GNU Info

(librep.info)Predicate Functions

---

Next: Local Functions Prev: Anonymous Functions Up: Functions

---

Enter node , (file) or (file)node

Predicate Functions
-------------------

   In Lisp, a function which returns a boolean `true' or boolean `false'
value is called a "predicate". As is the convention in Lisp a value of
`()' means false, anything else means true. The symbols `nil' and `t'
are often used to represent constant false and true values (Note: nil
and t).

   Another Lisp convention is that the names of predicate functions
should name the quality that the predicate is testing followed by
either a `p' or `-p' string. The `p' variant is used when the first
string does not contain any hyphens.

   For example, the predicate to test for the quality "const-variable"
(a variable which has a constant value, Note: Defining Variables) is
called `const-variable-p'. On the other hand the predicate to test for
the quality "cons" (a Cons cell) is called `consp'.