dr Robert Kowalczyk WMiI UŁ Summer School Haskell 1 dr Robert Kowalczyk WMiI UŁ

Distribution of programming paradygms Programming languages Imperative Declarative Object-oriented Structured Generic Concurrent Functional Logical SQL, XML Procedural dr Robert Kowalczyk WMiI UŁ

dr Robert Kowalczyk WMiI UŁ Definitions Imperative programming: telling the „machine” (computer) how to do something, and as a result what you want to happen will happen. Declarative programming: telling the „machine” (computer) what you would like to happen, and let the computer figure out how to do it. dr Robert Kowalczyk WMiI UŁ

Imperative vs. declarative programming - example We wish to double all the numbers in an array. Imperative style of programming: var numbers = [1,2,3,4,5] var doubled = [] for(var i = 0; i < numbers.length; i++) { var newNumber = numbers[i] * 2 doubled.push(newNumber) } console.write(doubled) //=> [2,4,6,8,10] Declarative style of programming: var doubled = numbers.map(function(n) { return n * 2 }) console.log(doubled) //=> [2,4,6,8,10] dr Robert Kowalczyk WMiI UŁ

Declarative vs. imperative programming dr Robert Kowalczyk WMiI UŁ

Functional programming In functional programming (special type of declarative programming), programs are executed by evaluating expressions, in contrast with imperative programming where programs are composed of many statements which change global state when executed. dr Robert Kowalczyk WMiI UŁ

Functional programming languages Lisp - a family of programming languages: Common Lisp , Scheme i Clojure Ocaml ML Haskell Erlang Scala Python dr Robert Kowalczyk WMiI UŁ

dr Robert Kowalczyk WMiI UŁ Haskell language First version of Haskell („Haskell 1.0”) was defined in 1990. Haskell is purely functional programming language. Haskell has lazy evaluation. Haskell is a strongly typed programming language. dr Robert Kowalczyk WMiI UŁ

dr Robert Kowalczyk WMiI UŁ Haskell compiler To write Haskell programs, you need a program called a Haskell compiler. A compiler is a program that takes code written in Haskell and translates it into machine code, a more primitive language that the computer understands. dr Robert Kowalczyk WMiI UŁ

dr Robert Kowalczyk WMiI UŁ Haskell platform dr Robert Kowalczyk WMiI UŁ

First program/script in Haskell After you have installed the Haskell Platform, it's now time to write your first Haskell code. You can: write script haskel.hs with code: main = do putStrLn "Hello World" and compile it ghc haskel.hs run ghci (interactive console) and write or load script :l haskel.hs dr Robert Kowalczyk WMiI UŁ

dr Robert Kowalczyk WMiI UŁ Help in Haskell :? or :help – display list of commands :l or :load – load module :cd – change directory :! – run the shell command :! cd – change local directory dr Robert Kowalczyk WMiI UŁ

Haskell as a calculator dr Robert Kowalczyk WMiI UŁ

Functions in Haskell (embedded) abs x => |x| floor x => [x] div => / mod => % max a b => maximum(a,b) min a b => minimum(a,b) sqrt x => x^(1/2) exp x => exp(x) log x => ln(x) sin x => sin(x) cos x => cos(x) tan x => tan(x) pi => 3.141592653589793 dr Robert Kowalczyk WMiI UŁ

Prefix and infix notation in Haskell dr Robert Kowalczyk WMiI UŁ

Comments and a simple function -- one line comment {- block comment -} Prelude>let square x = x * x Prelude>square 3 9 Prelude>square (-3) Write script first.hs square x = x * x then load script :l first.hs and run function square 4 dr Robert Kowalczyk WMiI UŁ

dr Robert Kowalczyk WMiI UŁ Variables Prelude>let r=4 Prelude>r 4 Prelude>let area = pi*r*r error – why? Prelude>let r=4.0 Prelude>area 50.26548245743669 Prelude>let r=5.0 why? 78.53981633974483 dr Robert Kowalczyk WMiI UŁ

Checking the type of objects To check the type of object we can use the command :t or :type Prelude>:t "dog" "dog"::[Char] Prelude>:t 'e' 'e'::Char Prelude>let i=5 Prelude>:t i i::Integer Prelude>lez z=5.6 Prelude>:t z z::Double dr Robert Kowalczyk WMiI UŁ

dr Robert Kowalczyk WMiI UŁ Classtypes in Haskell dr Robert Kowalczyk WMiI UŁ

Simple types in Haskell Int - numbers in the range -229…229-1 Integer - very big integer numbers Float - real numbers Double - real numbers Char - (Unicode) character Bool - logical type dr Robert Kowalczyk WMiI UŁ

dr Robert Kowalczyk WMiI UŁ Logical operators Prelude>True && (1<4) True Prelude>False || False False Prelude>2 /= 3 Prelude>not True dr Robert Kowalczyk WMiI UŁ

dr Robert Kowalczyk WMiI UŁ Lists Prelude> let names = ["Jane", "George", "Kate"] Prelude> let numbers = [-2,-1,0,1,2] Prelude> -3 : numbers [-3,-2,-1,0,1,2] Prelude> numbers [-2,-1,0,1,2] Prelude> head numbers -2 Prelude> tail numbers [-1,0,1,2] dr Robert Kowalczyk WMiI UŁ

Lists and lazy evaluation Prelude>['H','a','s','k','e','l','l'] "Haskell" Prelude>'H':'a':'s':'k':'e':'l':'l':[] Prelude>[1,2..10] [1,2,3,4,5,6,7,8,9,10] Prelude>[5,3..(-1)] [5,3,1,-1] Prelude>[1,2..] 1,2,3,4,…. let pitagoras = [(a, b, c)| c <- [1 ..] , b <- [1 .. c] , a <- [1 .. b] , a^2 + b^2 == c^2] Prelude> [[1,2,3],[2,3,4],[3,4,5]] [[1,2,3],[2,3,4],[3,4,5]] dr Robert Kowalczyk WMiI UŁ

dr Robert Kowalczyk WMiI UŁ Functions on lists (!!) :: [a] -> Int -> a (!!) [-2,-1,0,1,2] 1 -1 length :: [a] -> Int length [-2,-1,0,1,2] 5 (++) :: [a] -> [a] -> [a] (++) [1,2,3] [4,5,6] [1,2,3,4,5,6] drop: Int -> [a] -> [a] drop 3 [-2,-1,0,1,2] [1,2] sum :: (Num a) => [a] -> a sum [-2,-1,0,1,2] 0 dr Robert Kowalczyk WMiI UŁ

dr Robert Kowalczyk WMiI UŁ Functions on lists product :: (Num a) => [a] -> a product [-2,-1,0,1,2] 0 map :: (a->b) -> [a] -> [b] map (+2) [-2,-1,0,1,2] [0,1,2,3,4] filter :: (a -> Bool) -> [a] -> [a] filter (>0) [-2,-1,0,1,2] [1,2] null :: [a] -> Bool null [] true take :: Int -> [a] -> [a] take 2 [-2,-1,0,1,2] [-2,-1] dr Robert Kowalczyk WMiI UŁ

The comprehension list [exp(x) | x <- list, cond(x)] Prelude> [x^2 | x <- [1..10], even x] [4,16,36,64,100] Prelude> [a+b| a<-[1..5], b<-[-5..(-1)]] [-4,-3,-2,-1,0,-3,-2,-1,0,1,-2,-1,0,1,2,-1,0,1,2,3,0,1,2,3,4] Prelude> [head x | x <- [[1..10],[20..30],[30..40]] [1,20,30] dr Robert Kowalczyk WMiI UŁ

dr Robert Kowalczyk WMiI UŁ Tuppels Prelude>let p = (2,3) Prelude>fst p 2 Prelude>snd p 3 Prelude>(”Martin”,”Depp”,23) (”Martin”,”Depp”,23) Prelude> zip [1,2,3,4,5] [5,5,5,5,5] [(1,5),(2,5),(3,5),(4,5),(5,5)] Prelude>unzip [(1,5),(2,5),(3,5),(4,5),(5,5)] ([1,2,3,4,5],[5,5,5,5,5]) dr Robert Kowalczyk WMiI UŁ

dr Robert Kowalczyk WMiI UŁ Excercise 1 Define the function writeS x which writes string x on the screen. For example, if you run writeS "Robert" you should see on the screen: "Hello Robert!" Hint: If you want to add two strings you can use ++ operator. dr Robert Kowalczyk WMiI UŁ

dr Robert Kowalczyk WMiI UŁ Excercise 2 Using the list comprehension construction write the function unitaryN which will build unitary matrix. For example, if you run unitaryN 5 you should see on the screen: [[1,0,0,0,0],[0,1,0,0,0],[0,0,1,0,0],[0,0,0,1,0],[0,0,0,0,1]] Hint: use if command: if condition then instruction1 else instruction2 Simpler version: using the list comprehension construction write the function vectorNM which will build unitary vector as in the following example: vectorNM 5 7 returns vector: [0,0,0,0,1,0,0] dr Robert Kowalczyk WMiI UŁ

dr Robert Kowalczyk WMiI UŁ Excercise 3 Build a list of integer numbers such that: numbers are three digits numbers are divisible by 3 all digits in each number are different. Of course these are the numbers: 102,105,…,987 dr Robert Kowalczyk WMiI UŁ

dr Robert Kowalczyk WMiI UŁ Coffee break  dr Robert Kowalczyk WMiI UŁ