LISP - Data Types


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In LISP, variables are not typed, but data objects are.

LISP data types can be categorized as.

  • Scalar types - for example, number types, characters, symbols etc.

  • Data structures - for example, lists, vectors, bit-vectors, and strings.

Any variable can take any LISP object as its value, unless you have declared it explicitly.

Although, it is not necessary to specify a data type for a LISP variable, however, it helps in certain loop expansions, in method declarations and some other situations that we will discuss in later chapters.

The data types are arranged into a hierarchy. A data type is a set of LISP objects and many objects may belong to one such set.

The typep predicate is used for finding whether an object belongs to a specific type.

The type-of function returns the data type of a given object.

Type Specifiers in LISP

Type specifiers are system-defined symbols for data types.

array fixnum package simple-string
atom float pathname simple-vector
bignum function random-state single-float
bit hash-table ratio standard-char
bit-vector integer rational stream
character keyword readtable string
[common] list sequence [string-char]
compiled-function long-float short-float symbol
complex nill signed-byte t
cons null simple-array unsigned-byte
double-float number simple-bit-vector vector

Apart from these system-defined types, you can create your own data types. When a structure type is defined using defstruct function, the name of the structure type becomes a valid type symbol.

Example 1

Create new source code file named main.lisp and type the following code in it.

(setq x 10)
(setq y 34.567)
(setq ch nil)
(setq n 123.78)
(setq bg 11.0e+4)
(setq r 124/2)

(print x)
(print y)
(print n)
(print ch)
(print bg)
(print r)

When you click the Execute button, or type Ctrl+E, LISP executes it immediately and the result returned is:

10 
34.567 
123.78 
NIL 
110000.0 
62

Example 2

Next let's check the types of the variables used in the previous example. Create new source code file named main. lisp and type the following code in it.

(defvar x 10)
(defvar y 34.567)
(defvar ch nil)
(defvar n 123.78)
(defvar bg 11.0e+4)
(defvar r 124/2)

(print (type-of x))
(print (type-of y))
(print (type-of n))
(print (type-of ch))
(print (type-of bg))
(print (type-of r))

When you click the Execute button, or type Ctrl+E, LISP executes it immediately and the result returned is:

(INTEGER 0 281474976710655) 
SINGLE-FLOAT 
SINGLE-FLOAT 
NULL 
SINGLE-FLOAT 
(INTEGER 0 281474976710655)


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