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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 otherwise, since variables in Lisp are not declared to be of a certain type, as they are in other programming languages. It is therefore up to the individual function to test whether each actual argument belongs to a type that can be used by the function.

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 + which it cannot handle:

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

Many functions, called type predicates, are provided to test whether an object is a member of a given type. (Following a convention of long standing, the names of most Emacs Lisp predicates end in `p'.)

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

atom

see section Predicates on Lists

arrayp

see section Functions that Operate on Arrays

bufferp

see section Buffer Basics

byte-code-function-p

see section Byte-Code Function Type

case-table-p

see section The Case Table

char-or-string-p

see section The Predicates for Strings

commandp

see section Interactive Call

consp

see section Predicates on Lists

floatp

see section Type Predicates for Numbers

frame-live-p

see section Deleting Frames

framep

see section Frames

integer-or-marker-p

see section Predicates on Markers

integerp

see section Type Predicates for Numbers

keymapp

see section Creating Keymaps

listp

see section Predicates on Lists

markerp

see section Predicates on Markers

natnump

see section Type Predicates for Numbers

nlistp

see section Predicates on Lists

numberp

see section Type Predicates for Numbers

number-or-marker-p

see section Predicates on Markers

overlayp

see section Overlays

processp

see section Processes

sequencep

see section Sequences

stringp

see section The Predicates for Strings

subrp

see section Accessing Function Cell Contents

symbolp

see section Symbols

syntax-table-p

see section Syntax Tables

user-variable-p

see section Defining Global Variables

vectorp

see section Vectors

window-configuration-p

see section Window Configurations

window-live-p

see section Deleting Windows

windowp

see section Basic Concepts of Emacs Windows

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