Common Lisp the Language, 2nd Edition

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# 11.7. Package System Functions and Variables

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`.

Implementation note: In the past, some Lisp compilers have read the entire file into Lisp before processing any of the forms. Other compilers have arranged for the loader to do all of its intern operations before evaluating any of the top-level forms. Neither of these techniques will work in a straightforward way in Common Lisp because of the presence of multiple packages.

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.

Rationale: Many existing implementations of Common Lisp happen to have violated the first edition specification, providing as the default not only the `lisp` package but also (or instead) a package containing implementation-dependent language extensions. This is for good reason: usually it is much more convenient to the user for the default `:use` list to be the entire, implementation-dependent, extended language rather than only the facilities specified in this book. The X3J13 vote simply legitimizes existing practice.

[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:

`:internal`
  The symbol was directly present in the package as an internal symbol.

`:external`
  The symbol was directly present as an external symbol.

`:inherited`
  The symbol was inherited via `use-package` (which implies that the symbol is internal).

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).

Compatibility note: Conceptually, `intern` translates a string to a symbol. In MacLisp and several other dialects of Lisp, `intern` can take either a string or a symbol as its argument; in the latter case, the symbol's print name is extracted and used as the string. However, this leads to some confusing issues about what to do if `intern` finds a symbol that is not `eq` to the argument symbol. To avoid such confusion, Common Lisp requires the argument to be a string.

[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).

Compatibility note: The equivalent of this in MacLisp is `remob`.

[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.

`(:size integer)`
  This specifies approximately the number of symbols expected to be in the package. This is purely an efficiency hint to the storage allocator, so that implementations using hash tables as part of the package data structure (the usual technique) will not have to incrementally expand the package as new symbols are added to it (for example, as a large file is read while ``in'' that package).

`(:nicknames {package-name}*)`
  The specified names become nicknames of the package being defined. If any of the specified nicknames already refers to an existing package, a continuable error is signaled exactly as for the function `make-package`.

`(:shadow {symbol-name}*)`
  Symbols with the specified names are created as shadows in the package being defined, just as with the function `shadow`.

`(:shadowing-import-from package-name {symbol-name}*)`
  Symbols with the specified names are located in the specified package. These symbols are imported into the package being defined, shadowing other symbols if necessary, just as with the function `shadowing-import`. In no case will symbols be created in a package other than the one being defined; a continuable error is signaled if for any symbol-name there is no symbol of that name accessible in the package named package-name.

`(:use {package-name}*)`
  The package being defined is made to ``use'' (inherit from) the packages specified by this option, just as with the function `use-package`. If no `:use` option is supplied, then a default list is assumed as for `make-package`.

  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.

`(:import-from package-name {symbol-name}*)`
  Symbols with the specified names are located in the specified package. These symbols are imported into the package being defined, just as with the function `import`. In no case will symbols be created in a package other than the one being defined; a continuable error is signaled if for any symbol-name there is no symbol of that name accessible in the package named package-name.

`(:intern {symbol-name}*)`
  Symbols with the specified names are located or created in the package being defined, just as with the function `intern`. Note that the action of this option may be affected by a `:use` option, because an inherited symbol will be used in preference to creating a new one.

`(:export {symbol-name}*)`
  Symbols with the specified names are located or created in the package being defined and then exported, just as with the function `export`. Note that the action of this option may be affected by a `:use`, `:import-from`, or `:shadowing-import-from` option, because an inherited or imported symbol will be used in preference to creating a new one.

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:

1. `:shadow` and `:shadowing-import-from`
2. `:use`
3. `:import-from` and `:intern`
4. `:export`

Shadows are established first in order to avoid spurious name conflicts when use links are established. Use links must occur before importing and interning so that those operations may refer to normally inherited symbols rather than creating new ones. Exports are performed last so that symbols created by any of the other options, in particular, shadows and imported symbols, may be exported. Note that exporting an inherited symbol implicitly imports it first (see section 11.4).

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.

Implementation note: An implementation of `defpackage` might choose to transform all the package-name and symbol-name arguments into strings at macro expansion time, rather than at the time the resulting expansion is executed, so that even if source code is expressed in terms of strange symbols in the `defpackage` form, the binary file resulting from compiling the source code would contain only strings. The purpose of this is simply to minimize the creation of useless symbols in production code. This technique is permitted as an implementation strategy but is not a behavior required by the specification of `defpackage`.

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.

Rationale: This facility is a bit more flexible in some ways than `do-symbols` and friends. In particular, it makes it possible to implement `loop` clauses for iterating over packages in a way that is both portable and efficient (see chapter 26).

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