Functions

A function is a Lisp object which, when applied to a sequence of argument values, produces a value -- the function's result. It may also produce side-effects. All Lisp functions return results -- there is nothing like a procedure in Pascal.

Functions are the main building-block in Lisp programs, each program is usually a system of inter-related functions.

There are two types of function: primitive functions are functions written in the C language, these are sometimes called built-in functions, the object containing the C code itself is called a subr. All other functions are written in Lisp.

Function: functionp object

Returns t if object is a function (i.e. it can be used as the function argument of funcall.

(functionp 'set)
    => t

(functionp 'setq)
    => nil

(functionp #'(lambda (x) (+ x 2)))
   => t

• Lambda Expressions: Structure of a function object
• Named Functions: Functions can be named by symbols,
• Anonymous Functions: Or they can be un-named
• Predicate Functions: Functions which return boolean values
• Defining Functions: How to write a function definition
• Calling Functions: Functions can be called by hand
• Mapping Functions: Map a function to the elements of a list

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