456 lines
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456 lines
24 KiB
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<!Converted with LaTeX2HTML 0.6.5 (Tue Nov 15 1994) by Nikos Drakos (nikos@cbl.leeds.ac.uk), CBLU, University of Leeds >
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<HEAD>
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<TITLE>8.5. Environments</TITLE>
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<meta name="description" value=" Environments">
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<meta name="resource-type" value="document">
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<P>
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<b>Common Lisp the Language, 2nd Edition</b>
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<BR> <HR><A NAME=tex2html2792 HREF="node103.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html2790 HREF="node97.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html2786 HREF="node101.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html2794 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html2795 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html2793 HREF="node103.html"> Declarations</A>
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<B>Up:</B> <A NAME=tex2html2791 HREF="node97.html"> Macros</A>
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<B> Previous:</B> <A NAME=tex2html2787 HREF="node101.html"> Compiler Macros</A>
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<HR> <P>
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<H1><A NAME=SECTION001250000000000000000>8.5. Environments</A></H1>
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<P>
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<img align=bottom alt="change_begin" src="gif/change_begin.gif"><br>
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X3J13 voted in June 1989 (SYNTACTIC-ENVIRONMENT-ACCESS) <A NAME=8713> </A> to add some facilities for obtaining information
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from environment objects of the kind received as arguments
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by macro expansion functions, <tt>*macroexpand-hook*</tt> functions,
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and <tt>*evalhook*</tt> functions.
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There is a minimal set of accessors <tt>(variable-information</tt>,
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<tt>function-information</tt>, and <tt>declaration-information)</tt> and a constructor
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<tt>(augment-environment)</tt> for environments.
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<P>
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All of the standard declaration specifiers, with the exception of <tt>special</tt>,
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can be defined fairly easily using <tt>define-declaration</tt>. It also
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seems to be able to handle most extended declarations.
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<P>
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The function <tt>parse-macro</tt> is provided so that
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users don't have to write their
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own code to destructure macro arguments.
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This function is not entirely necessary since X3J13 voted
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in March 1989 (DESTRUCTURING-BIND) <A NAME=8723> </A>
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to add <tt>destructuring-bind</tt> to the language.
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However, <tt>parse-macro</tt> is worth having anyway, since any program-analyzing
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program is going to need to define it, and the implementation isn't completely
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trivial even with <tt>destructuring-bind</tt> to build upon.
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<P>
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The function <tt>enclose</tt> allows expander functions to be defined in a non-null
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lexical environment, as required by the vote of X3J13 in
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March 1989 (DEFINING-MACROS-NON-TOP-LEVEL) <A NAME=8728> </A> . It
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also provides a mechanism by which a program processing
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the body of an <tt>(eval-when (:compile-toplevel) ...)</tt> form
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can execute it in the enclosing environment (see issue
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(EVAL-WHEN-NON-TOP-LEVEL) <A NAME=8730> </A> ).
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<P>
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In all of these functions the argument named <i>env</i> is an environment
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object. (It is not required that implementations
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provide a distinguished representation for such objects.) Optional <i>env</i>
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arguments default to <tt>nil</tt>, which represents the local null lexical environment
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(containing only global definitions and proclamations that are present in the
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run-time environment). All of these functions should signal an error of type
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<tt>type-error</tt> if the value of an environment argument is not a syntactic
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environment object.
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<P>
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The accessor functions <tt>variable-information</tt>, <tt>function-information</tt>, and
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<tt>declaration-information</tt> retrieve information about
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declarations that are in
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effect in the environment. Since implementations are permitted to ignore
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declarations (except for <tt>special</tt> declarations and <tt>optimize safety</tt>
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declarations if they ever compile unsafe code), these accessors are required
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only to return information about declarations that were explicitly added to
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the environment using <tt>augment-environment</tt>. They might also return
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information about declarations recognized and added to the environment by the
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interpreter or the compiler, but that is at the discretion of the
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implementor. Implementations are also permitted to canonicalize
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declarations, so the information returned by the accessors might not be
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identical to the information that was passed to <tt>augment-environment</tt>.
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<P>
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<BR><b>[Function]</b><BR>
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<tt>variable-information</tt> <tt><i>variable</i></tt> <tt>&optional</tt> <tt><i>env</i></tt><P> This function returns information about the interpretation of the symbol
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<i>variable</i> when it appears as a variable within the lexical environment <i>env</i>.
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Three values are returned.
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<P>
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The first value indicates the type of definition or binding for <i>variable</i>
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in <i>env</i>:
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<DL COMPACT>
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<DT><tt>nil</tt>
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<DD>
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There is no apparent definition or binding for <i>variable</i>.
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<P>
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<DT><tt>:special</tt>
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<DD>
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The <i>variable</i> refers to a special variable, either declared or proclaimed.
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<P>
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<DT><tt>:lexical</tt>
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<DD>
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The <i>variable</i> refers to a lexical variable.
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<P>
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<DT><tt>:symbol-macro</tt>
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<DD>
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The <i>variable</i> refers to a <tt>symbol-macrolet</tt> binding.
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<P>
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<DT><tt>:constant</tt>
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<DD>
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Either the <i>variable</i> refers to a named constant defined by
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<tt>defconstant</tt> or the <i>variable</i> is a keyword symbol.
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<P>
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</DL>
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<P>
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The second value indicates whether there is a local binding of the name. If
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the name is locally bound, the second value is true; otherwise, the second value
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is <tt>nil</tt>.
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<P>
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The third value is an a-list containing information about declarations
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that apply to the apparent binding of the <i>variable</i>. The keys in the a-list
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are symbols that name declaration specifiers, and the format of the
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corresponding value in the <i>cdr</i> of each pair depends on the particular
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declaration name involved. The standard declaration names
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that might appear as keys in this a-list are:
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<DL COMPACT>
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<DT><tt>dynamic-extent</tt>
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<DD>
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A non-<tt>nil</tt> value indicates that the <i>variable</i> has been
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declared <tt>dynamic-extent</tt>. If the value is <tt>nil</tt>, the pair
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might be omitted.
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<P>
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<DT><tt>ignore</tt>
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<DD>
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A non-<tt>nil</tt> value indicates that the <i>variable</i> has been declared
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<tt>ignore</tt>. If the value is <tt>nil</tt>, the pair might be omitted.
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<P>
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<DT><tt>type</tt>
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<DD>
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The value is a type specifier associated with the <i>variable</i> by a <tt>type</tt>
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declaration or an abbreviated declaration such as
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<tt>(fixnum <i>variable</i>)</tt>.
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If no explicit association exists, either by <tt>proclaim</tt> or
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<tt>declare</tt>, then the type specifier is <tt>t</tt>. It is permissible for
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implementations to use a type specifier that is equivalent
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to or a supertype of the one appearing in the original
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declaration. If the value is <tt>t</tt>, the pair might be
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omitted.
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<P>
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</DL>
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If an implementation supports additional declaration specifiers that
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apply to variable bindings, those declaration names might also
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appear in the a-list. However, the corresponding key must not
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be a symbol that is external in any package defined in the standard
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or that is otherwise accessible in the <tt>common-lisp-user</tt> package.
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<P>
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The a-list might contain multiple entries for a given key.
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The consequences of destructively modifying the list
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structure of this a-list or its elements (except for values that
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appear in the a-list as a result of <tt>define-declaration</tt>) are undefined.
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<P>
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Note that the global binding might differ from the
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local one and can be retrieved by calling <tt>variable-information</tt>
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with a null lexical environment.
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<P>
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<BR><b>[Function]</b><BR>
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<tt>function-information</tt> <tt><i>function</i></tt> <tt>&optional</tt> <tt><i>env</i></tt><P> This function returns information about the interpretation of the function-name
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<i>function</i> when it appears in a functional position within lexical
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environment <i>env</i>. Three values are returned.
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<P>
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The first value indicates the type of definition or binding of the function-name
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which is apparent in <i>env</i>:
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<DL COMPACT>
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<DT><tt>nil</tt>
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<DD> There is no apparent definition for <i>function</i>.
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<P>
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<DT><tt>:function</tt>
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<DD> The <i>function</i> refers to a function.
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<P>
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<DT><tt>:macro</tt>
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<DD> The <i>function</i> refers to a macro.
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<P>
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<DT><tt>:special-form</tt>
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<DD> The <i>function</i> refers to a special form.
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<P>
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</DL>
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Some function-names can refer to both a global macro and a global special
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form. In such a case the macro takes precedence and <tt>:macro</tt> is returned as
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the first value.
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<P>
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The second value specifies whether the definition is local or global. If
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local, the second value is true; it is <tt>nil</tt> when the definition is
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global.
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<P>
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The third value is an a-list containing information about declarations
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that apply to the apparent binding of the function. The keys in the a-list
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are symbols that name declaration specifiers, and the format of the
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corresponding values in the <i>cdr</i> of each pair depends on the particular
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declaration name involved. The standard declaration names
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that might appear as keys in this a-list are:
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<DL COMPACT>
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<DT><tt>dynamic-extent</tt>
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<DD>
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A non-<tt>nil</tt> value indicates that the function has been
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declared <tt>dynamic-extent</tt>. If the value is <tt>nil</tt>, the pair
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might be omitted.
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<P>
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<DT><tt>inline</tt>
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<DD>
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The value is one of the symbols <tt>inline</tt>, <tt>notinline</tt>, or <tt>nil</tt> to indicate
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whether the function-name has been declared <tt>inline</tt>,
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declared <tt>notinline</tt>, or neither, respectively.
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If the value is <tt>nil</tt>, the pair might be omitted.
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<P>
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<DT><tt>ftype</tt>
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<DD>
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The value is the type specifier associated with the function-name in the
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environment, or the symbol <tt>function</tt> if there is no functional
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type declaration or proclamation associated with the function-name.
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This value might not include all the apparent <tt>ftype</tt>
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declarations for the function-name. It is permissible for
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implementations to use a type specifier that is equivalent
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to or a supertype of the one that appeared in the original
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declaration. If the value is <tt>function</tt>, the pair might be
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omitted.
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<P>
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</DL>
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If an implementation supports additional declaration specifiers that
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apply to function bindings, those declaration names might also
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appear in the a-list. However, the corresponding key must not be
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a symbol that is external in any package defined in the standard or
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that is otherwise accessible in the <tt>common-lisp-user</tt> package.
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<P>
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The a-list might contain multiple entries for a given key.
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In this case the value associated with the first entry has
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precedence. The consequences of destructively modifying the list
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structure of this a-list or its elements (except for values
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that appear in the a-list as a result of <tt>define-declaration</tt>) are
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undefined.
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<P>
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Note that the global binding might differ from the local
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one and can be retrieved by calling <tt>function-information</tt> with a null
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lexical environment.
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<P>
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<BR><b>[Function]</b><BR>
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<tt>declaration-information</tt> <tt><i>decl-name</i></tt> <tt>&optional</tt> <tt><i>env</i></tt><P> This function returns information about declarations named by the
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symbol <i>decl-name</i> that are in force in the environment <i>env</i>.
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Only declarations that do not apply to function or variable bindings
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can be accessed with this function. The format of the information
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that is returned depends on the <i>decl-name</i> involved.
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<P>
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It is required that this function recognize <tt>optimize</tt> and <tt>declaration</tt> as
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<i>decl-name</i>s. The values returned for these two cases are as follows:
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<DL COMPACT>
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<DT><tt>optimize</tt>
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<DD>
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A single value is returned,
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a list whose entries are of the form <tt>(<i>quality</i> <i>value</i>)</tt>, where
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<i>quality</i> is one of the standard optimization qualities
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<tt>(speed</tt>, <tt>safety</tt>, <tt>compilation-speed</tt>, <tt>space</tt>, <tt>debug)</tt>
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or some implementation-specific optimization quality, and
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<i>value</i> is an integer in the range 0 to 3 (inclusive).
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The returned list
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always contains an entry for each of the standard qualities and
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for each of the implementation-specific qualities. In the
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absence of any previous declarations, the associated values are
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implementation-dependent. The list might contain multiple
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entries for a quality, in which case the first such entry
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specifies the current value.
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The consequences of destructively modifying this list or
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its elements are undefined.
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<P>
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<DT><tt>declaration</tt>
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<DD>
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A single value is returned,
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a list of the declaration names that have been proclaimed as
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valid through the use of the <tt>declaration</tt> proclamation.
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The consequences of destructively modifying this list or
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its elements are undefined.
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<P>
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</DL>
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If an implementation is extended to recognize additional
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declaration specifiers in <tt>declare</tt> or <tt>proclaim</tt>, it is required that
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either the <tt>declaration-information</tt> function should recognize those
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declarations also or the implementation should provide a similar accessor that is
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specialized for that declaration specifier. If <tt>declaration-information</tt>
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is used to return the information, the corresponding <i>decl-name</i> must not
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be a symbol that is external in any package defined in the standard or
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that is otherwise accessible in the <tt>common-lisp-user</tt> package.
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<P>
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<BR><b>[Function]</b><BR>
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<pre>
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augment-environment <i>env</i> &key :variable :symbol-macro
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:function :macro :declare
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</pre>
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<P> This function returns a new environment containing the information present in
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<i>env</i> augmented with the information provided by the keyword arguments. It is
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intended to be used by program analyzers that perform a code walk.
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<P>
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The arguments are supplied as follows.
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<DL COMPACT><DT><tt>:variable</tt>
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<DD>
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The argument is a list of symbols that will be visible as bound variables in
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the new environment. Whether each binding is to be interpreted
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as special or lexical depends on <tt>special</tt> declarations recorded
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in the environment or provided in the <tt>:declare</tt> argument.
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<P>
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<DT><tt>:symbol-macro</tt>
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<DD>
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The argument is a list of symbol macro definitions, each of the form
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<tt>(<i>name</i> <i>definition</i>)</tt>; that is, the argument is
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in the same format as the
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<i>cadr</i> of a <tt>symbol-macrolet</tt> special form. The new environment
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will have local symbol-macro bindings of each symbol to the
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corresponding expansion, so that <tt>macroexpand</tt> will be able to
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expand them properly. A type declaration in the <tt>:declare</tt>
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argument that refers to a name in this list implicitly
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modifies the definition associated with the name. The effect
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is to wrap a <tt>the</tt> form mentioning the type around the
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definition.
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<P>
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<DT><tt>:function</tt>
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<DD>
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The argument is a list of function-names that will be visible as local
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function bindings in the new environment.
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<P>
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<DT><tt>:macro</tt>
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<DD>
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The argument is a list of local macro definitions,
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each of the form <tt>(<i>name</i> <i>definition</i>)</tt>.
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Note that the argument is <i>not</i>
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in the same format as the
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<i>cadr</i> of a <tt>macrolet</tt> special form.
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Each <i>definition</i> must be a function of two
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arguments (a form and an environment). The new environment
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will have local macro bindings of each name to the
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corresponding expander function, which will be returned by
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<tt>macro-function</tt> and used by <tt>macroexpand</tt>.
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<P>
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<DT><tt>:declare</tt>
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<DD>
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The argument is a list of declaration specifiers.
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Information about these declarations can
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be retrieved from the resulting environment using
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<tt>variable-information</tt>, <tt>function-information</tt>, and
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<tt>declaration-information</tt>.
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<P>
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</DL>
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The consequences of subsequently
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destructively modifying the list
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structure of any of the arguments to this function are undefined.
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<P>
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An error is signaled if any of the symbols naming a symbol macro in the
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<tt>:symbol-macro</tt> argument is also included in the <tt>:variable</tt> argument.
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An error is
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signaled if any symbol naming a symbol macro in the <tt>:symbol-macro</tt> argument is
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also included in a <tt>special</tt> declaration specifier in the <tt>:declare</tt> argument.
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An error is
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signaled if any symbol naming a macro in the <tt>:macro</tt> argument is also included
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in the <tt>:function</tt> argument.
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The condition type of each of these errors is <tt>program-error</tt>.
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<P>
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The extent of the returned environment is the same as the extent of the
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argument environment <i>env</i>. The result might share structure with <i>env</i>
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but <i>env</i> is not modified.
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<P>
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While an environment argument received by an <tt>*evalhook*</tt>
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function is permitted to be used as the
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environment argument to <tt>augment-environment</tt>, the consequences are undefined if an
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attempt is made to use the result of <tt>augment-environment</tt> as the environment
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argument for <tt>evalhook</tt>. The environment
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returned by <tt>augment-environment</tt> can be used only for syntactic analysis, that is,
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as an argument to
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the functions defined in this section and functions such as <tt>macroexpand</tt>.
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<P>
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<BR><b>[Macro]</b><BR>
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<tt>define-declaration</tt> <tt><i>decl-name</i></tt> <tt><i>lambda-list</i></tt> <tt>{<i>form</i>}*</tt><P> This macro defines a handler for the named declaration. It is the mechanism by which
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<tt>augment-environment</tt> is extended to support additional declaration
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specifiers. The function defined by this macro will be called with two
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arguments, a declaration specifier whose <i>car</i> is <i>decl-name</i>
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and the <i>env</i> argument to
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<tt>augment-environment</tt>. This function must return two values. The
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first value must be one of the following keywords:
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<DL COMPACT>
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<DT><tt>:variable</tt>
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<DD> The declaration applies to variable bindings.
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<DT><tt>:function</tt>
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<DD> The declaration applies to function bindings.
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<DT><tt>:declare</tt>
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<DD> The declaration does not apply to bindings.
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<P>
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</DL>
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If the first value is <tt>:variable</tt> or <tt>:function</tt>
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then the second value must be a list, the elements of which are lists of the
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form <tt>(<i>binding-name</i> <i>key</i> <i>value</i>)</tt>. If the corresponding information
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function (either <tt>variable-information</tt> or <tt>function-information</tt>) is applied to
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the <i>binding-name</i> and the augmented environment, the a-list returned
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by the information function as its third value will contain the <i>value</i>
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under the specified <i>key</i>.
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<P>
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If the first value is <tt>:declare</tt>, the second value must be a cons
|
|
of the form <tt>(<i>key</i> . <i>value</i>)</tt>. The function
|
|
<tt>declaration-information</tt> will return <i>value</i> when applied to the
|
|
<i>key</i> and the augmented environment.
|
|
<P>
|
|
<tt>define-declaration</tt> causes <i>decl-name</i> to be proclaimed to be a
|
|
declaration; it is as if its expansion included a call <tt>(proclaim
|
|
'(declaration <i>decl-name</i>))</tt>. As is the case with standard
|
|
declaration specifiers, the evaluator and compiler are permitted,
|
|
but not required, to add information about declaration specifiers
|
|
defined with <tt>define-declaration</tt> to the macro expansion and <tt>*evalhook*</tt>
|
|
environments.
|
|
<P>
|
|
The consequences are undefined if <i>decl-name</i> is a symbol that can
|
|
appear as the <i>car</i> of any standard declaration specifier.
|
|
<P>
|
|
The consequences are also undefined if the return value from a
|
|
declaration handler defined with <tt>define-declaration</tt> includes a <i>key</i> name
|
|
that is used by the corresponding accessor to return information about
|
|
any standard declaration specifier. (For example, if
|
|
the first return value from the handler is <tt>:variable</tt>, the second return
|
|
value may not use the symbols <tt>dynamic-extent</tt>, <tt>ignore</tt>, or <tt>type</tt>
|
|
as <i>key</i> names.)
|
|
<P>
|
|
The <tt>define-declaration</tt> macro does not have any special compile-time
|
|
side effects (see section <A HREF="node224.html#COMPILERSECTION">25.1</A>).
|
|
<P>
|
|
<BR><b>[Function]</b><BR>
|
|
<tt>parse-macro <i>name</i> <i>lambda-list</i> <i>body</i> &optional <i>env</i></tt><P> This function is used to process a macro definition in the same way
|
|
as <tt>defmacro</tt> and <tt>macrolet</tt>. It returns a lambda-expression that accepts
|
|
two arguments, a form and an environment. The <i>name</i>, <i>lambda-list</i>,
|
|
and <i>body</i> arguments correspond to the parts of a <tt>defmacro</tt> or <tt>macrolet</tt>
|
|
definition.
|
|
<P>
|
|
The <i>lambda-list</i> argument may include <tt>&environment</tt> and <tt>&whole</tt>
|
|
and may include destructuring.
|
|
The <i>name</i>
|
|
argument is used to enclose the <i>body</i> in an implicit <tt>block</tt> and might also
|
|
be used for implementation-dependent purposes (such as including the name of
|
|
the macro in error messages if the form does not match the <i>lambda-list</i>).
|
|
<P>
|
|
<BR><b>[Function]</b><BR>
|
|
<tt>enclose <i>lambda-expression</i> &optional <i>env</i></tt><P> This function returns an object of type <tt>function</tt> that is equivalent to what
|
|
would be obtained by evaluating <tt>`(function ,<i>lambda-expression</i>)</tt>
|
|
in a syntactic
|
|
environment <i>env</i>. The <i>lambda-expression</i> is permitted to reference only the
|
|
parts of the environment argument <i>env</i> that are relevant only to syntactic
|
|
processing, specifically declarations and the definitions of macros and
|
|
symbol macros. The consequences are undefined if the <i>lambda-expression</i>
|
|
contains any references to variable or function bindings that are
|
|
lexically visible in <i>env</i>, any <tt>go</tt> to a tag that is lexically visible in
|
|
<i>env</i>, or any <tt>return-from</tt> mentioning a block name that is lexically
|
|
visible in <i>env</i>.
|
|
<P>
|
|
<br><img align=bottom alt="change_end" src="gif/change_end.gif">
|
|
|
|
<P>
|
|
<BR> <HR><A NAME=tex2html2792 HREF="node103.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html2790 HREF="node97.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html2786 HREF="node101.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html2794 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html2795 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<HR>
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<P><ADDRESS>
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AI.Repository@cs.cmu.edu
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