# Functional Programming and Haskell - cs.usyd.edu.au tdaw3088/misc/haskell-  · Intro Functional Programming…

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• Intro Functional Programming Haskell List Processing Examples Wrap-Up

Tim Dawborn

University of Sydney, AustraliaSchool of Information Technologies

Tim Dawborn Functional Programming and Haskell 1/22

• Intro Functional Programming Haskell List Processing Examples Wrap-Up

A programming paradigm is a fundamental style of programmingregarding how solutions to problems are to be formulated in aprogramming language

Declarative Programming describes what you want, rather thanhow to do itSQL, XSLT

Imperative Programming computation is a set of statementswhich change the programs overall stateFortran, Pascal, C

Logic Programming use of mathematical logic for computerprogrammingProlog, Datalog, Godel

Object Oriented Programming models real-world objects andtheir interactions to design programsJava, C#, OCaml

Tim Dawborn Functional Programming and Haskell 2/22

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Functional Programming

Treats computation as a series of mathematical functions

Avoids states and mutable data

Foundation concepts include:

pure functionscalculus

Tim Dawborn Functional Programming and Haskell 3/22

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Pure Functions

Have no side effects

Retain referential transparencythus pure functions are thread safe by definition

Example pure functions:

sin(x)length("hello world")encrypt(msg)

Example impure functions:

random()global x = 4; void doit(){ x = 42; }

Tim Dawborn Functional Programming and Haskell 4/22

• Intro Functional Programming Haskell List Processing Examples Wrap-Up

Lambda Calculus (calculus)

calculus is a formal system used within computer scienceand formal logic theory

Can be used to define what a computable function is

Concerned with entities called termsv variable name v .E1 abstraction (anonymous function)(E1 E2) terms

Checking for equivalence between two calculus expressionsis undecidable

Easy to think of as an anonymous pure function (impliesreferential transparency)

e.g. square = y . y y

Tim Dawborn Functional Programming and Haskell 5/22

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Lambda Calculus (calculus)

What if we have a list of numbers L and we wanted to create anew list L = [ n | n3 + 3n + 3 n L]? (in Python)

def doit ( items ) :r = [ ]for i in items :

r . append (i3 + 3i + 3)return r

doit ( [ 1 , 2 , 3 , 4 , 5 ] )

def doit (n ) :return n3 + 3n + 3

map (doit , [ 1 , 2 , 3 , 4 , 5 ] )

map ( lambda x : x3 + 3x + 3 , [ 1 , 2 , 3 , 4 , 5 ] )

Tim Dawborn Functional Programming and Haskell 6/22

• Intro Functional Programming Haskell List Processing Examples Wrap-Up

Functional, interpreted language

Lazy call-by-name evaluation does not evaluate thefunction calls; knows the relation and will evaluate whenneeded

List makeList ( ) {List m = new List ( ) ;m . value = 2 ;m . next = makeList ( ) ;return m ;

}

makeList = 2 : makeList

Tim Dawborn Functional Programming and Haskell 7/22

• Intro Functional Programming Haskell List Processing Examples Wrap-Up

All functions are pure (no side effects)

Does not support destructive updates: the code

int x = 5 ;x = x + 1 ;

Easy to read (and understand ?) code as a result

qsort : : ( Ord a ) => [ a ] > [ a ]qsort [ ] = [ ]qsort (x : xs ) = qsort ( filter (< x ) xs )

++ [ x ]++ qsort ( filter (>= x ) xs )

Tim Dawborn Functional Programming and Haskell 8/22

• Intro Functional Programming Haskell List Processing Examples Wrap-Up

Syntax

No parenthesiss used for invoking functions

Arbitrary precision integers

> 35 + 116

> 2501125899906842624

> sqrt 21.4142135623730951

> 5 == 4.5 (0.5)True

Tim Dawborn Functional Programming and Haskell 9/22

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Lists [ ]

Homogeneous

Cons and concatenation operators are : and ++respectively

> 0 : [ 1 , 2 , 3 , 4 , 5 ][ 0 , 1 , 2 , 3 , 4 , 5 ]

This displayed pre-formed list syntax as shown above is in-factsyntactic sugar the compiler breaks this down into individualelements consd together with a concluding empty list. i.e.

> 0 : 1 : 2 : 3 : 4 : 5 : [ ][ 0 , 1 , 2 , 3 , 4 , 5 ]

Strings are lists of Chars

Some useful functions for lists include head, tail, length,and null

Tim Dawborn Functional Programming and Haskell 10/22

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Tuples ( )

Heterogeneous k-tuples, (k N > 0)

Special inbuilt functions for pairs (2-tuple)

fst gets 1st item in a pairsnd gets 2nd item in a pair

Using a combination of standard tuple operations, how do you getout the character from ((1, a), "foo") ?

snd ( fst ( ( 1 , a ) , "foo" ) )

Tim Dawborn Functional Programming and Haskell 11/22

• Intro Functional Programming Haskell List Processing Examples Wrap-Up

List Processing

Four key standard list processing functions: map, filter, foldl,and foldr

> map Char . toUpper "Hello World""HELLO WORLD"

> map Char . isUpper "Hello World"[ True , False , False , False , False , False , True , False ,False , False , False ]

> filter Char . isLower "Hello World""elloorld"

Tim Dawborn Functional Programming and Haskell 12/22

• Intro Functional Programming Haskell List Processing Examples Wrap-Up

List Processing Folding

Folding is a key concept used a lot in functional programming

Syntax is: foldr function init val list

Replaces occurrences of cons with the function parameter,and replaces the empty list constructor (end of the conssequence) with the initial value

Associativity is crucial in folding: a (b c) = (a b) c?foldr is right associativefoldl is (surprise surprise) left associative

Tim Dawborn Functional Programming and Haskell 13/22

• Intro Functional Programming Haskell List Processing Examples Wrap-Up

List Processing Folding

> foldr () 1 [ 4 , 8 , 5 ]> 4 ( f o l d r () 1 8 : 5 : [ ] )> 4 (8 ( f o l d r () 1 5 : [ ] ) )> 4 (8 (5 ( f o l d r () 1 [ ] ) ) )> 4 (8 (5 ( 1 ) ) )

0

> foldl () 1 [ 4 , 8 , 5 ]> ( f o l d l () 1 4 : 8 : [ ] ) 5> ( ( f o l d l () 1 4 : [ ] ) 8) 5> ( ( ( f o l d l () 1 [ ] ) 4) 8) 5> ( ( ( 1 ) 4) 8) 5

16

Tim Dawborn Functional Programming and Haskell 14/22

• Intro Functional Programming Haskell List Processing Examples Wrap-Up

Questions / Examples

How can I compute the number of lowercase letters in a String?

length ( filter Char . isLower "aBCde" )

( length . filter Char . isLower ) "aBCde"

Given that max compares two numbers and returns the max,write a function using folding that returns the max item in a list ofintegers, or zero if the list is empty

foldr max 0 [ 5 , 1 0 , 2 , 8 , 1 ]

Tim Dawborn Functional Programming and Haskell 15/22

• Intro Functional Programming Haskell List Processing Examples Wrap-Up

Code Snippets

Implement the built-in filter function

my_filter : : (a > Bool ) > [ a ] > [ a ]my_filter _ [ ] = [ ]my_filter f (x : xs ) =

if f xthen x : ( my_filter f xs )else my_filter f xs

Implement the built-in map function

my_map : : (a > b ) > [ a ] > [ b ]my_map _ [ ] = [ ]my_map f (x : xs ) = (f x ) : my_map f xs

Tim Dawborn Functional Programming and Haskell 16/22

• Intro Functional Programming Haskell List Processing Examples Wrap-Up

Code Snippets

Implementation of a prime sieve:

primes = sieve [ 2 . . ] wheresieve (p : xs ) =

p : sieve ( filter (\x > x mod p > 0) xs )

take 20 primes

Implementation of a uniq function for a list:

uniq : : (Eq a ) => [ a ] > [ a ]uniq [ ] = [ ]uniq (x : xs ) = x : ( uniq . filter (/=x ) ) xs

Tim Dawborn Functional Programming and Haskell 17/22

• Intro Functional Programming Haskell List Processing Examples Wrap-Up

Code Snippets

Implementation of the built-in foldr function

foldr : : (a > b > b ) > b > [ a ] > bfoldr _ s [ ] = sfoldr f s (x : xs ) = f x ( foldr f s xs )

Non-optimised implementation of quicksort

qsort : : ( Ord a ) => [ a ] > [ a ]qsort [ ] = [ ]qsort (x : xs ) = qsort ( filter (< x ) xs )

++ [ x ]++ qsort ( filter (>= x ) xs )

Tim Dawborn Functional Programming and Haskell 18/22

• Intro Functional Programming Haskell List Processing Examples Wrap-Up

Code Snippets

Three different ways to implement factorial: two recursiveand one itterative

fact : : ( Integral a ) => a > afact 0 = 1fact n = n fact (n1)

fact : : ( Integral a ) => a > afact n | n a > afact n = foldr ( ) 1 [ 1 . . n ]

Tim Dawborn Functional Programming and Haskell 19/22

• Intro Functional Programming Haskell List Processing Examples Wrap-Up

Code Snippets

Implement the built-in zip function

zip : : [ a ] > [ b ] > [ ( a , b ) ]zip [ ] _ = [ ]zip _ [ ] = [ ]zip (y : xy ) (z : xz ) = (y , z ) : zip xy xz

Does the following implementation also work?

zip : : [ a ] > [ b ] > [ ( a , b ) ]zip ay@ (y : xy ) az@ (z : xz ) =

| null ay | | null az = [ ]| otherwise = (y , z ) : zip xy xz

Tim Dawborn Functional Programming and Haskell 20/22

• Intro Functional Programming Haskell List Processing Examples Wrap-Up

Conclusion

This was a very basic and brief intro into FunctionalProgramming in Haskell

For a good introduction into Haskell, have a go at YetAnother Haskell Tutorial (YAHT)

Tim Dawborn Functional Programming and Haskell 21/22

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References

http://www.csse.monash.edu.au/lloyd/tildeFP/Lambda/Ch/

Tim Dawborn Functional Programming and Haskell 22/22