5.2.2 Canonicalization

     

When given a type specifier, Python will often rewrite it into a different (but equivalent) type. This is the mechanism that Python uses for detecting type equivalence. For example, in Python's canonical representation, these types are equivalent:

(or list (member :end))  (or cons (member nil :end))

This has two implications for the user:

• The standard symbol type specifiers for atom, null, fixnum, etc., are in no way magical. The  null type is actually defined to be (member nil),  list is (or cons null), and  fixnum is (signed-byte 30).
• When the compiler prints out a type, it may not look like the type specifier that originally appeared in the program. This is generally not a problem, but it must be taken into consideration when reading compiler error messages.