Type Specifiers

Type specifiers can be symbols, classes, or lists. Figure~4–2 lists symbols that are standardized atomic type specifiers, and Figure~4–3 lists standardized compound type specifier names. For syntax information, see the dictionary entry for the corresponding type specifier. It is possible to define new type specifiers using defclass, define-condition, defstruct, or deftype.

If a type specifier is a list, the car of the list is a symbol, and the rest of the list is subsidiary type information. Such a type specifier is called a compound type specifier . Except as explicitly stated otherwise, the subsidiary items can be unspecified. The unspecified subsidiary items are indicated by writing *. For example, to completely specify a vector, the type of the elements and the length of the vector must be present.

 (vector double-float 100)

The following leaves the length unspecified:

 (vector double-float *)

The following leaves the element type unspecified:

 (vector * 100)                                      

Suppose that two type specifiers are the same except that the first has a * where the second has a more explicit specification. Then the second denotes a subtype of the type denoted by the first.

If a list has one or more unspecified items at the end, those items can be dropped. If dropping all occurrences of * results in a singleton list, then the parentheses can be dropped as well (the list can be replaced by the symbol in its car). For example, (vector double-float *) can be abbreviated to (vector double-float), and (vector * *) can be abbreviated to (vector) and then to vector.

Figure 4–4 show the defined names that can be used as compound type specifier names but that cannot be used as atomic type specifiers.

New type specifiers can come into existence in two ways.

• Defining a structure by using defstruct without using the :type specifier or defining a class by using defclass or define-condition automatically causes the name of the structure or class to be a new type specifier symbol.

• deftype can be used to define derived type specifiers , which act as ‘abbreviations’ for other type specifiers.

A class object can be used as a type specifier. When used this way, it denotes the set of all members of that class.

Figure 4–5 shows some defined names relating to types and declarations.

Figure 4–6 shows all defined names that are type specifier names, whether for atomic type specifiers or compound type specifiers; this list is the union of the lists in Figure~4–2 and Figure~4–3.