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 - Using scalar data types

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

main.lisp

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

; print values of variables
(print x)
(print y)
(print n)
(print ch)
(print bg)
(print r)

Output

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 - Checking types of variables

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.

main.lisp

; define variables and set values
(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 all variables
(print (type-of x))
(print (type-of y))
(print (type-of n))
(print (type-of ch))
(print (type-of bg))
(print (type-of r))

Output

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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