ANSI Common Lisp   2 Syntax   2.3 Interpretation of Tokens

2.3.6 Package System Consistency Rules

• Read-read consistency

  Reading the same symbol name always results in the same symbol.

• Print-read consistency

  An interned symbol always prints as a sequence of characters that, when read back in, yields the same symbol.

  For information about how the Lisp printer treats symbols, see Section 22.1.3.3 Printing Symbols.

• Print-print consistency

  If two interned symbols are not the same, then their printed representations will be different sequences of characters.

These rules are true regardless of any implicit interning. As long as the current package is not changed, results are reproducible regardless of the order of loading files or the exact history of what symbols were typed in when. If the value of *package* is changed and then changed back to the previous value, consistency is maintained. The rules can be violated by changing the value of *package*, forcing a change to symbols or to packages or to both by continuing from an error, or calling one of the following functions: unintern, unexport, shadow, shadowing-import, or unuse-package.

An inconsistency only applies if one of the restrictions is violated between two of the named symbols. shadow, unexport, unintern, and shadowing-import can only affect the consistency of symbols with the same names (under string=) as the ones supplied as arguments.