Slide 8.a: Lisp Slide 8.c: Basic functions (cont.) Home

Definitions and Basic Functions

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Definitions

• An atom is either an integer or an identifier.


• A list is a left parenthesis, followed by zero or more S-expressions, followed by a right parenthesis.


• An S-expression is an atom or a list.


• NIL means “false.” NIL is also the name for the empty list, and may be written as ( ).


• T means “true,” but actually anything that is not NIL is “true.”

Basic Functions

• DEFUN is used to define new functions.


• QUOTE takes a single S-expression as a parameter, and returns that S-expression, unevaluated, as its result.


• The CAR of a nonempty list is the first element in that list.


• The CDR of a nonempty list is the list with its first element removed.


• The CONS of an S-expression to a list is the list with the S-expression inserted at the beginning, as a new first element. Note that the CAR of this new list is the given S-expression, while the CDR is the original list.


• EQ compares two atoms for equality (identity) and returns “true” if the atoms are the same, NIL if they are different.


• ATOM of an S-expression is “true” if the S-expression is an atom, and NIL otherwise.


• NULL of an S-expression is “true” if the S-expression is the empty list (that is, NIL), and NIL otherwise.