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GNU Emacs Lisp Reference Manual

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2.6 Type Predicates

The Emacs Lisp interpreter itself does not perform type checking on the actual arguments passed to functions when they are called. It could not do so, since function arguments in Lisp do not have declared data types, as they do in other programming languages. It is therefore up to the individual function to test whether each actual argument belongs to a type that the function can use.

All built-in functions do check the types of their actual arguments when appropriate, and signal a wrong-type-argument error if an argument is of the wrong type. For example, here is what happens if you pass an argument to + that it cannot handle:

 
(+ 2 'a)
     error--> Wrong type argument: number-or-marker-p, a

If you want your program to handle different types differently, you must do explicit type checking. The most common way to check the type of an object is to call a type predicate function. Emacs has a type predicate for each type, as well as some predicates for combinations of types.

A type predicate function takes one argument; it returns t if the argument belongs to the appropriate type, and nil otherwise. Following a general Lisp convention for predicate functions, most type predicates' names end with `p'.

Here is an example which uses the predicates listp to check for a list and symbolp to check for a symbol.

 
(defun add-on (x)
  (cond ((symbolp x)
         ;; If X is a symbol, put it on LIST.
         (setq list (cons x list)))
        ((listp x)
         ;; If X is a list, add its elements to LIST.
         (setq list (append x list)))
        (t
         ;; We handle only symbols and lists.
         (error "Invalid argument %s in add-on" x))))

Here is a table of predefined type predicates, in alphabetical order, with references to further information.

atom
See section atom.

arrayp
See section arrayp.

bool-vector-p
See section bool-vector-p.

bufferp
See section bufferp.

byte-code-function-p
See section byte-code-function-p.

case-table-p
See section case-table-p.

char-or-string-p
See section char-or-string-p.

char-table-p
See section char-table-p.

commandp
See section commandp.

consp
See section consp.

display-table-p
See section display-table-p.

floatp
See section floatp.

frame-configuration-p
See section frame-configuration-p.

frame-live-p
See section frame-live-p.

framep
See section framep.

functionp
See section functionp.

integer-or-marker-p
See section integer-or-marker-p.

integerp
See section integerp.

keymapp
See section keymapp.

keywordp
See section 11.2 Variables that Never Change.

listp
See section listp.

markerp
See section markerp.

wholenump
See section wholenump.

nlistp
See section nlistp.

numberp
See section numberp.

number-or-marker-p
See section number-or-marker-p.

overlayp
See section overlayp.

processp
See section processp.

sequencep
See section sequencep.

stringp
See section stringp.

subrp
See section subrp.

symbolp
See section symbolp.

syntax-table-p
See section syntax-table-p.

user-variable-p
See section user-variable-p.

vectorp
See section vectorp.

window-configuration-p
See section window-configuration-p.

window-live-p
See section window-live-p.

windowp
See section windowp.

The most general way to check the type of an object is to call the function type-of. Recall that each object belongs to one and only one primitive type; type-of tells you which one (see section 2. Lisp Data Types). But type-of knows nothing about non-primitive types. In most cases, it is more convenient to use type predicates than type-of.

Function: type-of object
This function returns a symbol naming the primitive type of object. The value is one of the symbols symbol, integer, float, string, cons, vector, char-table, bool-vector, hash-table, subr, compiled-function, marker, overlay, window, buffer, frame, process, or window-configuration.

 
(type-of 1)
     => integer
(type-of 'nil)
     => symbol
(type-of '())    ; () is nil.
     => symbol
(type-of '(x))
     => cons


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