📝Common Lisp: packages and symbols

• symbols

  • symbol without a colon is an unqualified name
  • symbol with one colon can only refer to exported (public) symbols
  • symbol with double-colon can refer to unexported symbols
  • keyword symbols start with colon and are interned in KEYWORD package and automatically exported
  • uninterned symbols are written with a leading #:. Every time the reader reads a name starting with #:, it creates a new symbol
• accessible: all the symbols that can be found in a given package with find-symbol (= symbols that can be referred to with unqualified names when the package is current)
• present: symbol is contained in package’s name-to-symbol table
• The package in which a symbol is first interned (by the reader) is home package
• A package inherits symbols from other packages by using the other packages. Only external symbols are inherited. A symbol is made external by exporting (also makes it accessible via single-colon name).
• For each name, there can only be one symbol max. A symbol can be made shadowing which makes it shadow other symbols with the same name. Each package maintains a list of shadowing symbols.
• A symbol can be imported by adding it to the name-to-symbol table

• A symbol can be uninterned from a package—removed from its name-to-symbol and shadowing tables

  • a symbol that is not present in any package is called an uninterned symbol, and can no longer be read by the reader
• Package system in common lisp is only exporting symbols, not functions or variables.

;; define package
(defpackage :package.name
  (:use :common-lisp :package2)
  ;; export symbols
  (:export
   :symbol1
   :symbol2)
  ;; import symbols
  (:import-from :other.package.name :symbol3 :symbol4)
  ;; shadow a symbol (e.g., from :use above)
  ;;
  ;; This creates a new 'symbol5 and adds it to package-to-name table
  ;; of package.name and to shadowing list
  (:shadow :symbol5)
  ;; Shadowing import (makes symbol-name from other packages
  ;; inaccessible)
  (:shadowing-import-from :package-name :symbol-name))