoted function return a <tt>RECORD</tt>
rather than a plain value.
This <tt>RECORD</tt> contains all arguments with "out" direction
including the return value of the C function (if there is one).
If more than one argument is "out"
then the function <b>must</b> have the <tt>multiretval</tt> feature activated,
but it is explicitly requested from the user to prevent mistakes.</td>
</tr>
<tr>
<td>constnumeric</td>
<td><tt>%constnumeric(12) twelve;</tt> or
<tt>%feature("constnumeric","12") twelve;</tt></td>
<td>This feature can be used to tell Modula-3's back-end of SWIG
the value of an identifier.
This is necessary in the cases
where it was defined by a non-trivial C expression.
This feature is used by the
<tt>-generateconst</tt> <a href="#options">option</a>.
In future it may be generalized to other kind of values
such as strings.
</td>
</tr>
</table>
<H3><a name="pragmas"></a>23.5.2 Pragmas</H3>
<table border summary="Modula-3 pragmas">
<tr>
<th>Pragma</th>
<th>Example</th>
<th>Description</th>
</tr>
<tr>
<td>unsafe</td>
<td><tt>%pragma(modula3) unsafe="true";</tt></td>
<td>Mark the raw interface modules as <tt>UNSAFE</tt>.
This will be necessary in many cases.</td>
</tr>
<tr>
<td>library</td>
<td><tt>%pragma(modula3) library="m3fftw";</tt></td>
<td>Specifies the library name for the wrapper library to be created.
It should be distinct from the name of the library to be wrapped.</td>
</tr>
</table>
<H2><a name="remarks"></a>23.6 Remarks</H2>
<ul>
<li>
The Modula-3 part of SWIG doesn't try to generate nicely formatted code.
Use <tt>m3pp</tt> to postprocess the Modula files,
it does a very good job here.
</li>
</ul>
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