Just about where the trouble begins.
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Gerd Heber
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c6d958e6a0
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0d29b9e591
@ -1120,3 +1120,215 @@
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lists maintained in the packages play a "dual" role in the packaging system,
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the list of /external/ symbols and the list of /used/ packages. The following
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function gives the list of the external symbols of a package.
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#+BEGIN_SRC lisp :session pcl :exports both :package "P1" :results value verbatim
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*package*
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#+END_SRC
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#+BEGIN_SRC lisp :session pcl :exports both :package "P1" :results value verbatim
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(defun package-external-symbols (package)
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(let ((rslt nil))
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(do-external-symbols (s package)
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(push s rslt))
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rslt))
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#+END_SRC
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The evaluation of the first expression is just to confirm that the default
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package remains ="P1"=. Let us see what the lists of external symbols of the
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packages ="P1"=, ="CL-USER"= and ="COMMON-LISP"= are:
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#+BEGIN_SRC lisp :session pcl :exports both :package "P1" :results value verbatim
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(package-external-symbols "P1")
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#+END_SRC
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#+BEGIN_SRC lisp :session pcl :exports both :package "P1" :results value verbatim
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(package-external-symbols "CL-USER")
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#+END_SRC
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#+BEGIN_SRC lisp :session pcl :exports both :package "P1" :results value verbatim
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(subseq (package-external-symbols "COMMON-LISP") 0 10)
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#+END_SRC
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#+BEGIN_SRC lisp :session pcl :exports both :package "P1" :results value verbatim
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(length (package-external-symbols "COMMON-LISP"))
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#+END_SRC
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There are no external symbols in ="P1"= and ="CL-USER"=, but 978 such symbols
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in the package ="COMMON-LISP"=. (Only the first ten are being displayed.) A
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point of concern might be the following: these symbols are accessible via the
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package ="COMMON-LISP"= and yet the double-colon notation
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=common-lisp::simple-error= was not used. The explanation has two "dual"
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parts:
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1. The symbol =simple-error= is external in the package ="COMMON-LISP"=.
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2. The package ="COMMON-LISP"= is in the =package-use-list= of the package
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="P1"=:
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#+BEGIN_SRC lisp :session pcl :exports both :package "P1" :results value verbatim
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(multiple-value-list (find-symbol "SIMPLE-ERROR" "COMMON-LISP"))
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#+END_SRC
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#+BEGIN_SRC lisp :session pcl :exports both :package "P1" :results value verbatim
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(package-use-list "P1")
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#+END_SRC
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#+BEGIN_SRC lisp :session pcl :exports both :package "P1" :results value verbatim
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(package-nicknames (find-package "COMMON-LISP"))
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#+END_SRC
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="CL"= is the nickname of the package whose official name is ="COMMON-LISP"=,
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and which is not to be confused with ="COMMON-LISP-USER"=.
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#+BEGIN_SRC lisp :session pcl :exports both :package "P1" :results value verbatim
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(eq (find-package "CL")
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(find-package "COMMON-LISP"))
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#+END_SRC
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#+BEGIN_SRC lisp :session pcl :exports both :package "P1" :results value verbatim
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(eq (print (find-package "CL"))
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(print (find-package "CL-USER")))
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#+END_SRC
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Applied to a package $p$, the Lisp function [[http://clhs.lisp.se/Body/f_pkg_us.htm][=package-use-list=]] returns the
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list of all the packages used by $p$. If $p_1$ uses $p_2$, then it is said
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that $p_2$ is used by $p_1$. Whether this relation is reflexive can be
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doubted (What does "$p$ uses $p$" mean?), and it is neither symmetric
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("$p_1$ uses $p_2$" does not imply "$p_2$ uses $p_1$") nor transitive
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("$p_1$ uses $p_2$" and "$p_2$ uses $p_3$” does not imply
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"$p_1$ uses $p_3$"). The oriented graph of the /use/ relation is totally
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arbitrary.
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* First toy examples.
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Let's start over with two "fresh" packages ="P1"= and ="P2"=.
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#+BEGIN_SRC lisp :session pcl :exports both :package "P1" :results value verbatim
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(in-package "CL-USER")
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#+END_SRC
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#+BEGIN_SRC lisp :session pcl :exports both :results value verbatim
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(delete-package (find-package "P1"))
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#+END_SRC
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#+BEGIN_SRC lisp :session pcl :exports both :results value verbatim
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(defpackage "P1" (:use "CL"))
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#+END_SRC
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#+BEGIN_SRC lisp :session pcl :exports both :results value verbatim
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(defpackage "P2" (:use "CL"))
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#+END_SRC
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We switch the default package to ="P1"=:
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#+BEGIN_SRC lisp :session pcl :exports both :results value verbatim
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(in-package "P1")
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#+END_SRC
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and we assume that, /in this environment/, we define the functions
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=package-internal-symbols= and =package-external-symbols= as explained
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before.
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#+BEGIN_SRC lisp :session pcl :package "P1" :exports both :results value verbatim
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(defun package-internal-symbols (package)
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(let ((rslt))
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(do-symbols (s package)
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(when (eq (second
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(multiple-value-list
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(find-symbol (symbol-name s) package)))
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:internal)
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(push s rslt)))
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rslt))
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#+END_SRC
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#+BEGIN_SRC lisp :session pcl :package "P1" :exports both :results value verbatim
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(defun package-external-symbols (package)
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(let ((rslt))
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(do-external-symbols (s package)
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(push s rslt))
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rslt))
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#+END_SRC
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What about the internal symbols of ="P1"=?
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#+BEGIN_SRC lisp :session pcl :package "P1" :exports both :results value verbatim
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(package-internal-symbols "P1")
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#+END_SRC
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We see the symbols naming our functions and also the symbols =rslt= and =s=
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used to define them. All the other symbols used in their definitions are in
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fact external in ="COMMON-LISP"= and absent here. We move now to the package
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="P2"= from where we cannot access the symbol =package-internal-symbols=
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directly.
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#+BEGIN_SRC lisp :session pcl :package "P1" :exports both :results value verbatim
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(in-package "P2")
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#+END_SRC
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#+BEGIN_SRC lisp :session pcl :package "P2" :exports both :results value verbatim
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(multiple-value-list (find-symbol "PACKAGE-INTERNAL-SYMBOLS" "P2"))
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#+END_SRC
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We have already seen the use of the ='::'= notation to make reference to the
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symbol at once. We can also /import/ the symbol, in which case it becomes
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directly accessible from ="P2"= permanently:
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#+BEGIN_SRC lisp :session pcl :package "P2" :exports both :results value verbatim
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(import 'p1::package-internal-symbols "P2")
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#+END_SRC
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#+BEGIN_SRC lisp :session pcl :package "P2" :exports both :results value verbatim
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(package-internal-symbols "P2")
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#+END_SRC
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Another solution requires /two/ steps:
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1. Make the symbol =external= in some package, which is the role of the Lisp
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function [[http://clhs.lisp.se/Body/f_export.htm][=export=]].
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2. Declare that the current package /uses/ the package where the symbol is
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external.
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Let us use this method to make the symbol =package-external-symbols= directly
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visible from ="P2"=.
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#+BEGIN_SRC lisp :session pcl :package "P2" :exports both :results value verbatim
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(export 'p1::package-external-symbols "P1")
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#+END_SRC
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#+BEGIN_SRC lisp :session pcl :package "P2" :exports both :results value verbatim
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(use-package "P1" "P2")
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#+END_SRC
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#+BEGIN_SRC lisp :session pcl :package "P2" :exports both :results value verbatim
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(package-external-symbols "P2")
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#+END_SRC
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#+BEGIN_SRC lisp :session pcl :package "P2" :exports both :results value verbatim
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(package-external-symbols "P1")
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#+END_SRC
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An =export= expression has the same form as an =import= one:
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=(export= $sm$ $st$ =)=, where $sm$ is a symbol and $st$ is a string naming a
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package. The symbol $sm$ must be readable from the current default package,
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for us ="P2"=, which explains the (required) prefix ='p1::'=. The argument
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$st$ is a string naming the package where the symbol $sm$ is to become
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external. Again, the terminology is somewhat misleading: the function
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=export= does not export anything. All it does is to make some connection
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$st \rightarrow sm$ "exportable" to other packages via =use-package=
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expressions. The 978 external symbols of ="COMMON-LISP"= contain all the
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standard Lisp constants, variables and functions. Because an allocated
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package is always initialized with the package ="COMMON-LISP"= in its
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=package-use-list=, these basic Lisp objects will be accessible from any
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package. If we remove ="COMMON-LISP"= from the =use-package-list= of the
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current default package, via =unuse-package=, the situation becomes a little
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tricky.
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#+BEGIN_SRC lisp :session pcl :package "P2" :exports both :results value verbatim
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(common-lisp::import
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'(common-lisp::progn common-lisp::ignore-errors common-lisp::multiple-value-list common-lisp::+)
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"P2")
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#+END_SRC
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#+BEGIN_SRC lisp :session pcl :package "P2" :exports both :results value verbatim
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(unuse-package "COMMON-LISP" "P2")
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#+END_SRC
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