CLHS: Section 3.2.4.2.2

3.2.4.2.2 Definition of Similarity

Two objects S (in source code) and C (in compiled code) are defined to be similar if and only if they are both of one of the types listed here (or defined by the implementation) and they both satisfy all additional requirements of similarity indicated for that type.

number

Two numbers S and C are similar if they are of the same type and represent the same mathematical value.

character

Two simple characters S and C are similar if they have similar code attributes.

Implementations providing additional, implementation-defined attributes must define whether and how non-simple characters can be regarded as similar.

symbol

Two apparently uninterned symbols S and C are similar if their names are similar.

Two interned symbols S and C are similar if their names are similar, and if either S is accessible in the current package at compile time and C is accessible in the current package at load time, or C is accessible in the package that is similar to the home package of S.

(Note that similarity of symbols is dependent on neither the current readtable nor how the function read would parse the characters in the name of the symbol.)

package

Two packages S and C are similar if their names are similar.

Note that although a package object is an externalizable object, the programmer is responsible for ensuring that the corresponding package is already in existence when code referencing it as a literal object is loaded. The loader finds the corresponding package object as if by calling find-package with that name as an argument. An error is signaled by the loader if no package exists at load time.

random-state

Two random states S and C are similar if S would always produce the same sequence of pseudo-random numbers as a copy[5] of C when given as the random-state argument to the function random, assuming equivalent limit arguments in each case.

(Note that since C has been processed by the file compiler, it cannot be used directly as an argument to random because random would perform a side effect.)

cons

Two conses, S and C, are similar if the car[2] of S is similar to the car[2] of C, and the cdr[2] of S is similar to the cdr[2] of C.

array

Two one-dimensional arrays, S and C, are similar if the length of S is similar to the length of C, the actual array element type of S is similar to the actual array element type of C, and each active element of S is similar to the corresponding element of C.

Two arrays of rank other than one, S and C, are similar if the rank of S is similar to the rank of C, each dimension[1] of S is similar to the corresponding dimension[1] of C, the actual array element type of S is similar to the actual array element type of C, and each element of S is similar to the corresponding element of C.

In addition, if S is a simple array, then C must also be a simple array. If S is a displaced array, has a fill pointer, or is actually adjustable, C is permitted to lack any or all of these qualities.

hash-table

Two hash tables S and C are similar if they meet the following three requirements:

They both have the same test (e.g., they are both eql hash tables).
There is a unique one-to-one correspondence between the keys of the two hash tables, such that the corresponding keys are similar.
For all keys, the values associated with two corresponding keys are similar.

If there is more than one possible one-to-one correspondence between the keys of S and C, the consequences are unspecified. A conforming program cannot use a table such as S as an externalizable constant.

pathname

Two pathnames S and C are similar if all corresponding pathname components are similar.

function

Functions are not externalizable objects.

structure-object and standard-object

A general-purpose concept of similarity does not exist for structures and standard objects. However, a conforming program is permitted to define a make-load-form method for any class K defined by that program that is a subclass of either structure-object or standard-object. The effect of such a method is to define that an object S of type K in source code is similar to an object C of type K in compiled code if C was constructed from code produced by calling make-load-form on S.

The following X3J13 cleanup issues, not part of the specification, apply to this section: