Книга: Practical Common Lisp

Importing Individual Names

Importing Individual Names

Now suppose you find a third-party library of functions for manipulating e-mail messages. The names used in the library's API are exported from the package COM.ACME.EMAIL, so you could :use that package to get easy access to those names. But suppose you need to use only one function from this library, and other exported symbols conflict with names you already use (or plan to use) in our own code.[231] In this case, you can import the one symbol you need with an :import-from clause in the DEFPACKAGE. For instance, if the name of the function you want to use is parse-email-address, you can change the DEFPACKAGE to this:

(defpackage :com.gigamonkeys.email-db
(:use :common-lisp :com.gigamonkeys.text-db)
(:import-from :com.acme.email :parse-email-address))

Now anywhere the name parse-email-address appears in code read in the COM.GIGAMONKEYS.EMAIL-DB package, it will be read as the symbol from COM.ACME.EMAIL. If you need to import more than one symbol from a single package, you can include multiple names after the package name in a single :import-from clause. A DEFPACKAGE can also include multiple :import-from clauses in order to import symbols from different packages.

Occasionally you'll run into the opposite situation—a package may export a bunch of names you want to use and a few you don't. Rather than listing all the symbols you do want to use in an :import-from clause, you can instead :use the package and then list the names you don't want to inherit in a :shadow clause. For instance, suppose the COM.ACME.TEXT package exports a bunch of names of functions and classes used in text processing. Further suppose that most of these functions and classes are ones you'll want to use in your code, but one of the names, build-index, conflicts with a name you've already used. You can make the build-index from COM.ACME.TEXT inaccessible by shadowing it.

(defpackage :com.gigamonkeys.email-db
(:use
:common-lisp
:com.gigamonkeys.text-db
:com.acme.text)
(:import-from :com.acme.email :parse-email-address)
(:shadow :build-index))

The :shadow clause causes a new symbol named BUILD-INDEX to be created and added directly to COM.GIGAMONKEYS.EMAIL-DB's name-to-symbol map. Now if the reader reads the name BUILD-INDEX, it will translate it to the symbol in COM.GIGAMONKEYS.EMAIL-DB's map, rather than the one that would otherwise be inherited from COM.ACME.TEXT. The new symbol is also added to a shadowing symbols list that's part of the COM.GIGAMONKEYS.EMAIL-DB package, so if you later use another package that also exports a BUILD-INDEX symbol, the package system will know there's no conflict—that you want the symbol from COM.GIGAMONKEYS.EMAIL-DB to be used rather than any other symbols with the same name inherited from other packages.

A similar situation can arise if you want to use two packages that export the same name. In this case the reader won't know which inherited name to use when it reads the textual name. In such situations you must resolve the ambiguity by shadowing the conflicting names. If you don't need to use the name from either package, you could shadow the name with a :shadow clause, creating a new symbol with the same name in your package. But if you actually want to use one of the inherited symbols, then you need to resolve the ambiguity with a :shadowing-import-from clause. Like an :import-from clause, a :shadowing-import-from clause consists of a package name followed by the names to import from that package. For instance, if COM.ACME.TEXT exports a name SAVE that conflicts with the name exported from COM.GIGAMONKEYS.TEXT-DB, you could resolve the ambiguity with the following DEFPACKAGE:

(defpackage :com.gigamonkeys.email-db
(:use
:common-lisp
:com.gigamonkeys.text-db
:com.acme.text)
(:import-from :com.acme.email :parse-email-address)
(:shadow :build-index)
(:shadowing-import-from :com.gigamonkeys.text-db :save))

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