Lisp

Summary #

is a family of programming languages with fully parenthesized prefix notation. LISP: LISt Processor lispcookbook

Prefix Notation #

LISP uses what is called a prefix notation, i.e. instead of writing 10+20 (this is called the infix notation) you have to write + 10 20.

pdf ref

S-expressions #

In the usual parenthesized syntax of Lisp, an S-expression is classically defined[1] as

1. an atom, or
2. 3. an of the form (x . y) where x and y are S-expressions.

Data-types #

Two fundamental data types: #

1. Atoms
2. Lists

e.g: (1 2 foo) is list with atoms 1, 2 and foo

Quote or “’” or apostrophe #

Difference is applying function to symbol(quote) vs to value.

(atom? ’turkey) #

“atom?” is a function and “’turkey” is the argument

(atom? 'turkey) ;; is same as
(atom? (quote turkey))
;; in the above s-expression
;; atom? gets evaluated to a function and "turkey" to a symbol not function.
;; so the function is applied to the symbol

(atom? turkey) #

(atom? turkey)
;; atom? evaluates to function
;; turkey is variable, evaluating turkey gives the value that is bound to it.
;; function is applied to the value

Dictionary or hastable #

http://www.cs.cmu.edu/Groups/AI/html/cltl/clm/node154.html

(setq a (make-hash-table))
(setf (gethash 'color a) 'brown)
(setf (gethash 'name a) 'fred)
(gethash 'color a) => brown
(gethash 'name a) => fred
(gethash 'pointy a) => nil

4 ways to implement dictionary #

stackoverflow Common Lisp has at least four different ways to do that (key value storage):

property lists (:foo 1 :bar 2) #

assoc lists ((:foo . 1) (:bar . 2)) #

hash tables #

CLOS objects #

CLOS objects (slot-value foo ‘bar) to get and (setf (slot-value foo ‘bar) 42) to set. The slot name can be stored in a variable: (let ((name ‘bar)) (slot-value foo name)) .