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(elisp)Lambda Components

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Components of a Lambda Expression
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   A function written in Lisp (a "lambda expression") is a list that
looks like this:

     (lambda (ARG-VARIABLES...)
       [DOCUMENTATION-STRING]
       [INTERACTIVE-DECLARATION]
       BODY-FORMS...)

   The first element of a lambda expression is always the symbol
`lambda'.  This indicates that the list represents a function.  The
reason functions are defined to start with `lambda' is so that other
lists, intended for other uses, will not accidentally be valid as
functions.

   The second element is a list of symbols--the argument variable names.
This is called the "lambda list".  When a Lisp function is called, the
argument values are matched up against the variables in the lambda
list, which are given local bindings with the values provided.  Note:
Local Variables.

   The documentation string is a Lisp string object placed within the
function definition to describe the function for the Emacs help
facilities.  Note: Function Documentation.

   The interactive declaration is a list of the form `(interactive
CODE-STRING)'.  This declares how to provide arguments if the function
is used interactively.  Functions with this declaration are called
"commands"; they can be called using `M-x' or bound to a key.
Functions not intended to be called in this way should not have
interactive declarations.  Note: Defining Commands, for how to write
an interactive declaration.

   The rest of the elements are the "body" of the function: the Lisp
code to do the work of the function (or, as a Lisp programmer would say,
"a list of Lisp forms to evaluate").  The value returned by the
function is the value returned by the last element of the body.