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<?xml version="1.0" encoding="utf-8"?>
<!DOCTYPE book [
<!ENTITY % eclent SYSTEM "ecl.ent">
%eclent;
]>
<book xmlns="http://docbook.org/ns/docbook" version="5.0" xml:lang="en">
<chapter  xml:id="Macros">
<title>Macros</title>
<para>A <firstterm>defmacro lambda-list</firstterm> is a lambda-list-like construct that is used as
the third element in the <literal>defmacro</literal> form,</para>
<screen>
(defmacro <replaceable>name</replaceable> <replaceable>efmacro-lambda-list</replaceable> [<replaceable>declaration</replaceable> | <replaceable>doc-string</replaceable>] {<replaceable>form</replaceable>}*)
</screen>
<para>The description of defmacro lambda-lists in &Steele84; is quite
ambiguous.  &ECL; employs the following syntax.</para>
<para>The complete syntax of a defmacro lambda-list is:</para>
<screen>
( [&amp;whole <replaceable>var</replaceable>]
[&amp;environment <replaceable>var</replaceable>]
[<replaceable>pseudo-var</replaceable>]
[&amp;optional {var | ( pseudo-var [initform [pseudo-var]] )}]
{[{&amp;rest | &amp;body} pseudo-var]
[&amp;key {var
| ({var | (keyword pseudo-var)} [initform [pseudo-var]])}*
[&amp;allow-other-keys]]
[&amp;aux {var | (pseudo-var [initform])}*]
| . var }
)
</screen>
<para role="continues">where <replaceable>pseudo-var</replaceable> is either a symbol or a list of the following
form:</para>
<screen>
( {pseudo-var}*
[&amp;optional {var | (pseudo-var [initform [pseudo-var]])}*]
{[{&amp;rest | &amp;body} pseudo-var]
[&amp;key {var | ({var | (keyword pseudo-var)}
[initform [pseudo-var]])}*
[&amp;allow-other-keys]]
[&amp;aux {var | (pseudo-var [initform])}]
| . var }
)
</screen>
<para>The defmacro lambda-list keyword <literal>&amp;whole</literal> may appear only at the
top-level, first in the defmacro lambda-list.  It is not allowed within
<replaceable>pseudo-var</replaceable>.  Use of the <literal>&amp;whole</literal> keyword does not affect the
processing of the rest of the defmacro lambda-list:</para>
<programlisting>
(defmacro foo (&amp;whole w x y) ...)
</programlisting>
<para>and</para>
<programlisting>
(defmacro foo (x y) ...)
</programlisting>
<para role="continues">both bind the variables <literal>x</literal> and <literal>y</literal> to the second and the
third elements, respectively, of macro forms of <literal>foo</literal>.</para>
<para>The defmacro lambda-list keyword <literal>&amp;environment</literal> may appear only at the
top-level, first in the defmacro lambda-list if <literal>&amp;whole</literal> is not supplied,
or immediately after the variable that follows <literal>&amp;whole</literal>, if <literal>&amp;whole</literal>
is supplied.  <literal>&amp;environment</literal> is not allowed within <replaceable>pseudo-var</replaceable>.  Like
<literal>&amp;whole</literal>, use of <literal>&amp;environment</literal> does not affect the processing of the
rest of the defmacro lambda-list.  If an <literal>&amp;environment</literal> parameter is
supplied and if this parameter is not used at all, then the &ECL; compiler
will issue a warning.  To suppress the warning, just remove the parameter from
the defmacro lambda-list, or add an <literal>ignore</literal> declaration.</para>
<para>The defmacro lambda-list keyword <literal>&amp;body</literal> is completely equivalent to the
&amp;rest keyword.  &ECL; takes no special action for <literal>&amp;body</literal> parameters.</para>
<para>Although useless, &ECL; allows supplied-p parameters to be destructured.
This is useless because supplied-p parameters can never be bound to a non-empty
list.  Our intention is to stick to the specification in the
&Steele84; as far as possible, even if it is silly to do so.</para>
<para>Like for ordinary lambda-lists, the interpreter detects invalid arguments to
macro expansion functions.  When a parameter is destructured, the structure of
the corresponding argument is also checked.  Such runtime argument checking may
or may not be embedded in compiled code, depending on the environment when the
code was generated.  If the code was generated while the <literal>safety</literal> optimize
level is zero (that is, while the value of <literal>(proclamation '(optimize
(safety 0)))</literal> is <replaceable>T</replaceable>), then the generated code does not perform argument
checking at all.  Otherwise, the compiled code does check the validity of
arguments.</para>
</chapter>
<!-- Keep this comment at the end of the file
  Local variables:
  sgml-parent-document: "ecl.xml"
  sgml-indent-step: 1
  nxml-child-indent: 1
  nxml-outline-child-indent: 1
  fill-column: 79
  End:
--></book>