LEARNING LISP

Contents | Atoms and Values | Defining Your Own Functions

Bag of Predicates

Congratulations. You have now mastered the basic concepts of atoms and lists. It is the purpose of this chapter to add to your "vocabulary" of Lisp functions. You will need a few more in order to do any real work. All of the new functions will be predicates. Remember that predicates ask questions about data and always return T for true or NIL for false.

The first function is a predicate that asks if an expression is an atom.

:(ATOM 'BOMB)

  T

:(SETQ BOMB 'KYAG)

  KYAG

:(ATOM BOMB)

  T

:(SETQ NIXON '(I AM NOT A COOK))

  (I AM NOT A COOK )

:(ATOM NIXON)

  NIL

:(ATOM '((AND) (EVE)))

  NIL

:(ATOM (CAR NIXON))

  T

:(ATOM ())

  T
As you will later see, testing for atom-icity is very important. [Is atom-icity a word? Probably not, but it is an atom.]

Another nice testing function is NULL. NULL says T if, and only if its argument is NIL. Now we have a way to test for NIL-icity [sorry].

:(NULL BOMB)

  NIL

:(NULL (SETQ ALIST '(LET THEM EAT CAKE)))

  NIL

:(NULL NIL)

  T

:(NULL ())

  T

:(NULL T)

  NIL

:(NULL (NULL T))

  T

:(NULL (NULL NIL))

  NIL

:(NULL (CAR ()))

  T

:(NULL (CDR ()))

  T

:(NULL BERRY-BUSH)

    ** ERROR: UNDEFINED ATOM **
      EVAL :: BERRY-BUSH

+()

  NIL
That last one is just to see if you're still awake.

It would be reasonable if you could test for the equality of two expressions. Since Lisp is always in a reasonable mood, there is such a predicate: EQUAL. EQUAL returns a T if and only if its arguments represent the same Lisp expression; the arguments may be atoms or lists, but there may only be two such expressions.

:(EQUAL '(A) '(A))

  T

:(EQUAL 1 (ADD 1 1)))

  NIL

:
:(EQUAL (CAR '((A DEEP)LIST)) '(A DEEP))

  T
Using the NULL and EQUAL predicates, we can make our own "not-equal" function.
:(NULL (EQUAL 1 (ADD 1 1)))

  T
That is, 1 does not equal 1 plus 1. Lisp has a function called NOT which does exactly the same thing as NULL. It would be preferable to use NOT here because it makes a little more sense if you read it out loud.

More equality:

:(EQUAL 'HEAD (CAR '(HEAD FOR THE NILS)))

  T

:(EQUAL 'SOLIPSIST)

    ** ERROR: TOO FEW ARGS **
      EQUAL :: ((QUOTE SOLIPSIST ) )

+()

  NIL

:(EQUAL 'THREE 'FOR 'ALL)

    ** ERROR: TOO MANY ARGS **
      EQUAL :: ((QUOTE THREE ) (QUOTE
      FOR ) (QUOTE ALL ) )

+()

  NIL

:(EQUAL T (EQUAL 4 (SUB 11 7))))

  T
We pulled a fast one in that last example. SUB is a new function. No problem, really, SUB does subtraction a la grade school. Play with it to make sure it does its homework.

Contents | Atoms and Values | Defining Your Own Functions