Fun never stops. introduction to haskell programming language

“Funneverstops.
IntroductiontoHaskell
Programminglanguage”
twitter: @rabbitonweb,
github: github.com/rabbitonweb

“Funneverstops.
IntroductiontoHaskell
Programminglanguage”
FOR SCALA
DEVELOPERS

TherearetwotypesofScaladevelopers:

1)thosewhoarehappytheynolongerprograminJava

2)ThoseWhoarepissedofftheyarenotyetusinghaskell

2)ThoseWhoarepissedofftheyarenotyetusinghaskell
Let’s get a bit of that Haskell taste...

WhylearnHaskell?
HaskellisPureFunctional
ProgrammingLanguage

❏ no assignment statements
WhatisFunctionalProgramming?

❏ no variables

❏ no variables
❏ once given a value, never change

❏ no variables
❏ no side-effects at all

❏ no variables
❏ no side-effects at all
“The functional programmer sounds
rather like a mediæval monk,
denying himself the pleasures of
life in the hope that it will make
him virtuous.”

WhyFunctionalProgrammingMatters?

What procedural programming,
dependency injection, actor
model, microservices and
clean architecture have in
common?

❏ We strive for modularity
What procedural programming,
dependency injection, actor
model, microservices and
clean architecture have in
common?

input f(input): output output g(input): output

input f(input): output output g(input): output output

h = g o f
input f(input): output output g(input): output output

❏ There is no concept of time

Whatwillthisfunctiondo?
/**
* This function returns a reversed list
* @param list A list to be reversed
* @return A reversed list
*/
def reverse[T](list: List[T]): List[T] = ???

/**
* @return A reversed list

/**

def reverse[T](list: List[T]): List[T] = list.sort

def rhrggahaham[A](a: A): A = ???

def rhrggahaham[A](a: A): A = ???
Ifwrittenbyfunctionalprogrammer

def rhrggahaham[A](a: A): A = a

def identity[A](a: A): A = a

def rhrggahaham(a: Int): Int = ???

def rhrggahaham(a: Int): Int = ???
Ifwrittenbyfunctionalprogrammer

def rhrggahaham(a: Int): Int = a
a + 10
20

❏ We can reason about our code
with ease

scala> def add(a: Int, b: Int) = a + b
add: (a: Int, b: Int)Int

scala> val curriedAdd = curry(add)
curriedAdd: Int => (Int => Int) = <function1>

scala> val curriedAdd = curry(add)
curriedAdd: Int => (Int => Int) = <function1>
scala> val add5 = curriedAdd(5)
scala> add5(7)
res1: Int = 12

def curry[A,B,C](f: (A, B) => C):

def curry[A,B,C](f: (A, B) => C): (A => (B => C)) =

(a: A) => ?

(a: A) => ( ? => ? )

(a: A) => ( (b: B) => ? )

(a: A) => ( (b: B) => f(a,b) )

with ease
❏ Just follow types

with ease
❏ Just follow types
❏ Lazy evaluation

World-> (World,A)
"aninteractiveprogramisapurefunctionthattakesthe
current'stateoftheworld'asitsarg.andproducesa
modifiedworldasresult"

World-> (World,A)
“Protip:Don'ttrytotrackreal-worldstateinyourcode.
Insteadtakeitasanargument,sinceonlytheworld
knowswhatstateit'sin.”

World->(World,A)
currentStateOfTheWorld = stateOfTheWorld
f

World->(World,A)
= f currentStateOfTheWorld
f

World->(World,A)
(newWorld, someValue) = f currentStateOfTheWorld
f

World->(World,A)
modifyWorldWith newWorld
f

World->(World,A)
modifyWorldWith newWorld
display someValue
f

HaskellHistoryinanutshell
❏ Miranda released 1985 - de facto FP ‘standard’

❏ At the FPCA'87 few formed a consensus that an open
standard should be defined for FP languages

❏ Haskell 1.0 defined in 1990

❏ Haskell 98 in late 1997

“(...) intended to specify a stable, minimal, portable
version of the language and an accompanying standard library
for teaching, and as a base for future extensions.”

❏ “The Haskell 98 Report” in 1999

❏ “The Haskell 2010 Report” in 2009/2010

❏ “The Haskell 2010 Report” in 2009/2010
❏ Further Haskell evolves rapidly around GHC

GHC-GlassgowGloriousHaskellCompiler

GHC-GlassgowGloriousHaskellCompiler
❏ > ghci
❏ :t
❏ +set :t
❏ :load
❏ Emacs + Haskell mode
❏ C-c C-b
❏ C-c C-l

FunWithFunctions
helloWorld = “Hello World”

FunWithFunctions
❏ function without parameters is called definition

FunWithFunctions
❏ function name must start with small letter

FunWithFunctions
❏ function name must start with small letter
> helloWorld
"Hello World"
it :: [Char]

FunWithFunctions
london u = “London “ ++ u

FunWithFunctions
> :t london
london :: [Char] -> [Char]

FunWithFunctions
> :t london
> london "baby"
"london baby"
it :: [Char]

FunWithFunctions
isA c = c == 'A'

FunWithFunctions
isA :: Char -> Bool
isA c = c == 'A'

FunWithFunctions
> isA 'd'
False
> isA 'A'
True
isA :: Char -> Bool
isA c = c == 'A'

FunWithFunctions
prefixMe c str = [c] ++ " " ++ str

FunWithFunctions
> prefixMe '_' "word"
"_ word"
it :: [Char]

FunWithFunctions
> :t prefixMe
prefixMe :: Char -> [Char] -> [Char]
"_ word"
it :: [Char]

FunWithFunctions
> :t prefixMe
prefixMe :: Char -> ([Char] -> [Char])
"_ word"
it :: [Char]

FunWithFunctions
answerToEverything = 42

FunWithFunctions
complexCalc x y z = x * y * z * answerToEverything

FunWithFunctions
> :t complexCalc
complexCalc :: Integer -> Integer -> Integer -> Integer

FunWithFunctions
> :t complexCalc
complexCalc :: Integer -> (Integer -> (Integer -> Integer))

FunWithFunctions
> :t complexCalc
> complexCalc 10 20 30

FunWithFunctions
> :t complexCalc
> complexCalc 10 20 30
> ((complexCalc 10) 20) 30

FunWithTypes
❏ We’ve already seen: Char, Integers & Bool

FunWithTypes
❏ Other build-in types:

FunWithTypes
❏ Int, Float, Doubles

FunWithTypes
❏ Tuples

FunWithTypes
❏ Tuples
> (False, "Is it?")
it :: (Bool, [Char])
> (10.4, True, "Values", 'Y')
it :: (Double, Bool, [Char], Char)

FunWithTypes
❏ Tuples
❏ Lists
> [False, False, True]
it :: [Bool]
> [10, 20, 30]
it :: [Integer]
> ["one", "two", "three"]
it :: [[Char]]

Polymorphic types
> length [False, False, True]
3

Polymorphic types
3
> length [10, 20, 30, 40]
4

Polymorphic types
3
> length [10, 20, 30, 40]
4
> length ["one", "two", "three"]
3

Polymorphic types
3
> length [10, 20, 30, 40]
4
3
length :: [a] -> Int

Polymorphic types
3
> length [10, 20, 30, 40]
4
3
length :: [a] -> Int
‘a’ is a type variable, which means that length can be
applied to list of any type

List101
numbers = [1, 3, 5, 7]

List101
numbers = [1, 3, 5, 7]
numbers !! 2 -> 5

List101
numbers = [1, 3, 5, 7]
numbers !! 2 -> 5
null numbers -> False

List101
numbers = [1, 3, 5, 7]
numbers !! 2 -> 5
head numbers -> 1

List101
numbers = [1, 3, 5, 7]
numbers !! 2 -> 5
head numbers -> 1
tail numbers -> [3, 5, 7]

List101
numbers = [1, 3, 5, 7]
numbers !! 2 -> 5
head numbers -> 1
init numbers -> [1, 3, 5]

List101
numbers = [1, 3, 5, 7]
numbers !! 2 -> 5
head numbers -> 1
last numbers -> 7

List101
numbers = [1, 3, 5, 7]
numbers !! 2 -> 5
head numbers -> 1
last numbers -> 7
drop 2 numbers -> [5, 7]

List101
numbers = [1, 3, 5, 7]
numbers !! 2 -> 5
head numbers -> 1
last numbers -> 7
take 2 numbers -> [1, 3]

List101
numbers = [1, 3, 5, 7]
numbers !! 2 -> 5
head numbers -> 1
last numbers -> 7
take 2 numbers -> [1, 3]
elem 5 numbers -> True

List101
[1..6] -> [1, 2, 3, 4, 5, 6]

List101
[1..6] -> [1, 2, 3, 4, 5, 6]
[1,3,.., 20] -> [1,3,5,7,9,11,13,15,17,19]

List101
[1..6] -> [1, 2, 3, 4, 5, 6]
[1,3,.., 20] -> [1,3,5,7,9,11,13,15,17,19]
[1..] -> [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, ...

List101
[1..6] -> [1, 2, 3, 4, 5, 6]
[1,3,.., 20] -> [1,3,5,7,9,11,13,15,17,19]
take 5 [1..] -> [1, 2, 3, 4, 5]

List101
[1..6] -> [1, 2, 3, 4, 5, 6]
[1,3,.., 20] -> [1,3,5,7,9,11,13,15,17,19]
take 5 [1..] -> [1, 2, 3, 4, 5]
(cycle [0, 1]) -> [0,1,0,1,0,1,0,1,0,1,0,1,0,1,0,1,..

List101
[1..6] -> [1, 2, 3, 4, 5, 6]
[1,3,.., 20] -> [1,3,5,7,9,11,13,15,17,19]
take 5 [1..] -> [1, 2, 3, 4, 5]
take 6 (cycle [0, 1]) -> [0,1,0,1,0,1]

List101
[1..6] -> [1, 2, 3, 4, 5, 6]
[1,3,.., 20] -> [1,3,5,7,9,11,13,15,17,19]
take 5 [1..] -> [1, 2, 3, 4, 5]
take 6 (cycle [0, 1]) -> [0,1,0,1,0,1]
take 3 (repeat 2) -> [2, 2, 2]

List101
[2 ^ x | x <- [1..5]] -> [2,4,8,16,32]

List101
[2 ^ x | x <- [1..5]] -> [2,4,8,16,32]
take 7 [2 ^ x | x <- [1..]] -> [2,4,8,16,32,64,128]

List101
[2 ^ x | x <- [1..5]] -> [2,4,8,16,32]
take 7 [2 ^ x | x <- [1..]] -> [2,4,8,16,32,64,128]
[(x,y) | x <- [1..5], y <- [1..5]] -> [(1,1),(1,2),(1,3), ...

List101
[2 ^ x | x <- [1..5]] -> [2,4,8,16,32]
take 7 [2 ^ x | x <- [1..]] -> [2,4,8,16,32,64,128]
[(x,y) | x <- [1..5], y <- [1..5]] -> [(1,1),(1,2),(1,3), …
[(x,y) | x <- [1..2], y <- [x..2]] -> [(1,1),(1,2),(2,2)]

List101
[2 ^ x | x <- [1..5]] -> [2,4,8,16,32]
take 7 [2 ^ x | x <- [1..]] -> [2,4,8,16,32,64,128]
[(x,y) | x <- [1..5], y <- [1..5]] -> [(1,1),(1,2),(1,3), ...
[(x,y) | x <- [1..2], y <- [x..2]] -> [(1,1),(1,2),(2,2)]
[x | x <- [1..7], x `mod` 2 == 0] -> [2, 4, 6]

List101
[2 ^ x | x <- [1..5]] -> [2,4,8,16,32]
take 7 [2 ^ x | x <- [1..]] -> [2,4,8,16,32,64,128]
[(x,y) | x <- [1..5], y <- [1..5]] -> [(1,1),(1,2),(1,3), ...
[(x,y) | x <- [1..2], y <- [x..2]] -> [(1,1),(1,2),(2,2)]
[x | x <- [1..7], x `mod` 2 == 0] -> [2, 4, 6]
zip "abc" [1,2,3] -> [('a',1),('b',2),('c',3)]

List101
[2 ^ x | x <- [1..5]] -> [2,4,8,16,32]
take 7 [2 ^ x | x <- [1..]] -> [2,4,8,16,32,64,128]
[(x,y) | x <- [1..5], y <- [1..5]] -> [(1,1),(1,2),(1,3), ...
[(x,y) | x <- [1..2], y <- [x..2]] -> [(1,1),(1,2),(2,2)]
[x | x <- [1..7], x `mod` 2 == 0] -> [2, 4, 6]
zip "abc" [1,2,3] -> [('a',1),('b',2),('c',3)]
zip "abc" (cycle [0,1]) -> [('a',0),('b',1),('c',0)]

List101
[2 ^ x | x <- [1..5]] -> [2,4,8,16,32]
take 7 [2 ^ x | x <- [1..]] -> [2,4,8,16,32,64,128]
[(x,y) | x <- [1..5], y <- [1..5]] -> [(1,1),(1,2),(1,3), ...
[(x,y) | x <- [1..2], y <- [x..2]] -> [(1,1),(1,2),(2,2)]
[x | x <- [1..7], x `mod` 2 == 0] -> [2, 4, 6]
zip "abc" [1,2,3] -> [('a',1),('b',2),('c',3)]
zip "abc" (cycle [0,1]) -> [('a',0),('b',1),('c',0)]
zipWith (+) [1,2,3] [4,5,6] -> [5,7,9]

Classes
caseclassFoo(...)???
No,notthoseclasses.Typeclasses.

TypeClasses
Type class is a set of types which support certain
collection of functions defined by the class.

TypeClasses
❏ Eq
(==) :: a -> a -> Bool; (/=) :: a -> a -> Bool

TypeClasses
❏ Eq
(==) :: a -> a -> Bool; (/=) :: a -> a -> Bool
❏ Ord

TypeClasses
❏ Eq
(==) :: a -> a -> Bool; (/=) :: a -> a -> Bool
❏ Ord
< , <= , > , >= :: a -> a -> Bool; min , max :: a -> a -> a

TypeClasses
❏ Eq
(==) :: a -> a -> Bool; (/=) :: a -> a -> Bool
❏ Ord
❏ Show

TypeClasses
❏ Eq
(==) :: a -> a -> Bool; (/=) :: a -> a -> Bool
❏ Ord
❏ Show
show :: a -> String

TypeClasses
❏ Eq
(==) :: a -> a -> Bool; (/=) :: a -> a -> Bool
❏ Ord
❏ Show
show :: a -> String
❏ Read

TypeClasses
❏ Eq
(==) :: a -> a -> Bool; (/=) :: a -> a -> Bool
❏ Ord
❏ Show
show :: a -> String
❏ Read
read :: String -> a

TypeClasses
> 10 == 20
False
> 10 < 20
True

TypeClasses
> 10 == 20
False
> 10 < 20
True
> show 1 ++ " is a number"
"1 is a number"

TypeClasses
> 10 == 20
False
> 10 < 20
True
"1 is a number"
> read "5" + 10
15

TypeClasses
> 10 == 20
False
> 10 < 20
True
"1 is a number"
> read "5" + 10
15
> read "5"
<interactive>:85:1:
No instance for (Read a0) arising from a use of `read'
*Main> read "5" :: Integer 5 it :: Integer

TypeClasses
> 10 == 20
False
> 10 < 20
True
"1 is a number"
> read "5" + 10
15
> read "5" :: Integer
5

Typeclasses&classconstraints
add a b = a + b

add :: Num a => a -> a -> a
add a b = a + b

add :: Num a => a -> a -> a
add a b = a + b
add 10 20

add :: Num a => a -> a -> a
add a b = a + b
add 10 20
add 5.0 7.0

add :: Num a => a -> a -> a
add a b = a + b
add 10 20
add 5.0 7.0
weirdo n d str = (n + 2, d / 2.0, str ++ "!!")

add :: Num a => a -> a -> a
add a b = a + b
add 10 20
add 5.0 7.0
weirdo :: (Fractional t1, Num t) =>
t -> t1 -> [Char] -> (t, t1, [Char])

add :: Num a => a -> a -> a
add a b = a + b
add 10 20
add 5.0 7.0
t -> t1 -> [Char] -> (t, t1, [Char])
weirdo 7 7.0 “kuku”

add :: Num a => a -> a -> a
add a b = a + b
add 10 20
add 5.0 7.0
t -> t1 -> [Char] -> (t, t1, [Char])
weirdo 7 7.0 “kuku”
weirdo 7.0 5.0 “mono”

FunWithFunctions,Part2
sum :: Num a => [a] -> a

sum :: Num a => [a] -> a
sum list = if null list then 0
else 1 + sum (tail list)

sum :: Num a => [a] -> a
sum [] = 0

sum :: Num a => [a] -> a
sum [] = 0
sum x:xs = 1 + sum xs

programmingLanguage :: [Char] -> [Char]

programmingLanguage "Scala" = "Good"

programmingLanguage "Haskell" = "Even better!"

programmingLanguage "Haskell" = "Even better!"
programmingLanguage other = "Meh..." ++ other ++ ", pf.."

first :: (a, b, c) -> a
first (x, _, _) = x
second :: (a, b, c) -> b
second (_, y, _) = y
third :: (a, b, c) -> c
third (_, _, z) = z

rankMe :: (Num a, Ord a) => a -> [Char]

rankMe lines

rankMe lines
| lines > 100 = "Senior"

rankMe lines
| lines > 30 = "Regular"

rankMe lines
| lines >= 0 = "Junior"

rankMe lines
| lines < 0 = "Congrats! You are a true Hacker!"

rankMe :: (Num a, Ord a) => a -> a -> a -> [Char]
rankMe added deleted modified

rankMe :: (Num a, Ord a) => a -> a -> a -> [Char]
rankMe added deleted modified
where lines = added - deleted

Creatingnewtypes-aliasing
type String = [Char]

type Point = (Int, Int)

type Point = (Int, Int)
type Parser a = String -> (a, String)

Creatingnewtypes-Datadeclarations

data OS = Windows | OSX | Linux | Unix

bootup :: OS -> [Char]

bootup Unix = "Up and running"

bootup Linux = "Well, I was actually never down..."

bootup OSX = "Non-sufficient Starbucks coffee, please refill"

bootup OSX = "Non-sufficient Starbucks coffee, please refill"
bootup Windows = "Upgrading to Windows 10, this should take a
second.."

class Bootable a where
boot :: a -> [Char]

class Bootable a where
boot :: a -> [Char]
instance Bootable OS where
boot Unix = "Up and running"
boot Linux = "Well, I was actually never down..."
boot OSX = "Non-sufficient Starbucks coffee, please refill"
boot Windows = "Upgrading to Windows 10, this should take a
second.."

instance Eq OS where
Unix == Unix = True
Linux == Linux = True
OSX == OSX = True
Windows == Windows = True
_ == _ = False

instance Eq OS where
Unix == Unix = True
Linux == Linux = True
OSX == OSX = True
Windows == Windows = True
_ == _ = False
> Unix == Windows
False

instance Show OS where
show Unix = "Unix"
show Linux = "Linux"
show OSX = "OSX"
show Windows = "Windows"

instance Show OS where
show Unix = "Unix"
show Linux = "Linux"
show OSX = "OSX"
show Windows = "Windows"
> show Unix
"Unix"

deriving (Eq, Ord, Show, Read)

> Windows == Unix
False

> Windows == Unix
False
> show Windows
"Windows"
> read "Windows"::OS
Windows
> Linux > Windows
True

data Shape = Circle Float | Rect Float Float
deriving Show
> Circle 10
Circle 10.0
data Option a = Nothing | Some a
deriving Show
> Some “one”
Some “one”

Quicksort
qs [] = []
qs (x : xs) = qs smaller ++ [x] ++ qs larger

Quicksort
qs [] = []
where smaller = [a | a <- xs, a <= x]
larger = [b | b <- xs, b > x]

Quicksort
qs [] = []
where smaller = [a | a <- xs, a <= x]
larger = [b | b <- xs, b > x]
> qs [3,7,1,9,3,10]
[1,3,3,7,9,10]

FizzBuzz
1,2,Fizz,4,Buzz,Fizz,7,8,Fizz,Buzz,11,12,13,

FizzBuzz
1,2,Fizz,4,Buzz,Fizz,7,8,Fizz,Buzz,11,12,13,14,FizzBuzz,..

FizzBuzz
fizzes = cycle ["", "", "fizz"]
buzzes = cycle ["", "", "", "", "buzz"]

FizzBuzz
pattern = zipWith (++) fizzes buzzes

FizzBuzz
> take 16 pattern
["","","fizz","","buzz","fizz","","","fizz","buzz","","
fizz","","","fizzbuzz",""]

FizzBuzz
fizzbuzz = zipWith combine pattern [1..]
where combine word number =
if null word then show number else word

FizzBuzz
fizzbuzz = zipWith combine pattern [1..]
where combine word number =
> take 16 pattern if null word then show number else word
["1","2","fizz","4","buzz","fizz","7","8","fizz","buzz","
11","fizz","13","14","fizzbuzz","16"]

Whatnext?
1. Read “Why Functional Programming Matters paper”

Whatnext?
2. Try to implement examples from that paper using Haskell

Whatnext?
3. Sign for FP101x

Whatnext?
3. Sign for FP101x
https://courses.edx.org/courses/course-v1:
DelftX+FP101x+3T2015/courseware/79633017711e44c9878df4f33701
4766/

Whatnext?
3. Sign for FP101x
4. Try out Frege (Haskell on the JVM)

Whatnext?
3. Sign for FP101x
4. Try out Frege (Haskell on the JVM)
https://github.com/Frege/frege

PawelSzulc

PawelSzulc
blog: rabbitonweb.com

PawelSzulc
THANKYOUVERYMUCH
blog: rabbitonweb.com

Fun never stops. introduction to haskell programming language

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