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
the argument belongs to the appropriate type, and
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.
The most general way to check the type of an object is to call the
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
This function returns a symbol naming the primitive type of object. The value is one of the symbols
window-configuration. However, if object is a record, the type specified by its first slot is returned; Records.(type-of 1) ⇒ integer (type-of 'nil) ⇒ symbol (type-of '()) ;
nil. ⇒ symbol (type-of '(x)) ⇒ cons (type-of (record 'foo)) ⇒ foo