CLHS: Section 2.3.5

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## 2.3.5 Valid Patterns for Tokens

The valid patterns for tokens are summarized in the next figure.

    nnnnn              a number
    xxxxx              a symbol in the current package
    :xxxxx             a symbol in the the KEYWORD package
    ppppp:xxxxx        an external symbol in the ppppp package
    ppppp::xxxxx       a (possibly internal) symbol in the ppppp package
    :nnnnn             undefined
    ppppp:nnnnn        undefined
    ppppp::nnnnn       undefined
    ::aaaaa            undefined
    aaaaa:             undefined
    aaaaa:aaaaa:aaaaa  undefined

Figure 2-17. Valid patterns for tokens

Note that nnnnn has number syntax, neither xxxxx nor ppppp has number syntax, and aaaaa has any syntax.

A summary of rules concerning package markers follows. In each case, examples are offered to illustrate the case; for presentational simplicity, the examples assume that the readtable case of the current readtable is `:upcase`.

1. If there is a single package marker, and it occurs at the beginning of the token, then the token is interpreted as a symbol in the `KEYWORD` package. It also sets the symbol-value of the newly-created symbol to that same symbol so that the symbol will self-evaluate.

  For example, `:bar`, when read, interns `BAR` as an external symbol in the `KEYWORD` package.

2. If there is a single package marker not at the beginning or end of the token, then it divides the token into two parts. The first part specifies a package; the second part is the name of an external symbol available in that package.

  For example, `foo:bar`, when read, looks up `BAR` among the external symbols of the package named `FOO`.

3. If there are two adjacent package markers not at the beginning or end of the token, then they divide the token into two parts. The first part specifies a package; the second part is the name of a symbol within that package (possibly an internal symbol).

  For example, `foo::bar`, when read, interns `BAR` in the package named `FOO`.

4. If the token contains no package markers, and does not have potential number syntax, then the entire token is the name of the symbol. The symbol is looked up in the current package.

  For example, `bar`, when read, interns `BAR` in the current package.

5. The consequences are unspecified if any other pattern of package markers in a token is used. All other uses of package markers within names of symbols are not defined by this standard but are reserved for implementation-dependent use.

For example, assuming the readtable case of the current readtable is `:upcase`, `editor:buffer` refers to the external symbol named `BUFFER` present in the package named `editor`, regardless of whether there is a symbol named `BUFFER` in the current package. If there is no package named `editor`, or if no symbol named `BUFFER` is present in `editor`, or if `BUFFER` is not exported by `editor`, the reader signals a correctable error. If `editor::buffer` is seen, the effect is exactly the same as reading `buffer` with the `EDITOR` package being the current package.

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