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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.