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 atom.

arrayp

See arrayp.

bool-vector-p

See bool-vector-p.

bufferp

See bufferp.

byte-code-function-p

See byte-code-function-p.

case-table-p

See case-table-p.

char-or-string-p

See char-or-string-p.

char-table-p

See char-table-p.

commandp

See commandp.

consp

See consp.

display-table-p

See display-table-p.

floatp

See floatp.

frame-configuration-p

See frame-configuration-p.

frame-live-p

See frame-live-p.

framep

See framep.

functionp

See functionp.

integer-or-marker-p

See integer-or-marker-p.

integerp

See integerp.

keymapp

See keymapp.

keywordp

See Constant Variables.

listp

See listp.

markerp

See markerp.

wholenump

See wholenump.

nlistp

See nlistp.

numberp

See numberp.

number-or-marker-p

See number-or-marker-p.

overlayp

See overlayp.

processp

See processp.

sequencep

See sequencep.

stringp

See stringp.

subrp

See subrp.

symbolp

See symbolp.

syntax-table-p

See syntax-table-p.

user-variable-p

See user-variable-p.

vectorp

See vectorp.

window-configuration-p

See window-configuration-p.

window-live-p

See window-live-p.

windowp

See 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 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.