Common Lisp the Language, 2nd Edition
Some of the functions and variables in this section are described in previous sections but are included here for completeness.
It is up to each implementation's compiler to ensure that when a
compiled file is loaded, all of the symbols in the file end up in the
same packages that they would occupy if the Lisp source file were
loaded. In most compilers, this will be accomplished by treating
certain package operations as though they are surrounded by
(eval-when (compile load eval) ...); see eval-when.
These operations are
make-package, in-package, shadow, shadowing-import,
export, unexport, use-package, unuse-package, and import.
To guarantee proper compilation in all Common Lisp
implementations, these functions should appear only at top level within
a file. As a matter of style, it is suggested that each file contain
only one package, and that all of the package setup forms appear near
the start of the file. This is discussed in more detail, with examples,
in section 11.9.
X3J13 voted in March 1989 (IN-PACKAGE-FUNCTIONALITY)
to cancel the specifications of the preceding paragraph
in order to support a model of file compilation in which the
compiler need never take special note of ordinary function calls;
only special forms and macros are recognized as affecting the state
of the compilation process.
As part of this change in-package was changed to be a macro
rather than a function and its functionality was restricted.
The actions of
shadow, shadowing-import, use-package,
import, intern, and export for compilation
purposes may be accomplished with the new macro defpackage.
For the functions described here, all optional arguments named package default to the current value of *package*. Where a function takes an argument that is either a symbol or a list of symbols, an argument of nil is treated as an empty list of symbols. Any argument described as a package name may be either a string or a symbol. If a symbol is supplied, its print name will be used as the package name; if a string is supplied, the user must take care to specify the same capitalization used in the package name, normally all uppercase.
[Variable]
*package*
The value of this variable must be a package; this package is said to be the current package. The initial value of *package* is the user package.
X3J13 voted in March 1989 (LISP-PACKAGE-NAME) to specify that
the forthcoming ANSI Common Lisp will use the package name common-lisp-user
instead of user.
The function load rebinds *package* to its current value. If some form in the file changes the value of *package* during loading, the old value will be restored when the loading is completed.
X3J13 voted in October 1988 (COMPILE-FILE-PACKAGE)
to require compile-file to rebind *package* to its current value.
[Function]
make-package package-name &key :nicknames :use
This creates and returns a new package with the specified package name. As described above, this argument may be either a string or a symbol. The :nicknames argument must be a list of strings to be used as alternative names for the package. Once again, the user may supply symbols in place of the strings, in which case the print names of the symbols are used. These names and nicknames must not conflict with any existing package names; if they do, a correctable error is signaled.
The :use argument is a list of packages or the names (strings or symbols) of packages whose external symbols are to be inherited by the new package. These packages must already exist. If not supplied, :use defaults to a list of one package, the lisp package.
X3J13 voted in March 1989 (LISP-PACKAGE-NAME) to specify that
the forthcoming ANSI Common Lisp will use the package name common-lisp
instead of lisp.
X3J13 voted in January 1989 (MAKE-PACKAGE-USE-DEFAULT) to change the specification of make-package so that the default value for the :use argument is unspecified. Portable code should specify :use '("COMMON-LISP") explicitly.
[Function]
in-package package-name &key :nicknames :use
The in-package function is intended to be placed at the start of a file containing a subsystem that is to be loaded into some package other than user.
If there is not already a package named package-name, this function is similar to make-package, except that after the new package is created, *package* is set to it. This binding will remain in force until changed by the user (perhaps with another in-package call) or until the *package* variable reverts to its old value at the completion of a load operation.
If there is an existing package whose name is package-name, the
assumption is that the user is re-loading a file after making some
changes. The existing package is augmented to reflect any new nicknames
or new packages in the :use list (with the usual error checking), and
*package* is then set to this package.
X3J13 voted in January 1989
(RETURN-VALUES-UNSPECIFIED)
to specify that in-package returns the new package, that is, the
value of *package* after the operation has been executed.
X3J13 voted in March 1989 (LISP-PACKAGE-NAME) to specify that the forthcoming ANSI Common Lisp will use the package name common-lisp-user instead of user.
X3J13 voted in March 1989 (IN-PACKAGE-FUNCTIONALITY) to restrict the functionality of in-package and to make it a macro. This is an incompatible change.
Making in-package a macro rather than a function means that there is no need to require compile-file to handle it specially. Since defpackage is also defined to have side effects on the compilation environment, there is no need to require any of the package functions to be treated specially by the compiler.
[Macro]
in-package name
This macro causes *package* to be set to the package named name,
which must be a symbol or string. The name is not evaluated.
An error is signaled if the
package does not already exist. Everything this macro does is also
performed at compile time if the call appears at top level.
[Function]
find-package name
The name must be a string that is the name or nickname for a package. This argument may also be a symbol, in which case the symbol's print name is used. The package with that name or nickname is returned; if no such package exists, find-package returns nil. The matching of names observes case (as in string=).
X3J13 voted in January 1989
(PACKAGE-FUNCTION-CONSISTENCY)
to allow find-package to accept a package object, in which case
the package is simply returned (see section 11.2).
[Function]
package-name package
The argument must be a package. This function returns the string that names that package.
X3J13 voted in January 1989
(PACKAGE-FUNCTION-CONSISTENCY)
to allow package-name to accept a package name or nickname, in which case
the primary name of the package so specified is returned
(see section 11.2).
X3J13 voted in January 1989
(PACKAGE-DELETION)
to add a function to delete packages.
One consequence of this vote is that package-name
will return nil instead of a package name if applied
to a deleted package object. See delete-package.
[Function]
package-nicknames package
The argument must be a package. This function returns the list of nickname strings for that package, not including the primary name.
X3J13 voted in January 1989
(PACKAGE-FUNCTION-CONSISTENCY)
to allow package-nicknames to accept a package name or nickname,
in which case the nicknames of the package so specified are returned
(see section 11.2).
[Function]
rename-package package new-name &optional new-nicknames
The old name and all of the old nicknames of package are eliminated and are replaced by new-name and new-nicknames. The new-name argument is a string or symbol; the new-nicknames argument, which defaults to nil, is a list of strings or symbols.
X3J13 voted in January 1989
(PACKAGE-FUNCTION-CONSISTENCY)
to clarify that the package argument may be either a package object
or a package name (see section 11.2).
X3J13 voted in January 1989
(RETURN-VALUES-UNSPECIFIED)
to specify that rename-package returns package.
[Function]
package-use-list package
A list of other packages used by the argument package is returned.
X3J13 voted in January 1989
(PACKAGE-FUNCTION-CONSISTENCY)
to clarify that the package argument may be either a package object
or a package name (see section 11.2).
[Function]
package-used-by-list package
A list of other packages that use the argument package is returned.
X3J13 voted in January 1989
(PACKAGE-FUNCTION-CONSISTENCY)
to clarify that the package argument may be either a package object
or a package name (see section 11.2).
[Function]
package-shadowing-symbols package
A list is returned of symbols that have been declared as shadowing symbols in this package by shadow or shadowing-import. All symbols on this list are present in the specified package.
X3J13 voted in January 1989
(PACKAGE-FUNCTION-CONSISTENCY)
to clarify that the package argument may be either a package object
or a package name (see section 11.2).
[Function]
list-all-packages
This function returns a list of all packages that currently exist in the Lisp system.
[Function]
delete-package package
X3J13 voted in January 1989 (PACKAGE-DELETION) to add the delete-package function, which deletes the specified package from all package system data structures. The package argument may be either a package or the name of a package.
If package is a name but there is currently no package of that name, a correctable error is signaled. Continuing from the error makes no deletion attempt but merely returns nil from the call to delete-package.
If package is a package object that has already been deleted, no error is signaled and no deletion is attempted; instead, delete-package immediately returns nil.
If the package specified for deletion is currently used by other packages, a correctable error is signaled. Continuing from this error, the effect of the function unuse-package is performed on all such other packages so as to remove their dependency on the specified package, after which delete-package proceeds to delete the specified package as if no other package had been using it.
If any symbol had the specified package as its home package before the call to delete-package, then its home package is unspecified (that is, the contents of its package cell are unspecified) after the delete-package operation has been completed. Symbols in the deleted package are not modified in any other way.
The name and nicknames of the package cease to be recognized package names. The package object is still a package, but anonymous; packagep will be true of it, but package-name applied to it will return nil.
The effect of any other package operation on a deleted package object is undefined. In particular, an attempt to locate a symbol within a deleted package (using intern or find-symbol, for example) will have unspecified results.
delete-package returns t if the deletion succeeds,
and nil otherwise.
[Function]
intern string &optional package
The package, which defaults to the current package, is searched for a symbol with the name specified by the string argument. This search will include inherited symbols, as described in section 11.4. If a symbol with the specified name is found, it is returned. If no such symbol is found, one is created and is installed in the specified package as an internal symbol (as an external symbol if the package is the keyword package); the specified package becomes the home package of the created symbol.
X3J13 voted in March 1989 (CHARACTER-PROPOSAL)
to specify that intern may in effect perform the
search using a copy of the argument string in which
some or all of the implementation-defined
attributes have been removed from the characters of the string.
It is implementation-dependent which attributes are removed.
Two values are returned. The first is the symbol that was found or created. The second value is nil if no pre-existing symbol was found, and takes on one of three values if a symbol was found:
X3J13 voted in January 1989 (PACKAGE-FUNCTION-CONSISTENCY) to clarify that the package argument may be either a package object or a package name (see section 11.2).
[Function]
find-symbol string &optional package
This is identical to intern, but it never creates a new symbol. If a symbol with the specified name is found in the specified package, directly or by inheritance, the symbol found is returned as the first value and the second value is as specified for intern. If the symbol is not accessible in the specified package, both values are nil.
X3J13 voted in January 1989
(PACKAGE-FUNCTION-CONSISTENCY)
to clarify that the package argument may be either a package object
or a package name (see section 11.2).
[Function]
unintern symbol &optional package
If the specified symbol is present in the specified package, it is removed from that package and also from the package's shadowing-symbols list if it is present there. Moreover, if the package is the home package for the symbol, the symbol is made to have no home package. Note that in some circumstances the symbol may continue to be accessible in the specified package by inheritance. unintern returns t if it actually removed a symbol, and nil otherwise.
unintern should be used with caution. It changes the state of the package system in such a way that the consistency rules do not hold across the change.
X3J13 voted in January 1989
(PACKAGE-FUNCTION-CONSISTENCY)
to clarify that the package argument may be either a package object
or a package name (see section 11.2).
[Function]
export symbols &optional package
The symbols argument should be a list of symbols, or possibly a single symbol. These symbols become accessible as external symbols in package (see section 11.4). export returns t.
By convention, a call to export listing all exported symbols is placed near the start of a file to advertise which of the symbols mentioned in the file are intended to be used by other programs.
X3J13 voted in January 1989
(PACKAGE-FUNCTION-CONSISTENCY)
to clarify that the package argument may be either a package object
or a package name (see section 11.2).
[Function]
unexport symbols &optional package
The argument should be a list of symbols, or possibly a single symbol. These symbols become internal symbols in package. It is an error to unexport a symbol from the keyword package (see section 11.4). unexport returns t.
X3J13 voted in January 1989
(PACKAGE-FUNCTION-CONSISTENCY)
to clarify that the package argument may be either a package object
or a package name (see section 11.2).
[Function]
import symbols &optional package
The argument should be a list of symbols, or possibly a single symbol. These symbols become internal symbols in package and can therefore be referred to without having to use qualified-name (colon) syntax. import signals a correctable error if any of the imported symbols has the same name as some distinct symbol already accessible in the package (see section 11.4). import returns t.
X3J13 voted in June 1987 (IMPORT-SETF-SYMBOL-PACKAGE)
to clarify that if any symbol to be imported has no home package
then import sets the home package of the symbol to the
package to which the symbol is being imported.
X3J13 voted in January 1989
(PACKAGE-FUNCTION-CONSISTENCY)
to clarify that the package argument may be either a package object
or a package name (see section 11.2).
[Function]
shadowing-import symbols &optional package
This is like import, but it does not signal an error even if the importation of a symbol would shadow some symbol already accessible in the package. In addition to being imported, the symbol is placed on the shadowing-symbols list of package (see section 11.5). shadowing-import returns t.
shadowing-import should be used with caution. It changes the state of the package system in such a way that the consistency rules do not hold across the change.
X3J13 voted in January 1989
(PACKAGE-FUNCTION-CONSISTENCY)
to clarify that the package argument may be either a package object
or a package name (see section 11.2).
[Function]
shadow symbols &optional package
The argument should be a list of symbols, or possibly a single symbol. The print name of each symbol is extracted, and the specified package is searched for a symbol of that name. If such a symbol is present in this package (directly, not by inheritance), then nothing is done. Otherwise, a new symbol is created with this print name, and it is inserted in the package as an internal symbol. The symbol is also placed on the shadowing-symbols list of the package (see section 11.5). shadow returns t.
X3J13 voted in March 1988 (SHADOW-ALREADY-PRESENT)
to change shadow to accept strings as well as well as symbols
(a string in the symbols list being treated as a print name),
and to clarify that if a symbol of specified name is already in
the package but is not yet on the shadowing-symbols list
for that package, then shadow does add it to the shadowing-symbols
list rather than simply doing nothing.
shadow should be used with caution. It changes the state of the package system in such a way that the consistency rules do not hold across the change.
X3J13 voted in January 1989
(PACKAGE-FUNCTION-CONSISTENCY)
to clarify that the package argument may be either a package object
or a package name (see section 11.2).
[Function]
use-package packages-to-use &optional package
The packages-to-use argument should be a list of packages or package names, or possibly a single package or package name. These packages are added to the use-list of package if they are not there already. All external symbols in the packages to use become accessible in package as internal symbols (see section 11.4). It is an error to try to use the keyword package. use-package returns t.
X3J13 voted in January 1989
(PACKAGE-FUNCTION-CONSISTENCY)
to clarify that the package argument may be either a package object
or a package name (see section 11.2).
[Function]
unuse-package packages-to-unuse &optional package
The packages-to-unuse argument should be a list of packages or package names, or possibly a single package or package name. These packages are removed from the use-list of package. unuse-package returns t.
X3J13 voted in January 1989
(PACKAGE-FUNCTION-CONSISTENCY)
to clarify that the package argument may be either a package object
or a package name (see section 11.2).
X3J13 voted in January 1989 (DEFPACKAGE) to add a macro defpackage to the language to make it easier to create new packages, alleviating the burden on the programmer to perform the various setup operations in exactly the correct sequence.
[Macro]
defpackage defined-package-name {option}*
This creates a new package, or modifies an existing one, whose name is defined-package-name. The defined-package-name may be a string or a symbol; if it is a symbol, only its print name matters, and not what package, if any, the symbol happens to be in. The newly created or modified package is returned as the value of the defpackage form.
Each standard option is a list of a keyword (the name of the option) and associated arguments. No part of a defpackage form is evaluated. Except for the :size option, more than one option of the same kind may occur within the same defpackage form.
The standard options for defpackage are as follows. In every case, any option argument called package-name or symbol-name may be a string or a symbol; if it is a symbol, only its print name matters, and not what package, if any, the symbol happens to be in.
X3J13 voted in January 1989 (MAKE-PACKAGE-USE-DEFAULT) to change the specification of make-package so that the default value for the :use argument is unspecified. This change affects defpackage as well. Portable code should specify (:use '("COMMON-LISP")) explicitly.
The order in which options appear in a defpackage form does not matter; part of the convenience of defpackage is that it sorts out the options into the correct order for processing. Options are processed in the following order:
If no package named defined-package-name already exists, defpackage creates it. If such a package does already exist, then no new package is created. The existing package is modified, if possible, to reflect the new definition. The results are undefined if the new definition is not consistent with the current state of the package.
An error is signaled if more than one :size option appears.
An error is signaled if the same symbol-name argument (in the sense of comparing names with string=) appears more than once among the arguments to all the :shadow, :shadowing-import-from, :import-from, and :intern options.
An error is signaled if the same symbol-name argument (in the sense of comparing names with string=) appears more than once among the arguments to all the :intern and :export options.
Other kinds of name conflicts are handled in the same manner that the underlying operations use-package, import, and export would handle them.
Implementations may support other defpackage options. Every implementation should signal an error on encountering a defpackage option it does not support.
The function compile-file should treat top-level defpackage forms in the same way it would treat top-level calls to package-affecting functions (as described at the beginning of section 11.7).
Here is an example of a call to defpackage that ``plays it safe'' by using only strings as names.
(cl:defpackage "MY-VERY-OWN-PACKAGE" (:size 496) (:nicknames "MY-PKG" "MYPKG" "MVOP") (:use "COMMON-LISP")
(:shadow "CAR" "CDR") (:shadowing-import-from "BRAND-X-LISP" "CONS") (:import-from "BRAND-X-LISP" "GC" "BLINK-FRONT-PANEL-LIGHTS") (:export "EQ" "CONS" "MY-VERY-OWN-FUNCTION"))
The preceding defpackage example is designed to operate correctly even if the package current when the form is read happens not to ``use'' the common-lisp package. (Note the use in this example of the nickname cl for the common-lisp package.) Moreover, neither reading in nor evaluating this defpackage form will ever create any symbols in the current package. Note too the use of uppercase letters in the strings.
Here, for the sake of contrast, is a rather similar use of defpackage that ``plays the whale'' by using all sorts of permissible syntax.
(defpackage my-very-own-package (:export :EQ common-lisp:cons my-very-own-function) (:nicknames "MY-PKG" #:MyPkg) (:use "COMMON-LISP") (:shadow "CAR") (:size 496) (:nicknames mvop) (:import-from "BRAND-X-LISP" "GC" Blink-Front-Panel-Lights) (:shadow common-lisp::cdr) (:shadowing-import-from "BRAND-X-LISP" CONS))
This example has exactly the same effect on the newly created package but may create useless symbols in other packages. The use of explicit package tags is particularly confusing; for example, this defpackage form will cause the symbol cdr to be shadowed in the new package; it will not be shadowed in the package common-lisp. The fact that the name ``CDR'' was specified by a package-qualified reference to a symbol in the common-lisp package is a red herring. The moral is that the syntactic flexibility of defpackage, as in other parts of Common Lisp, yields considerable convenience when used with commonsense competence, but unutterable confusion when used with Malthusian profusion.
Note that defpackage is not capable by itself of defining mutually recursive packages, for example two packages each of which uses the other. However, nothing prevents one from using defpackage to perform much of the initial setup and then using functions such as use-package, import, and export to complete the links.
The purpose of defpackage is to encourage the user to put the entire definition of a package and its relationships to other packages in a single place. It may also encourage the designer of a large system to place the definitions of all relevant packages into a single file (say) that can be loaded before loading or compiling any code that depends on those packages. Such a file, if carefully constructed, can simply be loaded into the common-lisp-user package.
Implementations and programming environments may also be better able
to support the programming process (if only by providing better
error checking) through global knowledge of the intended package setup.
[Function]
find-all-symbols string-or-symbol
find-all-symbols searches every package in the Lisp system to find every symbol whose print name is the specified string. A list of all such symbols found is returned. This search is case-sensitive. If the argument is a symbol, its print name supplies the string to be searched for.
[Macro]
do-symbols (var [package [result-form]])
{declaration}* {tag | statement}*
do-symbols provides straightforward iteration over the symbols of a package. The body is performed once for each symbol accessible in the package, in no particular order, with the variable var bound to the symbol. Then result-form (a single form, not an implicit progn) is evaluated, and the result is the value of the do-symbols form. (When the result-form is evaluated, the control variable var is still bound and has the value nil.) If the result-form is omitted, the result is nil. return may be used to terminate the iteration prematurely. If execution of the body affects which symbols are contained in the package, other than possibly to remove the symbol currently the value of var by using unintern, the effects are unpredictable.
X3J13 voted in January 1989
(PACKAGE-FUNCTION-CONSISTENCY)
to clarify that the package argument may be either a package object
or a package name (see section 11.2).
X3J13 voted in March 1988 (DO-SYMBOLS-DUPLICATES) to specify that the body of a do-symbols form may be executed more than once for the same accessible symbol, and users should take care to allow for this possibility.
The point is that the same symbol might be accessible via more than one chain of inheritance, and it is implementationally costly to eliminate such duplicates. Here is an example:
(setq *a* (make-package 'a)) ;Implicitly uses package common-lisp
(setq *b* (make-package 'b)) ;Implicitly uses package common-lisp
(setq *c* (make-package 'c :use '(a b)))
(do-symbols (x *c*) (print x)) ;Symbols in package common-lisp
; might be printed once or twice here
X3J13 voted in January 1989 (MAPPING-DESTRUCTIVE-INTERACTION) to restrict user side effects; see section 7.9.
Note that the loop construct provides a kind of for clause that
can iterate over the symbols of a package (see chapter 26).
[Macro]
do-external-symbols (var [package [result]])
{declaration}* {tag | statement}*
do-external-symbols is just like do-symbols, except that only the external symbols of the specified package are scanned. The clarification voted by X3J13 in March 1988 for do-symbols (DO-SYMBOLS-DUPLICATES) , regarding redundant executions of the body for the same symbol, applies also to do-external-symbols.
X3J13 voted in January 1989
(PACKAGE-FUNCTION-CONSISTENCY)
to clarify that the package argument may be either a package object
or a package name (see section 11.2).
X3J13 voted in January 1989
(MAPPING-DESTRUCTIVE-INTERACTION)
to restrict user side effects; see section 7.9.
[Macro]
do-all-symbols (var [result-form])
{declaration}* {tag | statement}*
This is similar to do-symbols but executes the body once for every symbol contained in every package. (This will not process every symbol whatsoever, because a symbol not accessible in any package will not be processed. Normally, uninterned symbols are not accessible in any package.) It is not in general the case that each symbol is processed only once, because a symbol may appear in many packages.
The clarification voted by X3J13
in March 1988 for do-symbols
(DO-SYMBOLS-DUPLICATES) ,
regarding redundant executions of the body for the same symbol,
applies also to do-all-symbols.
X3J13 voted in January 1989 (PACKAGE-FUNCTION-CONSISTENCY) to clarify that the package argument may be either a package object or a package name (see section 11.2).
X3J13 voted in January 1989 (MAPPING-DESTRUCTIVE-INTERACTION) to restrict user side effects; see section 7.9.
X3J13 voted in January 1989 (HASH-TABLE-PACKAGE-GENERATORS) to add a new macro with-package-iterator to the language.
[Macro]
with-package-iterator (mname package-list {symbol-type}+)
{form}*
The name mname is bound and defined as if by macrolet, with the body forms as its lexical scope, to be a ``generator macro'' such that each invocation of (mname) will return a symbol and that successive invocations will eventually deliver, one by one, all the symbols from the packages that are elements of the list that is the value of the expression package-list (which is evaluated exactly once).
Each element of the package-list value may be either a package or the name of a package. As a further convenience, if the package-list value is itself a package or the name of a package, it is treated as if a singleton list containing that value had been provided. If the package-list value is nil, it is considered to be an empty list of packages.
At each invocation of the generator macro, there are two possibilities. If there is yet another unprocessed symbol, then four values are returned: t, the symbol, a keyword indicating the accessibility of the symbol within the package (see below), and the package from which the symbol was accessed. If there are no more unprocessed symbols in the list of packages, then one value is returned: nil.
When the generator macro returns a symbol as its second value, the fourth value is always one of the packages present or named in the package-list value, and the third value is a keyword indicating accessibility: :internal means present in the package and not exported; :external means present and exported; and :inherited means not present (thus not shadowed) but inherited from some package used by the package that is the fourth value.
Each symbol-type in an invocation of with-package-iterator is not evaluated. More than one may be present; their order does not matter. They indicate the accessibility types of interest. A symbol is not returned by the generator macro unless its actual accessibility matches one of the symbol-type indicators. The standard symbol-type indicators are :internal, :external, and :inherited, but implementations are permitted to extend the syntax of with-package-iterator by recognizing additional symbol accessibility types. An error is signaled if no symbol-type is supplied, or if any supplied symbol-type is not recognized by the implementation.
The order in which symbols are produced by successive invocations of the generator macro is not necessarily correlated in any way with the order of the packages in the package-list. When more than one package is in the package-list, symbols accessible from more than one package may be produced once or more than once. Even when only one package is specified, symbols inherited in multiple ways via used packages may be produced once or more than once.
The implicit interior state of the iteration over the list of packages and the symbols within them has dynamic extent. It is an error to invoke the generator macro once the with-package-iterator form has been exited.
Any number of invocations of with-package-iterator and related macros may be nested, and the generator macro of an outer invocation may be called from within an inner invocation (provided, of course, that its name is visible or otherwise made available).
X3J13 voted in January 1989 (MAPPING-DESTRUCTIVE-INTERACTION) to restrict user side effects; see section 7.9.