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elisp: Lisp Data Types

 
 2 Lisp Data Types
 *****************
 
 A Lisp “object” is a piece of data used and manipulated by Lisp
 programs.  For our purposes, a “type” or “data type” is a set of
 possible objects.
 
    Every object belongs to at least one type.  Objects of the same type
 have similar structures and may usually be used in the same contexts.
 Types can overlap, and objects can belong to two or more types.
 Consequently, we can ask whether an object belongs to a particular type,
 but not for _the_ type of an object.
 
    A few fundamental object types are built into Emacs.  These, from
 which all other types are constructed, are called “primitive types”.
 Each object belongs to one and only one primitive type.  These types
 include “integer”, “float”, “cons”, “symbol”, “string”, “vector”,
 “hash-table”, “subr”, and “byte-code function”, plus several special
 types, such as “buffer”, that are related to editing.  (Editing
 Types.)
 
    Each primitive type has a corresponding Lisp function that checks
 whether an object is a member of that type.
 
    Lisp is unlike many other languages in that its objects are
 “self-typing”: the primitive type of each object is implicit in the
 object itself.  For example, if an object is a vector, nothing can treat
 it as a number; Lisp knows it is a vector, not a number.
 
    In most languages, the programmer must declare the data type of each
 variable, and the type is known by the compiler but not represented in
 the data.  Such type declarations do not exist in Emacs Lisp.  A Lisp
 variable can have any type of value, and it remembers whatever value you
 store in it, type and all.  (Actually, a small number of Emacs Lisp
 variables can only take on values of a certain type.  Variables
 with Restricted Values.)
 
    This chapter describes the purpose, printed representation, and read
 syntax of each of the standard types in GNU Emacs Lisp.  Details on how
 to use these types can be found in later chapters.
 

Menu

 
· Printed Representation      How Lisp objects are represented as text.
· Comments                    Comments and their formatting conventions.
· Programming Types           Types found in all Lisp systems.
· Editing Types               Types specific to Emacs.
· Circular Objects            Read syntax for circular structure.
· Type Predicates             Tests related to types.
· Equality Predicates         Tests of equality between any two objects.
 

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