Slide 8.e: How to write functions

Slide 8.d: COND
Slide 8.f: Programming Exercise II: calculating a string expression
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How to Write Functions

DEFUN defines the new function and return as its value the name of the function. It takes three parameters, which are implicitly quoted. The form of the function definition is as follows:

   (DEFUN function_name  parameter_list
      function_body )

where:

function_name is an identifier, as an atom;
parameter_list is a list (possibly empty) of identifiers, as a list of atoms; and
function_body is an S-expression representing the body of the function.

Given an atom A and a list of atoms LAT, the following code determines whether A occurs in LAT:


(defun check (a lat) (cond ((null lat) nil) ((eq a (car lat)) T) (T (check a (cdr lat))) ) ) (check ' ')

Actually, the above code check( ) is the same as the Lisp built-in function MEMBER. The following error message will be displayed if the you try to redefine the MEMBER by replacing check( ) by member( ):

   ** - Continuable Error
   DEFUN/DEFMACRO(MEMBER): # is locked
   If you continue (by typing 'continue'): 
      Ignore the lock and proceed