Dealing w/ packages in Org.
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Gerd Heber
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@ -586,3 +586,173 @@
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package should in principle be void of symbols. To make our experiments more
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understandable it is better to allocate and use toy packages. The
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[[http://clhs.lisp.se/Body/m_defpkg.htm][=defpackage=]] function (a macro) allocates a package of a given name.
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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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A package ="P1"= is now present in our environment, without any internal
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symbols.
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#+BEGIN_SRC lisp :session pcl :exports both :results value verbatim
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(package-internal-symbols "P1")
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#+END_SRC
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(The symbol =nil= also represents the empty list.) Let's allocate a few
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symbols and make them internal in the package ="P1"=! The current default
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package is ="COMMON-LISP-USER"= and we switch it to ="P1"= like this:
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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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#+BEGIN_SRC lisp :session pcl :exports both :package "P1"
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*package*
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#+END_SRC
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From now on, the default package is ="P1"=. If we write down a symbol then it
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will be understood by the Lisp reader as accessed via the package ="P1"=,
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that is, unless we use a special syntax. Let us create a list of three
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symbols and assign it to the symbol =list=, and then examine the internal
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symbols of the package ="P1"=:
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#+BEGIN_SRC lisp :session pcl :exports both :package "P1"
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(multiple-value-list
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(ignore-errors
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(setf (symbol-value 'list) '(symb1 symb2 symb3))))
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#+END_SRC
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The reader probably would prefer =(defvar list)= followed by
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=(setf list ...)=, which is rejected by some Lisp implementations, in
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particular SBCL, because in principle the symbol =list=, owned by the package
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="COMMON-LISP"=, is "locked". It is possible to unlock such a symbol, which
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is a little technical, but using =(symbol-value 'list)= as above gets the job
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done.
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#+BEGIN_SRC lisp :session pcl :exports both :package "P1"
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(multiple-value-list
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(ignore-errors
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(package-internal-symbols "P1")))
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#+END_SRC
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Oops! What happened? We've successfully used the function
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=package-internal-symbols= earlier and suddenly it seems to have disappeared
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from the environment. The point is that it was /defined when/
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="COMMON-LISP-USER"= was the default package. Let us examine the situation!
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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 "PACKAGE-INTERNAL-SYMBOLS" "COMMON-LISP-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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(multiple-value-list (find-symbol "PACKAGE-INTERNAL-SYMBOLS" "P1"))
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#+END_SRC
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A symbol with the given name is found in both packages. Before encountering
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the undefined functional value error, Lisp ( more precisely, the Lisp reader)
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had interned a /new/ symbol with the name ="PACKAGE-INTERNAL-SYMBOLS"= in the
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current default package, ="P1"=, for the sole reason that it was present in
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the source text. Let us verify that both symbols are different!
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#+BEGIN_SRC lisp :session pcl :exports both :package "P1"
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(eq (find-symbol "PACKAGE-INTERNAL-SYMBOLS" "COMMON-LISP-USER")
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(find-symbol "PACKAGE-INTERNAL-SYMBOLS" "P1"))
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#+END_SRC
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The symbols are different. Compare that with the status of =nil=, where the
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same symbol =nil= was accessible via ="COMMON-LISP"= and via
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="COMMON-LISP-USER"=. In our present situation both symbols are /different/,
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and so are their respective properties. For example, the first symbol does
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have a functional value and the second does not, which can be tested with the
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function [[http://clhs.lisp.se/Body/f_fbound.htm][=fboundp=]]x:
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#+BEGIN_SRC lisp :session pcl :exports both :package "P1"
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(fboundp (find-symbol "PACKAGE-INTERNAL-SYMBOLS" "COMMON-LISP-USER"))
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#+END_SRC
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#+BEGIN_SRC lisp :session pcl :exports both :package "P1"
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(fboundp (find-symbol "PACKAGE-INTERNAL-SYMBOLS" "P1"))
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#+END_SRC
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This is the first time we have seen the double-colon =‘::’= notation: When
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the symbol =package-internal-symbols= was searched via ="COMMON-LISP-USER"=,
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the answer was positive but the returned symbol was a little lengthy:
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=COMMON-LISP-USER::PACKAGE-INTERNAL-SYMBOLS=. The current default package is
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="P1"=, via which /this/ symbol is not accessible. However, we may access it
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using the double-colon notation:
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#+BEGIN_CENTER
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=package-name::symbol-name=
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#+END_CENTER
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This is one of the methods which allows the user or Lisp to refer to a symbol
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not accessible via the default package. The Lisp reader is told: "We mean the
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symbol =symbol-name= via the package =package-name=." In contrast, for the
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symbol with the same name but interned in the different ="P1"= package,
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because ="P1"= is the current default package, the double colon notation is
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not necessary. The longer notation lets us access a symbol interned in the
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="COMMON-LISP-USER"= package and its functional value as follows:
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#+BEGIN_SRC lisp :session pcl :exports both :package "P1" :results value verbatim
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(cl-user::package-internal-symbols "P1")
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#+END_SRC
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The =cl-user::= prefix tells Lisp to look for the symbol via the ="CL-USER"=
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package (Remember that ="CL-USER"= is a nickname for ="COMMON-LISP-USER"=?)
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and, because of its position just after a left parenthesis, to use its
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functional value. Currently, there are four internal symbols in the package
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="P1"= and they are returned in an arbitrary order. By the way, what about
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the symbol =list=, to which we assigned a list (!) of symbols?
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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 "LIST" "P1"))
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#+END_SRC
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The symbol is accessible via the package ="P1"= but with a different status,
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the status /inherited/. We will examine this situation later. In fact, the
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symbol =list=, one of the most important symbols in Lisp, is already present
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in the package ="COMMON-LISP"= at the beginning of a session, and by default
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accessible from any package.
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We must mention also that our function $\rho$, or the function =find-symbol=
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if you prefer, is (almost) injective /with respect to/ $st$. Let $p_1$ and
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$p_2$ be two packages, $st_1$ and $st_2$ two strings. Then if
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$\rho(p_1, st_1) = \rho(p_2, st_2)\neq\mathtt{nil}$, then $st_1 = st_2$. Pay
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attention to the fact that the equality is an equality between /symbols/.
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This is a consequence of $\mathtt{symbol-name}(\rho(p, st)) = st$; in other
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words, there is no possibility of nicknames for a symbol name. In particular,
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there is a 1-1 correspondence between the internal symbols of a package and
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their respective names.
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* Importing a symbol.
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Instead of using the lengthy notation =cl-user::xxx= to make an internal
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symbol in ="CL-USER"= accessible from ="P1"=, we may opt to /import/ this
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symbol in the package ="P1"=. However, in doing so there is another problem
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due to a name conflict, a point which will be examined in detail later:
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#+BEGIN_SRC lisp :session pcl :exports both :package "P1" :results value verbatim
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(multiple-value-list
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(ignore-errors
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(import 'cl-user::package-internal-symbols "P1")))
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#+END_SRC
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The problem is a "collision" between two /different/ symbols, which cannot
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coexist as internal symbols in the /same/ package ="P1"= with the /same/
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name. Importing the symbol internal in ="CL-USER"= into the package ="P1"= is
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not compatible with the current presence of /another/ symbol in ="P1"= with
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the same =symbol-name=. We must first [[http://clhs.lisp.se/Body/f_uninte.htm][=unintern=]] the accidental, pre-existing
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symbol in ="P1"= to "make room" for the symbol from ="CL-USER"=.
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#+BEGIN_SRC lisp :session pcl :exports both :package "P1" :results value verbatim
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(unintern 'package-internal-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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(import 'cl-user::package-internal-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-internal-symbols "P1")
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#+END_SRC
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