Added section on importing external symbols.
This commit is contained in:
Gerd Heber
parent
b048835e4d
commit
7df5bb41ab
@ -1421,3 +1421,206 @@
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(... assuming that no name conflict is generated.)
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(... assuming that no name conflict is generated.)
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Let's continue our journey in ="P1"=!
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Let's continue our journey in ="P1"=!
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#+BEGIN_SRC lisp :session pcl :package "P2" :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 :package "P1" :exports both :results value verbatim
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(export (intern "SYMB1" "P2") "P2")
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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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(multiple-value-list (find-symbol "SYMB1" "P2"))
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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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(multiple-value-list (find-symbol "SYMB1" "P1"))
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#+END_SRC
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We intern a symbol =p2::symb1= in ="P2"= and make it external immediately. As
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it stands this symbol is not accessible via ="P1"=. The fact that it is
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external in ="P2"= does not prevent us from importing it in ="P1"=.
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#+BEGIN_SRC lisp :session pcl :package "P1" :exports both :results value verbatim
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(import 'p2:symb1 "P1")
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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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(multiple-value-list (find-symbol "SYMB1" "P1"))
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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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(multiple-value-list (find-symbol "SYMB1" "P2"))
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#+END_SRC
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Once the importation is done, the symbol is accessible via ="P1"= with the
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status /internal/. The /external/ status with respect to ="P2"= is unchanged,
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which underlines that the status of a symbol does not depend on anything in
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the symbol itself. It is the /package/ which must be examined to determine
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the status of a symbol /with respect to this package/.
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Curiously, the last =find-symbol= returns the symbol =symb1=, without the
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prefix ='p2:'=. Lisp prints a symbol according to the following rule:
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#+BEGIN_QUOTE
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First, Lisp determines whether a symbol is accessible via the current default
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package, in which case it is printed without using the colon notation.
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Otherwise, Lisp examines the home package, where the symbol is certainly
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internal or external, and it is printed with the double or simple colon
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notation. The only exception to this rule appears to be a /keyword/, that is,
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a symbol of the package ="KEYWORD"=. (Though the ANSI definition is somewhat
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incomplete for the ="KEYWORD"= package, we can assume that it is forbidden to
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change anything about the scope of keywords.) Keywords are printed simply as
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=':xxx'=, without any prefix. Finally, if the symbol is homeless, it is
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printed as =#:xxx=.
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#+END_QUOTE
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The take-home message here is that an importation does not change anything
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for the relevant symbol; it is just a matter of updating a list of internal
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symbols in some package, that is, a list of respective pointers toward these
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symbols. Installing a new signpost in San Francisco that reads "New York"
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does not change anything in New York. Once more:
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#+BEGIN_SRC lisp :session pcl :package "P1" :exports both :results value verbatim
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(eq (find-symbol "SYMB1" "P1")
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(find-symbol "SYMB1" "P2"))
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#+END_SRC
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A novice could believe that adding ="P2"= to the =package-use-list= of ="P1"=
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could generate a name conflict, because the symbols =p1::symb1= and
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=p2::symb1= would become both accessible via ="P1"=. This is not the case:
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both notations represent one and the same symbol, and there is no ambiguity.
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#+BEGIN_SRC lisp :session pcl :package "P1" :exports both :results value verbatim
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(use-package "P2" "P1")
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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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(multiple-value-list (find-symbol "SYMB1" "P1"))
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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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(multiple-value-list (find-symbol "SYMB1" "P2"))
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#+END_SRC
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Note that because our symbol is internal in ="P1"=, the function
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=find-symbol= reports its status as =:internal=. This is in a sense preferred
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to the status =:inherited=, which after all would be possible. A name
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conflict occurs only if two /different/ symbols with the same name could be
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accessible via the same package, possibly via another package used by the
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first one. Two symmetrical illustrations:
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#+BEGIN_SRC lisp :session pcl :package "P1" :exports both :results value verbatim
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(export (intern "SYMB2" "P2") "P2")
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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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(multiple-value-list (intern "SYMB2" "P1") )
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#+END_SRC
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This is not an error: The second =intern= /does not/ allocate a new symbol, a
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symbol of name ="SYMB2"= being already accessible via ="P1"=, which uses
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="P2"=. In particular the second =intern= does not import the pre-existing
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symbol.
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We have to come up with something else to really allocate a new symbol.
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#+BEGIN_SRC lisp :session pcl :package "P1" :exports both :results value verbatim
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(multiple-value-list
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(ignore-errors
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(import (make-symbol "SYMB2") "P1")))
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#+END_SRC
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The function [[http://clhs.lisp.se/Body/f_mk_sym.htm][=make-symbol=]] allocates a /homeless/ symbol of a given name,
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that's its definition. A homeless symbol cannot cause a name conflict,
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because it is /not/ accessible via any package. Nothing prevents us from
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trying to =import= this symbol in ="P1"=, but this generates a name conflict
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between two different symbols: the freshly allocated =#:symb2= and the symbol
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external in ="P2"= accessible via ="P1"= which uses ="P2"=.
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Here's another way to cause a name conflict:
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#+BEGIN_SRC lisp :session pcl :package "P1" :exports both :results value verbatim
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(multiple-value-list (intern "SYMB3" "P1"))
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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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(multiple-value-list (intern "SYMB3" "P2"))
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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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(multiple-value-list
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(ignore-errors
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(export 'p2::symb3 "P2")))
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#+END_SRC
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The package ="P1"= uses the package ="P2"=, but this does not prevent them
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from having two different internal symbols with the same name ="SYMB3"=, for
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="P1"= does not see the symbol internal in ="P2"=. If we try to make this
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symbol external in ="P2"=, a name conflict arises.
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Another sort of name conflict can occur between two used packages:
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#+BEGIN_SRC lisp :session pcl :package "P1" :exports both :results value verbatim
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(export (intern "SYMB4" "P2") "P2")
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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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(export (intern "SYMB4" (defpackage :p3)) "P3")
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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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(multiple-value-list
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(ignore-errors
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(use-package "P3" "P1")))
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#+END_SRC
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Compare that with:
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#+BEGIN_SRC lisp :session pcl :package "P1" :exports both :results value verbatim
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(export (intern "SYMB5" "P2") "P2")
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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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(export (import 'p2:symb5 "P3"))
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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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(unexport 'p3:symb4 "P3")
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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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(use-package "P3" "P1")
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#+END_SRC
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Two symbols of name ="SYMB5"= are external in ="P2"= and ="P3"=, but the
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second one is in fact the same as the first one: it has been imported and
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immediately made external in ="P3"=.
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This time the =use-package=, which makes the external symbols of ="P2"= /and/
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="P3"= visible from ="P1"=, does not generate a name conflict, since we are
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dealing with two different paths which lead to the same /symbol/. This works
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as long as we do not forget to first =unexport= the symbol ="SYMB4"= of
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="P3"=, i.e., to downgrade its status from /external/ to /internal/.
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* A symbol can shadow another symbol.
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In the matter of name conflicts, the Lisp creators could have adopted another
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strategy and decided that the symbols /present/ in a package, that is,
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/internal/ or /external/ in this package, have precedence over /inherited/,
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possibly different, symbols. However, they reasoned that this could easily
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generate true programming errors because of possible perverse collisions of
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identifiers, and turn into a source of obscure bugs. Nevertheless, in line
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with the general spirit of Lisp, if a programmer really wants to use such a
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strategy, she can do it on a case-by-case basis and explicitly describe the
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intended exemptions. This is the role of the [[http://clhs.lisp.se/Body/f_pkg_sh.htm][=package-shadowing-symbols=]]
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list, which is maintained in every package $p$ and denoted by $ShgS(p)$.
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This list is empty initially. $ShgS(p)$ can contain symbols present in the
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concerned package, internal or external. These symbols will automatically
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/shadow/ any other competing symbol, such as symbols exported in other used
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packages and with the same =symbol-name=.
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The function [[http://clhs.lisp.se/Body/f_shadow.htm][=shadow=]] takes a symbol name (or a list of symbol names) and a
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package name, allocates a symbol with this name if it is not /present/ in
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this package, and records it in the shadowing list of the package.
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