Basic Lisp Techniques

(reading notes)

(It's a free book, just google the title)

The CL Language

Overview

CL uses a generalized prefix notation

(+ 2 2)

CL evaluates an expression according to the following rules:

if the expression is a number, it evaluates to itself
if the expression looks like a string, it also evaluates to itself
if the expression looks like a literal symbol, it evaluates to that symbol
if the expression looks like a list, CL assumes the first element is a symbol which names a function or macro, and the rest of the elements are arguments

Turning Off Evaluation

(quote (+ 1 2))

'(+ 1 2)

Data Types

Numbers
Strings
Symbols
Lists

Functions

define function

(defun hello()
    (list 'hello 'world))

Global and Local Variables

global

(defparameter *todays-temp* 90)
(defvar *todays-humidity* 70)

defparameter will overwrite existing symbol but defvar will not.

value can be changed by setq:

(setq *todays-humidity* 30)

local

(let ((a 20)
    (b 30))
    (+ a b))

List as a Data Structure

accessing elements of a list

(first '(a b c)) => A
(second '(a b c)) => B
(third '(a b c)) => C
(nth 0 '(a b c)) => A
(rest '(a b c)) => (B C)

the empty list

NIL, 'NIL, and () all evalute to the empty list.

are you a list?

(listp '(a b c)) => T
(listp 80) => NIL
(listp nil) => T

the conditional if

(if (> 3 4)
    "yes"
    "no") => "no"

length of a list

(length '(a b c)) => 3

to implement a length function:

(defun our-length (list)
    (if (null list)
        0
        (+ 1 (our-length (rest list))))

member of a list

(member 'b '(a b c)) => (B C)

member uses eql to test equality, and return the rest of the list starting from the found element.

to implement a member function:

(defun our-member (elem list)
    (if (null list)
        nil
        (if (eql elem (first list))
            list
            (our-member elem (rest list)))))

getting part of a list

(subseq '(a b c d) 1 3) => (B C)
(subseq '(a b c d) 1) => (B C D)

appending lists

(append '(1 2 3) '(4 5 6)) => (1 2 3 4 5 6)

adding elements to a list

(cons 'a '(b c)) => (A B C)

removing elements from a list

(remove 'c '(a b c d)) => (A B D)

append, cons, remove are non-destructive functions, use setq to achieve the new value:

(setq data (remove 10 data))

sorting list

(sort '(2 4 1 3 5) #'<) => (1 2 3 4 5)
(sort '(2 4 1 3 5) #'>) => (5 4 3 2 1)

sort will modify its sequence argument:

(setq data '(2 4 1 3 5)) => (2 4 1 3 5)
(sort data #'<) => (1 2 3 4 5)
data => (2 3 4 5)

always use setq to get the result:

(setq data (sort data #'<)) => (1 2 3 4 5)
data => (1 2 3 4 5)

implement a safe sort:

(defun safe-sort (list predicate)
    (let ((new-list (copy-list list)))
        (sort new-list predicate)))

treating a list as a set

(union '(1 2 3) '(2 3 4)) => (1 2 3 4)
(intersection '(1 2 3) '(2 3 4)) => (3 2)
(set-difference '(1 2 3 4) '(2 3)) => (4 1)

mapping a function to a list

(defun twice (num)
    (* num 2))
(mapcar #'twice '(1 2 3 4)) => (2 4 6 8)

property lists

example of a plist:

(:a "apple" :b "ball" :c "cat")

use getf to access value

(getf '(:a "apple" :b "ball" :c "cat") :b) => "ball"

Control of Execution

use progn to group multiple expressions

When

when you know the else-clause is NIL, use when instead of if

also when doesn't need progn, it can take any number of then-forms

Logical Operators

and, or and not

Cond

as soon as a non-NIL form is found, cond will be finished after evaluted the expressions.

no need for an explicit break statement.

use T as the default or catch-all clause:

(let ((n 4))
    (cond ((> n 10) "It's a lot")
        ((= n 10) "It's kind of a lot")
        (t "It's not a lot")))
=> "It's not a lot"

Case

case use eql to do matching, it will not work with strings.

the symbol otherwise has special meaning: it indicates a default or catch-all case:

(let ((color :orange))
    (case color
        (:blue "Blue")
        (:red "Red")
        (:green "Green)
        (otherwise "Unknown")))
=> "Unknown"

Iteration

do, dolist, dotimes, and loop

(let ((result 0))
    (dolist (elem '(4 5 6 7) result)
        (setq result (+ result elem))))
=> 22
(let ((result 1))
    (dotimes (n 10 result)
        (setq result (+ result n))))
=> 46

you can exit an iteration by calling return explicitly

Functions as Objects

named functions

use symbol-function to access the actual function object:

(symbol-function '+)

use funcall to call a function object:

(funcall (symbol-function '+) 1 2)
(setq plus (symbol-function '+))
(funcall plus 1 2)

use #' shorthand

(funcall #'+ 1 2)

functional arguments

like the example with mapcar, sort

anonymous functions

use lambda

(mapcar #'(lambda (num) (+ num 3))
    '(2 3 4))
=> (5 6 7)

optional arguments

identify by the &optional symbol

(defun greeting-list (&optional (username "Jake"))
    (list 'hello 'there username))
(greeting-list) => (HELLO THERE "Jake")
(greeting-list "Joe") => (HELLO THERE "Joe")

keyword arguments

identify by the &key symbol

(defun greeting-list (&key (username "Jake")
    (greeting "hello"))
    (list 'hello 'there username greeting))
(greeting-list :greeting "you ok?") => (HELLO THERE "Jake" "you ok?")
(greeting-list :greeting-list "you ok?" :username "Joe") => (HELLO THERE "Joe" "you ok?")