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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>11.9. An Example</TITLE>
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<meta name="description" value=" An Example">
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<meta name="keywords" value="clm">
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<meta name="resource-type" value="document">
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<meta name="distribution" value="global">
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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=tex2html3017 HREF="node121.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html3015 HREF="node111.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html3011 HREF="node119.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html3019 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html3020 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html3018 HREF="node121.html"> Numbers</A>
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<B>Up:</B> <A NAME=tex2html3016 HREF="node111.html"> Packages</A>
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<B> Previous:</B> <A NAME=tex2html3012 HREF="node119.html"> Modules</A>
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<HR> <P>
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<H1><A NAME=SECTION001590000000000000000>11.9. An Example</A></H1>
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<P>
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<img align=bottom alt="old_change_begin" src="gif/old_change_begin.gif"><br>
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<A NAME=PACKAGEEXAMPLESECTION>Most</A>
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users will want to load and use packages but will never need to
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build one. Often a user will load a number of packages into the
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<tt>user</tt> package whenever using Common Lisp. Typically an implementation
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might provide some sort of initialization file mechanism to make such setup
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automatic when the Lisp starts up. Table <A HREF="node120.html#INITFILETABLE">11-1</A>
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shows such an initialization file, one that simply
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causes other facilities to be loaded.
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<br><img align=bottom alt="old_change_end" src="gif/old_change_end.gif">
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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 March 1989 (LISP-PACKAGE-NAME) <A NAME=11142> </A> to specify that
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the forthcoming ANSI Common Lisp will use the package name <tt>common-lisp-user</tt>
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instead of <tt>user</tt>.
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<br><img align=bottom alt="change_end" src="gif/change_end.gif">
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<P>
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<A NAME=INITFILETABLE> </A>
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<listing>
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----------------------------------------------------------------
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Table 11-1: An Initialization File
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;;;; Lisp init file for I. Newton.
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;;; Set up the USER package the way I like it.
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(require 'calculus) ;I use CALCULUS a lot; load it.
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(use-package 'calculus) ;Get easy access to its
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; exported symbols.
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(require 'newtonian-mechanics) ;Same thing for NEWTONIAN-MECHANICS
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(use-package 'newtonian-mechanics)
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;;; I just want a few things from RELATIVITY,
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;;; and other things conflict.
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;;; Import only what I need into the USER package.
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(require 'relativity)
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(import '(relativity:speed-of-light
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relativity:ignore-small-errors))
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;;; These are worth loading, but I will use qualified names,
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;;; such as PHLOGISTON:MAKE-FIRE-BOTTLE, to get at any symbols
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;;; I might need from these packages.
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(require 'phlogiston)
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(require 'alchemy)
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;;; End of Lisp init file for I. Newton.
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----------------------------------------------------------------
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</listing>
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<P>
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When each of two files uses some symbols from the other, the author
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of those files must be
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careful to arrange the contents of the file in the proper order.
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Typically each file contains a single package that is a complete module.
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The contents of such a file should include the following items, in
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order:
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<P>
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<OL><LI>
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A call to <tt>provide</tt> that announces the module name.
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<P>
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<LI>
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A call to <tt>in-package</tt> that establishes the package.
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<P>
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<LI>
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A call to <tt>shadow</tt> that establishes any local symbols that will shadow
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symbols that would otherwise be inherited from packages that this
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package will use.
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<P>
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<LI>
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A call to <tt>export</tt> that establishes all of this package's external
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symbols.
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<P>
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<LI>
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Any number of calls to <tt>require</tt> to load other modules that the
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contents of this file might want to use or refer to. (Because the
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calls to <tt>require</tt> follow the calls to <tt>in-package</tt>,
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<tt>shadow</tt>, and <tt>export</tt>, it is possible for the packages that may
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be loaded to refer to external symbols in this package.)
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<P>
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<LI>
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Any number of calls to <tt>use-package</tt>, to make external
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symbols from other packages accessible in this package.
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<P>
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<LI>
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Any number of calls to <tt>import</tt>, to make
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symbols from other packages present in this package.
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<P>
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<LI>
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Finally, the definitions making up the contents of this package/module.
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</OL>
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<P>
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The following mnemonic sentence may be helpful in remembering
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the proper order of these calls:<br>
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<blockquote>
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<b>Put in seven extremely random user interface commands.</b>
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</blockquote>
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<P>
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Each word of the sentence corresponds to one item in the above ordering:
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<PRE>
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Put <tt>Provide</tt>
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IN <tt>IN-package</tt>
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Seven <tt>Shadow</tt>
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EXtremely <tt>EXport</tt>
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Random <tt>Require</tt>
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USEr <tt>USE-package</tt>
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Interface <tt>Import</tt>
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COmmands COntents of package/module
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</PRE>
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<P>
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The sentence says what it helps you to do.
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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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The most distressing aspect of the X3J13 vote to eliminate
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<tt>provide</tt> and <tt>require</tt>
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(REQUIRE-PATHNAME-DEFAULTS) <A NAME=11192> </A>
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is of course that it completely ruins the mnemonic sentence.
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<br><img align=bottom alt="change_end" src="gif/change_end.gif">
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<P>
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Now, suppose for the sake of example
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that the <tt>phlogiston</tt> and <tt>alchemy</tt> packages are
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single-file, single-package modules as described above. The <tt>phlogiston</tt>
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package needs to use the <tt>alchemy</tt> package, and the <tt>alchemy</tt> package
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needs to use several
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external symbols from the <tt>phlogiston</tt> package.
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The definitions in the <tt>alchemy</tt> and <tt>phlogiston</tt> files
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(see tables <A HREF="node120.html#ALCHEMYFILETABLE">11-2</A> and <A HREF="node120.html#PHLOGISTONFILETABLE">11-3</A>)
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allow a
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user to specify <tt>require</tt> statements for either of these modules, or for
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both of them in either order, and all relevant information will be
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loaded automatically and in the correct order.
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<P>
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<A NAME=ALCHEMYFILETABLE> </A>
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<listing>
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----------------------------------------------------------------
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Table 11-2: File alchemy
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;;;; Alchemy functions, written and maintained by Merlin, Inc.
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(provide 'alchemy) ;The module is named ALCHEMY.
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(in-package 'alchemy) ;So is the package.
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;;; There is nothing to shadow.
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;;; Here is the external interface.
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(export '(lead-to-gold gold-to-lead
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antimony-to-zinc elixir-of-life))
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;;; This package/module needs a function from
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;;; the PHLOGISTON package/module.
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(require 'phlogiston)
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;;; We don't frequently need most of the external symbols from
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;;; PHLOGISTON, so it's not worth doing a USE-PACKAGE on it.
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;;; We'll just use qualified names as needed. But we use
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;;; one function, MAKE-FIRE-BOTTLE, a lot, so import it.
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;;; It's external in PHLOGISTON and so can be referred to
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;;; here using ":" qualified-name syntax.
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(import '(phlogiston:make-fire-bottle))
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;;; Now for the real contents of this file.
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(defun lead-to-gold (x)
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"Takes a quantity of lead and returns gold."
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(when (> (phlogiston:heat-flow 5 x x) ;Using a qualified symbol
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3)
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(make-fire-bottle x)) ;Using an imported symbol
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(gild x))
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;;; And so on ...
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----------------------------------------------------------------
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</listing>
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<P>
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<A NAME=PHLOGISTONFILETABLE> </A>
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<listing>
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----------------------------------------------------------------
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Table 11-3: File phlogiston
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;;;; Phlogiston functions, by Thermofluidics, Ltd.
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(provide 'phlogiston) ;The module is named PHLOGISTON.
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(in-package 'phlogiston) ;So is the package.
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;;; There is nothing to shadow.
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;;; Here is the external interface.
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(export '(heat-flow cold-flow mix-fluids separate-fluids
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burn make-fire-bottle))
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;;; This file uses functions from the ALCHEMY package/module.
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(require 'alchemy)
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;;; We use alchemy functions a lot, so use the package.
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;;; This will allow symbols exported from the ALCHEMY package
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;;; to be referred to here without the need for qualified names.
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(use-package 'alchemy)
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;;; No calls to IMPORT are needed here.
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;;; The real contents of this package/module.
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(defvar *feeling-weak* nil)
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(defun heat-flow (amount x y)
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"Make some amount of heat flow from x to y."
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(when *feeling-weak*
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(quaff (elixir-of-life))) ;No qualifier is needed.
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(push-heat amount x y))
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;;; And so on ...
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----------------------------------------------------------------
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</listing>
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<p>
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<img align=bottom alt="old_change_begin" src="gif/old_change_begin.gif"><br>
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For very large modules whose contents are spread over several files
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(the <tt>lisp</tt> package is an example), it is recommended that the user
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create the package and declare all of the shadows and external symbols
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in a separate file, so that this can be loaded before anything that
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might use symbols from this package.
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<br><img align=bottom alt="old_change_end" src="gif/old_change_end.gif">
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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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Indeed, the <tt>defpackage</tt> macro
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approved by X3J13 in January 1989
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(DEFPACKAGE) <A NAME=11210> </A>
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encourages the use of such a separate file.
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(By the way,
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X3J13 voted in March 1989 (LISP-PACKAGE-NAME) <A NAME=11211> </A> to specify that
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the forthcoming ANSI Common Lisp will use the package name <tt>common-lisp</tt>
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instead of <tt>lisp</tt>.)
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Let's take a look at a revision
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of I. Newton's files using <tt>defpackage</tt>.
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<P>
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The new version of the initialization file avoids using <tt>require</tt>;
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instead, we assume that <tt>load</tt> will do the job
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(see table <A HREF="node120.html#DEFPACKAGEINITFILETABLE">11-4</A>).
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<P>
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<A NAME=DEFPACKAGEINITFILETABLE> </A>
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<listing>
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----------------------------------------------------------------
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Table 11-4: An Initialization File When defpackage Is Used
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;;;; Lisp init file for I. Newton.
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;;; Set up the USER package the way I like it.
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(load "calculus") ;I use CALCULUS a lot; load it.
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(use-package 'calculus) ;Get easy access to its
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; exported symbols.
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(load "newtonian-mechanics") ;Ditto for NEWTONIAN-MECHANICS
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(use-package 'newtonian-mechanics)
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;;; I just want a few things from RELATIVITY,
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;;; and other things conflict.
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;;; Import only what I need into the USER package.
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(load "relativity")
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(import '(relativity:speed-of-light
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relativity:ignore-small-errors))
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;;; These are worth loading, but I will use qualified names,
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;;; such as PHLOGISTON:MAKE-FIRE-BOTTLE, to get at any symbols
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;;; I might need from these packages.
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(load "phlogiston")
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(load "alchemy")
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;;; End of Lisp init file for I. Newton.
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----------------------------------------------------------------
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</listing>
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<P>
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The other files have each been split into two parts, one that
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establishes the package and one that defines the contents.
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This example uses a simple convention that for any file
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named, say, ``<tt>foo</tt>'' the file named ``<tt>foo-package</tt>''
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contains the necessary <tt>defpackage</tt> and/or other package-establishing
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code. The idiom
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<P><pre>
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(unless (find-package "FOO")
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(load "foo-package"))
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</pre><P>
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is conventionally used to load a package definition but only if the package
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has not already been defined. (This is a bit clumsy, and there are other
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ways to arrange things so that a package is defined no more than once.)
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<P>
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The file <tt>alchemy-package</tt> is shown in
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table <A HREF="node120.html#DEFPACKAGEALCHEMYPACKAGETABLE">11-5</A>.
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The tricky point here is that the <tt>alchemy</tt> and <tt>phlogiston</tt>
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packages contain mutual references (each imports from the other),
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and so <tt>defpackage</tt> alone cannot do the job. Therefore
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the <tt>phlogiston</tt> package is not mentioned in a <tt>:use</tt> option
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in the <tt>defpackage</tt> for the <tt>alchemy</tt> package. Instead,
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the function <tt>use-package</tt> is called explicitly, after the package definition
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for <tt>phlogiston</tt> has been loaded. Note that this file has
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been coded with excruciating care so as to operate correctly even if
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the package current when the file is loaded does not inherit from
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the <tt>common-lisp</tt> package. In particular, the standard load-package-definition
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idiom has been peppered with package qualifiers:
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<P><pre>
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(cl:unless (cl:find-package "PHLOGISTON")
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(cl:load "phlogiston-package"))
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</pre><P>
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Note the use of the nickname <tt>cl</tt> for the <tt>common-lisp</tt> package.
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<P>
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The <tt>alchemy</tt> file, shown in table <A HREF="node120.html#DEFPACKAGEALCHEMYFILETABLE">11-6</A>,
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simply loads the <tt>alchemy</tt> package definition,
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makes that package current, and then defines the ``real contents''
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of the package.
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<P>
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<A NAME=DEFPACKAGEALCHEMYFILETABLE> </A>
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<listing>
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----------------------------------------------------------------
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Table 11-5: File alchemy-package Using defpackage
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;;;; Alchemy package, written and maintained by Merlin, Inc.
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(cl:defpackage "ALCHEMY"
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(:export "LEAD-TO-GOLD" "GOLD-TO-LEAD"
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"ANTIMONY-TO-ZINC" "ELIXIR-OF-LIFE")
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)
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;;; This package needs a function from the PHLOGISTON package.
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;;; Load the definition of the PHLOGISTON package if necessary.
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(cl:unless (cl:find-package "PHLOGISTON")
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(cl:load "phlogiston-package"))
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;;; We don't frequently need most of the external symbols from
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;;; PHLOGISTON, so it's not worth doing a USE-PACKAGE on it.
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;;; We'll just use qualified names as needed. But we use
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;;; one function, MAKE-FIRE-BOTTLE, a lot, so import it.
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;;; It's external in PHLOGISTON and so can be referred to
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;;; here using ":" qualified-name syntax.
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(cl:import '(phlogiston:make-fire-bottle))
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----------------------------------------------------------------
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</listing>
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<A NAME=DEFPACKAGEALCHEMYPACKAGETABLE> </A>
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<listing>
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----------------------------------------------------------------
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Table 11-6: File alchemy Using defpackage
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;;;; Alchemy functions, written and maintained by Merlin, Inc.
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(unless (find-package "ALCHEMY")
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(load "alchemy-package"))
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(in-package 'alchemy)
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(defun lead-to-gold (x)
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"Takes a quantity of lead and returns gold."
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(when (> (phlogiston:heat-flow 5 x x) ;Using a qualified symbol
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3)
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(make-fire-bottle x)) ;Using an imported symbol
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(gild x))
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;;; And so on ...
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----------------------------------------------------------------
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</listing>
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<P>
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The file <tt>phlogiston-package</tt> is shown in
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table <A HREF="node120.html#DEFPACKAGEPHLOGISTONPACKAGETABLE">11-7</A>.
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This one is a little more straightforward than the file <tt>alchemy-package</tt>,
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because the latter bears the responsibility for breaking the
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circular package references.
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This file simply makes sure that the <tt>alchemy</tt> package is defined
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and then performs a <tt>defpackage</tt> for the <tt>phlogiston</tt> package.
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<P>
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<A NAME=DEFPACKAGEPHLOGISTONPACKAGETABLE> </A>
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<listing>
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----------------------------------------------------------------
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Table 11-7: File phlogiston-package Using defpackage
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;;;; Phlogiston package definition, by Thermofluidics, Ltd.
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;;; This package uses functions from the ALCHEMY package.
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(cl:unless (cl:find-package "ALCHEMY")
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(cl:load "alchemy-package"))
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(cl:defpackage "PHLOGISTON"
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(:use "COMMON-LISP" "ALCHEMY")
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(:export "HEAT-FLOW"
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"COLD-FLOW"
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"MIX-FLUIDS"
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"SEPARATE-FLUIDS"
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"BURN"
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"MAKE-FIRE-BOTTLE")
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)
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----------------------------------------------------------------
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</listing>
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<P>
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The <tt>phlogiston</tt> file, shown in table <A HREF="node120.html#DEFPACKAGEPHLOGISTONFILETABLE">11-8</A>,
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simply loads the <tt>phlogiston</tt> package definition,
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makes that package current, and then defines the ``real contents''
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of the package.
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<P>
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<A NAME=DEFPACKAGEPHLOGISTONFILETABLE> </A>
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<listing>
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----------------------------------------------------------------
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Table 11-8: File phlogiston Using defpackage
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;;;; Phlogiston functions, by Thermofluidics, Ltd.
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(unless (find-package "PHLOGISTON")
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(load "phlogiston-package"))
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(in-package 'phlogiston)
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(defvar *feeling-weak* nil)
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(defun heat-flow (amount x y)
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"Make some amount of heat flow from x to y."
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(when *feeling-weak*
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(quaff (elixir-of-life))) ;No qualifier is needed.
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(push-heat amount x y))
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;;; And so on ...
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----------------------------------------------------------------
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</listing>
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<P>
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Let's look at the question of package circularity in
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this example a little more closely.
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Suppose that the file <tt>alchemy-package</tt> is loaded first.
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It defines the <tt>alchemy</tt> package and then loads
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file <tt>phlogiston-package</tt>. That file in turn finds that the
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package <tt>alchemy</tt> has already been defined and therefore does
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not attempt to load file <tt>alchemy-package</tt> again; it merely
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defines package <tt>phlogiston</tt>. The file <tt>alchemy-package</tt>
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then has a chance to import <tt>phlogiston:make-fire-bottle</tt> and everything
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is fine.
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<P>
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On the other hand, suppose that the file <tt>phlogiston-package</tt> is
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loaded first. It finds that the
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package <tt>alchemy</tt> has <i>not</i> already been defined, and therefore
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|
it immediately loads file <tt>alchemy-package</tt>.
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That file in turn defines the <tt>alchemy</tt> package; then it
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|
finds that package <tt>phlogiston</tt> is not yet defined and so
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|
loads file <tt>phlogiston-package</tt> <i>again</i> (indeed, in nested fashion).
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|
This time file <tt>phlogiston-package</tt> <i>does</i> find that the
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package <tt>alchemy</tt> has already been defined, so it simply defines
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|
package <tt>phlogiston</tt> and terminates.
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|
The file <tt>alchemy-package</tt> then imports <tt>phlogiston:make-fire-bottle</tt>
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|
and terminates.
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|
Finally, the outer loading of file <tt>phlogiston-package</tt>
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|
<i>re-defines</i> package <tt>phlogiston</tt>. Oh, dear. Fortunately the
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|
two definitions of package <tt>phlogiston</tt> agree in every detail, so
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|
everything ought to be all right. Still, it looks a bit dicey; <i>I</i>
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certainly don't have the same warm, fuzzy feeling that I would if
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no package were defined more than once.
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<P>
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Conclusion: <tt>defpackage</tt> goes a long way, but it certainly doesn't
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solve all the possible problems of package and file management.
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Neither did <tt>require</tt> and <tt>provide</tt>. Perhaps further experimentation
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will yield facilities appropriate for future standardization.
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<br><img align=bottom alt="change_end" src="gif/change_end.gif">
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<P>
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