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

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

3. 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

4. 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

5. Declarative vs. imperative programming
dr Robert Kowalczyk WMiI UŁ

6. 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

7. 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

8. 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

9. 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

10. dr Robert Kowalczyk WMiI UŁ
Haskell platform
dr Robert Kowalczyk WMiI UŁ

11. 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

12. 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

13. Haskell as a calculator
dr Robert Kowalczyk WMiI UŁ

14. 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 =>
dr Robert Kowalczyk WMiI UŁ

15. Prefix and infix notation in Haskell
dr Robert Kowalczyk WMiI UŁ

16. 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

17. 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 Prelude>let r=5.0 why?
dr Robert Kowalczyk WMiI UŁ

18. 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

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

20. 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

21. 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

22. 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

23. 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

24. 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

25. 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

26. 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

27. 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

28. 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

29. 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

30. 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

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