11.1.1 Introduction to Packages

A package establishes a mapping from names to symbols. At any given time, one package is current. The current package is the one that is the value of *package*. When using the Lisp reader, it is possible to refer to symbols in packages other than the current one through the use of package prefixes in the printed representation of the symbol.

The next figure lists some defined names that are applicable to packages. Where an operator 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 package argument may be either a string, a symbol, or a package. If a symbol is supplied, its name will be used as the package name.

11.1.1.1 Package Names and Nicknames

Each package has a name (a string) and perhaps some nicknames (also strings). These are assigned when the package is created and can be changed later.

There is a single namespace for packages. The function find-package translates a package name or nickname into the associated package. The function package-name returns the name of a package. The function package-nicknames returns a list of all nicknames for a package. rename-package removes a package's current name and nicknames and replaces them with new ones specified by the caller.

11.1.1.2 Symbols in a Package

11.1.1.2.1 Internal and External Symbols

The mappings in a package are divided into two classes, external and internal. The symbols targeted by these different mappings are called external symbols and internal symbols of the package. Within a package, a name refers to one symbol or to none; if it does refer to a symbol, then it is either external or internal in that package, but not both. External symbols are part of the package's public interface to other packages. Symbols become external symbols of a given package if they have been exported from that package.

A symbol has the same name no matter what package it is present in, but it might be an external symbol of some packages and an internal symbol of others.

11.1.1.2.2 Package Inheritance

Packages can be built up in layers. From one point of view, a package is a single collection of mappings from strings into internal symbols and external symbols. However, some of these mappings might be established within the package itself, while other mappings are inherited from other packages via use-package. A symbol is said to be present in a package if the mapping is in the package itself and is not inherited from somewhere else.

There is no way to inherit the internal symbols of another package; to refer to an internal symbol using the Lisp reader, a package containing the symbol must be made to be the current package, a package prefix must be used, or the symbol must be imported into the current package.

11.1.1.2.3 Accessibility of Symbols in a Package

A symbol becomes accessible in a package if that is its home package when it is created, or if it is imported into that package, or by inheritance via use-package.

If a symbol is accessible in a package, it can be referred to when using the Lisp reader without a package prefix when that package is the current package, regardless of whether it is present or inherited.

Symbols from one package can be made accessible in another package in two ways.

• Any individual symbol can be added to a package by use of import. After the call to import the symbol is present in the importing package. The status of the symbol in the package it came from (if any) is unchanged, and the home package for this symbol is unchanged. Once imported, a symbol is present in the importing package and can be removed only by calling unintern.

  A symbol is shadowed₃ by another symbol in some package if the first symbol would be accessible by inheritance if not for the presence of the second symbol. See shadowing-import.

• The second mechanism for making symbols from one package accessible in another is provided by use-package. All of the external symbols of the used package are inherited by the using package. The function unuse-package undoes the effects of a previous use-package.

11.1.1.2.4 Locating a Symbol in a Package

When a symbol is to be located in a given package the following occurs:

• The external symbols and internal symbols of the package are searched for the symbol.
• The external symbols of the used packages are searched in some unspecified order. The order does not matter; see the rules for handling name conflicts listed below.

11.1.1.2.5 Prevention of Name Conflicts in Packages

Within one package, any particular name can refer to at most one symbol. A name conflict is said to occur when there would be more than one candidate symbol. Any time a name conflict is about to occur, a correctable error is signaled.

The following rules apply to name conflicts:

• Name conflicts are detected when they become possible, that is, when the package structure is altered. Name conflicts are not checked during every name lookup.
• If the same symbol is accessible to a package through more than one path, there is no name conflict. A symbol cannot conflict with itself. Name conflicts occur only between distinct symbols with the same name (under string=).
• Every package has a list of shadowing symbols. A shadowing symbol takes precedence over any other symbol of the same name that would otherwise be accessible in the package. A name conflict involving a shadowing symbol is always resolved in favor of the shadowing symbol, without signaling an error (except for one exception involving import). See shadow and shadowing-import.
• The functions use-package, import, and export check for name conflicts.
• shadow and shadowing-import never signal a name-conflict error.
• unuse-package and unexport do not need to do any name-conflict checking. unintern does name-conflict checking only when a symbol being uninterned is a shadowing symbol .
• Giving a shadowing symbol to unintern can uncover a name conflict that had previously been resolved by the shadowing.
• Package functions signal name-conflict errors of type package-error before making any change to the package structure. When multiple changes are to be made, it is permissible for the implementation to process each change separately. For example, when export is given a list of symbols, aborting from a name conflict caused by the second symbol in the list might still export the first symbol in the list. However, a name-conflict error caused by export of a single symbol will be signaled before that symbol's accessibility in any package is changed.
• Continuing from a name-conflict error must offer the user a chance to resolve the name conflict in favor of either of the candidates. The package structure should be altered to reflect the resolution of the name conflict, via shadowing-import, unintern, or unexport.
• A name conflict in use-package between a symbol present in the using package and an external symbol of the used package is resolved in favor of the first symbol by making it a shadowing symbol, or in favor of the second symbol by uninterning the first symbol from the using package.
• A name conflict in export or unintern due to a package's inheriting two distinct symbols with the same name (under string=) from two other packages can be resolved in favor of either symbol by importing it into the using package and making it a shadowing symbol , just as with use-package.