FUNCTIONAL PROGRAMMING05 Functions

FUNCTIONS - GLOBAL FUNCTIONS

fboundp Tells whether there is a function with a given symbol as its

name > (fboundp ‘+)

T > (symbol-function ‘+)

#<SYSTEM-FUNCTION +> (setf (symbol-function ‘add2)

#’(lambda (x) (+ x 2))) > (add2 1)

3

FUNCTIONS - GLOBAL FUNCTIONS

(setf func_name) (defun primo (lst) (car lst)) (defun (setf primo) (val lst)

(setf (car lst) val)) > (let ((x (list ‘a ‘b ‘c)))

(setf (primo x) 480) x)(480 B C)

The first parameter : new value The remaining parameters : the arguments of this setf

function

FUNCTIONS - GLOBAL FUNCTIONS

Documentation string If you make a string the first expression in the body of a

function defined with defun, then this string will become the function’s documentation string

(defun foo (x) “Implements an enhanced paradigm of diversity.” x)

> (documentation ‘foo ‘function)“Implements an enhanced paradigm of diversity.”

FUNCTIONS - LOCAL FUNCTIONS

Functions defined via defun or setf of symbol-function are global functions You can access global functions anywhere

Local functions can be defined with labels, which is a kind of let for functions (labels ((add10 (x) (+ x 10))

(consa (x) (cons ‘a x))) (consa (add10 3)))(A . 13)

FUNCTIONS - LOCAL FUNCTIONS

> (labels ((len (lst) (if (null lst) 0 (+ (len (cdr lst)) 1)))) (len ‘(a b c)))3

FUNCTIONS - PARAMETER LISTS

&rest Insert &rest before the last variable in the parameter list of a

function This variable will be set to a list of all the remaining

arguments (defun our-funcall (fn &rest args)

(apply fn args))

FUNCTIONS - PARAMETER LISTS

&optional All the arguments after it could be omitted (defun philosoph (thing &optional property)

(list thing ‘is property)) > (philosoph ‘death)

(DEATH IS NIL) The explicit default of the optional parameter

(defun philosoph (thing &optional (property ‘fun)) (list thing ‘is property))

> (philosoph ‘death)(DEATH IS FUN)

FUNCTIONS - PARAMETER LISTS

&key A keyword parameter is a more flexible kind of optional

parameter These parameters will be identified not by their position, but

by symbolic tags that precede them > (defun keylist (a &key x y z)

(list a x y z))KEYLIST

> (keylist 1 :y 2)(1 NIL 2 NIL)

> (keylist 1 :y 3 :x 2)(1 2 3 NIL)

FUNCTIONS – EXAMPLE: UTILITIES

> (single? ‘(a))T → returns true when its argument is a list of one element

> (append1 ‘(a b c) ‘d)(A B C D) → adds an element to the end of the list

> (map-int #’identify 10)(0 1 2 3 4 5 6 7 8 9) → returns a list of the results of calling the function on the integers from 0 to n-1

FUNCTIONS – EXAMPLE: UTILITIES

> (map-int #’(lambda (x) (random 100)) 10)(85 40 73 64 28 21 40 67 5 32)

> (filter #’(lambda (x) (and (evenp x) (+ x 10))) ‘(1 2 3 4 5 6 7)(12 14 16) → returns all the non-nil values returned by the function as it is applied to the elements of the list

> (most #’length ‘((a b) (a b c) (a)))(A B C) → returns the element of a list with3 the highest score, according to some scoring function

FUNCTIONS – EXAMPLE: UTILITIES

(defun single? (lst)

(and (consp 1st) (null (cdr lst))))

(defun appendl (1st obj)

(append 1st (list obj)))

(defun map-int (fn n)

(let ((acc nil))

(dotimes (i n)

(push (funcall fn i) acc))

(nreverse acc)))

(defun filter (fn lst)

(let ((acc nil))

(dolist (x lst)

(let ((val (funcall fn x)))

(if val (push val acc))))

(nreverse acc)))

FUNCTIONS – EXAMPLE: UTILITIES

(defun most (fn 1st)

(if (null 1st)

(values nil nil)

(let* ((wins (car lst))

(max (funcall fn wins)))

(dolist (obj (cdr lst))

(let ((score (funcall fn obj)))

(when (> score max)

(setf wins obj

max score))))

(values wins max))))

FUNCTIONS – CLOSURES

(defun combiner (x) (typecase x (number #’+) (list #’append) (t #’list)))

(defun combine (&rest args) (apply (combiner (car args)) args))

> (combine 2 3)5

> (combine ‘(a b) ‘(c d))(A B C D)

FUNCTIONS – CLOSURES

When a function refers to a variable defined outside it, it is called a free variable

Closure A function that refers to a free variable (defun add-to-list (num lst)

(mapcar #’(lambda (x) (+ x num)) lst)) num is free

(defun make-adder (n) #’(lambda (x) (+ x n))) → returns a function

n is free

FUNCTIONS – CLOSURES

(setf add3 (make-adder 3))#<CLOSURE: LAMBDA (X) (+ X N)>

> (funcall add3 2)5

(setf add27 (make-adder 27)) #<CLOSURE: LAMBDA (X) (+ X N)>

> (funcall add27 2)29

FUNCTIONS – CLOSURES

Make several closures share variables (let ((counter 0))

(defun reset ( ) (setf counter 0)) (defun stamp ( ) (setf counter (+ counter 1))))

> (list (stamp) (stamp) (reset) (stamp))(1 2 0 1)

You can do the same thing which a global counter, but this way the counter is protected from unintended references

FUNCTIONS – EXAMPLE: FUNCTION BUILDERS

(defun disjoin (fn &rest fns)

(if (null fns)

fn

(let ((disj (apply #’disjoin fns)))

#’(lambda (&rest args)

(or (apply fn args) (apply disj args))))))

FUNCTIONS – EXAMPLE: FUNCTION BUILDERS

(defun conjoin (fn &rest fns)

(if (null fns)

fn

(let ((conj (apply #’conjoin fns)))

#’(lambda (&rest args)

(and (apply fn args) (apply conj args))))))

(defun curry (fn &rest args)

#’(lambda (&rest args2)

(apply fn (append args args2))))

(defun rcurry (fn &rest args)

#’(lambda (&rest args2)

(apply fn (append args2 args))))

(defun always (x) #’(lambda (&rest args) x))

FUNCTIONS – EXAMPLE: FUNCTION BUILDERS

> (mapcar (disjoin #’integerp #’symbolp) ‘(a “a” 2 3))(T NIL T T)

> (mapcar (conjoin #’integerp #’oddp) ‘(a “a” 2 3))(NIL NIL NIL T)

> (funcall (curry #’- 3) 2)1

> (funcall (rcurry #’- 3) 2)-1

FUNCTIONS – RECURSION

Recursion plays a greater role in Lisp than in most other languages Functional programming

Recursive algorithms are less likely to involve side-effects Recursive data structure

Lisp’s implicit use of pointers makes it easy to have recursively defined data structures

The most common is the list: a list is either nil, or a cons whose cdr is a list

Elegance Lisp programmers care a great deal about the beauty of their

programs Recursive algorithms are often more elegant than their iterative

counterparts

FUNCTIONS – RECURSION

To solve a problem using recursion, you have to Show how to solve the problem in the general case by

breaking it down into a finite number of similar, but smaller, problems

Show how to solve the smallest version of the problem - the base case - by some finite number of operations

<Example1> Finding the length of a proper list In the general case, the length of a proper list is the length of

its cdr plus 1 The length of an empty list is 0

FUNCTIONS – RECURSION

<Example２ > member Something is a member of a list if it is the first element, or a

member of the cdr Nothing is a member of the empty list

<Example3> copy-tree The copy-tree of a cons is a cons made of the copy-tree of its

car, and the copy-tree of its cdr The copy-tree of an atom is itself

FUNCTIONS – RECURSION

The obvious recursive algorithm is not always the most efficient E.g. Fibonacci function

1. Fib(0)=Fib(1)=12. Fib(n)=Fib(n-1)+Fib(n-2)

(defun fib (n) (if (<= n 1) 1 (+ (fib (- n 1)) (fib (- n 2)))))

The recursive version is appallingly inefficient The same computations are done over and over

FUNCTIONS – RECURSION

(defun fib (n) (do ((i n (- i 1)) (f1 1 (+ f1 f2)) (f2 1 f1)) ((<= i 1) f1)))

The iterative version is not as clear, but it is far more efficient However, this kind of thing happen very rarely in practice

FUNCTIONS

Homework (Due April 14) Use the lambda function and closures to write a function

called our-complement that takes a predicate and returns the opposite predicate. For example:> (mapcar (our-complement #’oddp) ‘(1 2 3 4 5 6))(NIL T NIL T NIL T)