Introduction to Haskell programming language that I did for London Scala User Group, Nov 2015

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

2. “Funneverstops.
IntroductiontoHaskell
Programminglanguage”
twitter: @rabbitonweb,
github: github.com/rabbitonweb
FOR SCALA
DEVELOPERS

3. WhylearnHaskell
???

4. TherearetwotypesofScaladevelopers:

5. TherearetwotypesofScaladevelopers:
1)thosewhoarehappytheynolongerprograminJava

6. TherearetwotypesofScaladevelopers:
1)thosewhoarehappytheynolongerprograminJava
2)ThoseWhoarepissedofftheyarenotyetusinghaskell

7. TherearetwotypesofScaladevelopers:
1)thosewhoarehappytheynolongerprograminJava
2)ThoseWhoarepissedofftheyarenotyetusinghaskell
Let’s get a bit of that Haskell taste...

9. WhylearnHaskell?

10. WhylearnHaskell?
HaskellisPureFunctional
ProgrammingLanguage

11. WhatisFunctionalProgramming?

12. ❏ no assignment statements
WhatisFunctionalProgramming?

13. ❏ no assignment statements
❏ no variables
WhatisFunctionalProgramming?

14. ❏ no assignment statements
❏ no variables
❏ once given a value, never change
WhatisFunctionalProgramming?

15. ❏ no assignment statements
❏ no variables
❏ once given a value, never change
❏ no side-effects at all
WhatisFunctionalProgramming?

16. ❏ no assignment statements
❏ no variables
❏ once given a value, never change
❏ no side-effects at all
WhatisFunctionalProgramming?

17. ❏ no assignment statements
❏ no variables
❏ once given a value, never change
❏ no side-effects at all
WhatisFunctionalProgramming?

18. ❏ no assignment statements
❏ no variables
❏ once given a value, never change
❏ no side-effects at all
WhatisFunctionalProgramming?

19. ❏ no assignment statements
❏ no variables
❏ once given a value, never change
❏ no side-effects at all
WhatisFunctionalProgramming?

20. WhatisFunctionalProgramming?
❏ no assignment statements
❏ no variables
❏ once given a value, never change
❏ 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.”

21. WhyFunctionalProgrammingMatters?

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

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

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

25. f(input): output

26. input f(input): output

27. input f(input): output output

28. input f(input): output output g(input): output

29. input f(input): output output g(input): output output

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

31. WhyFunctionalProgrammingMatters?
❏ We strive for modularity

32. WhyFunctionalProgrammingMatters?
❏ We strive for modularity
❏ There is no concept of time

33. 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] = ???

36. 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] = ???

37. 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] = ???

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

39. Whatwillthisfunctiondo?
/**
* This function returns a reversed list
def reverse[T](list: List[T]): List[T] = ???

40. Whatwillthisfunctiondo?
/**
def reverse[T](list: List[T]): List[T] = ???

41. Whatwillthisfunctiondo?
def reverse[T](list: List[T]): List[T] = ???

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

43. Whatwillthisfunctiondo?

44. Whatwillthisfunctiondo?
def rhrggahaham[A](a: A): A = ???

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

46. Whatwillthisfunctiondo?
def rhrggahaham[A](a: A): A = ???

47. Whatwillthisfunctiondo?
def rhrggahaham[A](a: A): A = a

48. Whatwillthisfunctiondo?
def identity[A](a: A): A = a

49. Whatwillthisfunctiondo?
def rhrggahaham[A](a: A): A = a

50. Whatwillthisfunctiondo?
def rhrggahaham[A](a: A): A = a

51. Whatwillthisfunctiondo?
def rhrggahaham[A](a: A): A = a

52. Whatwillthisfunctiondo?
def rhrggahaham(a: Int): Int = ???

53. Whatwillthisfunctiondo?
def rhrggahaham(a: Int): Int = ???
Ifwrittenbyfunctionalprogrammer

54. Whatwillthisfunctiondo?
def rhrggahaham(a: Int): Int = a
a + 10
20

55. WhyFunctionalProgrammingMatters?
❏ We strive for modularity
❏ There is no concept of time

56. WhyFunctionalProgrammingMatters?
❏ We strive for modularity
❏ There is no concept of time
❏ We can reason about our code
with ease

57. def curry

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

59. 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>

60. 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 add5 = curriedAdd(5)
scala> add5(7)
res1: Int = 12

61. def curry

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

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

64. def curry[A,B,C](f: (A, B) => C): (A => (B => C)) =
(a: A) => ?

65. def curry[A,B,C](f: (A, B) => C): (A => (B => C)) =
(a: A) => ( ? => ? )

66. def curry[A,B,C](f: (A, B) => C): (A => (B => C)) =
(a: A) => ( (b: B) => ? )

67. def curry[A,B,C](f: (A, B) => C): (A => (B => C)) =
(a: A) => ( (b: B) => f(a,b) )

68. WhyFunctionalProgrammingMatters?
❏ We strive for modularity
❏ There is no concept of time
❏ We can reason about our code
with ease

69. WhyFunctionalProgrammingMatters?
❏ We strive for modularity
❏ There is no concept of time
❏ We can reason about our code
with ease
❏ Just follow types

70. WhyFunctionalProgrammingMatters?
❏ We strive for modularity
❏ There is no concept of time
❏ We can reason about our code
with ease
❏ Just follow types
❏ Lazy evaluation

72. WhyFunctionalProgrammingMatters?
❏ We strive for modularity
❏ There is no concept of time
❏ We can reason about our code
with ease
❏ Just follow types
❏ Lazy evaluation

73. WhylearnHaskell
???

74. WhylearnHaskell?
HaskellisPureFunctional
ProgrammingLanguage

75. WhylearnHaskell?
HaskellisPureFunctional
ProgrammingLanguage

76. WhylearnHaskell?
HaskellisPureFunctional
ProgrammingLanguage

80. https://xkcd.com/1312/

81. World-> (World,A)

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

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

84. World->(World,A)

85. World->(World,A)
PURE

86. World->(World,A)

87. World->(World,A)

88. World->(World,A)

89. World->(World,A)

90. World->(World,A)

91. World->(World,A)

92. World->(World,A)

93. World->(World,A)

94. World->(World,A)

95. World->(World,A)

96. World->(World,A)

97. World->(World,A)

98. World->(World,A)

99. World->(World,A)

100. World->(World,A)
IMPURE

101. World->(World,A)

102. World->(World,A)
f

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

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

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

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

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

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

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

110. World->(World,A)
currentStateOfTheWorld = stateOfTheWorld
(newWorld, someValue) = f currentStateOfTheWorld
modifyWorldWith newWorld
f

111. World->(World,A)
currentStateOfTheWorld = stateOfTheWorld
(newWorld, someValue) = f currentStateOfTheWorld
modifyWorldWith newWorld
display someValue
f

112. JustaLittlebitof
history

113. HaskellHistoryinanutshell

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

115. 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

116. 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

117. 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

118. 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.”

119. 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

120. 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
❏ “The Haskell 98 Report” in 1999

121. 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
❏ “The Haskell 98 Report” in 1999
❏ “The Haskell 2010 Report” in 2009/2010

122. 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
❏ “The Haskell 98 Report” in 1999
❏ “The Haskell 2010 Report” in 2009/2010
❏ Further Haskell evolves rapidly around GHC

125. Showussomecode
already!

126. GHC-GlassgowHaskellCompiler

127. GHC-GlassgowGloriousHaskellCompiler

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

129. FunWithFunctions

130. FunWithFunctions
helloWorld = “Hello World”

131. FunWithFunctions
❏ function without parameters is called definition
helloWorld = “Hello World”

132. FunWithFunctions
❏ function without parameters is called definition
❏ function name must start with small letter
helloWorld = “Hello World”

133. FunWithFunctions
❏ function without parameters is called definition
❏ function name must start with small letter
> helloWorld
"Hello World"
it :: [Char]
helloWorld = “Hello World”

134. FunWithFunctions

135. FunWithFunctions
london u = “London “ ++ u

136. FunWithFunctions
> :t london
london :: [Char] -> [Char]
london u = “London “ ++ u

137. FunWithFunctions
> :t london
london :: [Char] -> [Char]
> london "baby"
"london baby"
it :: [Char]
london u = “London “ ++ u

138. FunWithFunctions
> :t london
london :: [Char] -> [Char]
> london "baby"
"london baby"
it :: [Char]
london :: [Char] -> [Char]
london u = “London “ ++ u

139. FunWithFunctions
isA c = c == 'A'

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

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

142. FunWithFunctions

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

144. FunWithFunctions
> prefixMe '_' "word"
"_ word"
it :: [Char]
prefixMe c str = [c] ++ " " ++ str

145. FunWithFunctions
> :t prefixMe
prefixMe :: Char -> [Char] -> [Char]
> prefixMe '_' "word"
"_ word"
it :: [Char]
prefixMe c str = [c] ++ " " ++ str

146. FunWithFunctions
> :t prefixMe
prefixMe :: Char -> ([Char] -> [Char])
> prefixMe '_' "word"
"_ word"
it :: [Char]
prefixMe c str = [c] ++ " " ++ str

147. FunWithFunctions

148. FunWithFunctions
answerToEverything = 42

149. FunWithFunctions
answerToEverything = 42
complexCalc x y z = x * y * z * answerToEverything

150. FunWithFunctions
> :t complexCalc
complexCalc :: Integer -> Integer -> Integer -> Integer
answerToEverything = 42
complexCalc x y z = x * y * z * answerToEverything

151. FunWithFunctions
> :t complexCalc
complexCalc :: Integer -> Integer -> Integer -> Integer
complexCalc :: Integer -> (Integer -> (Integer -> Integer))
answerToEverything = 42
complexCalc x y z = x * y * z * answerToEverything

152. FunWithFunctions
> :t complexCalc
complexCalc :: Integer -> Integer -> Integer -> Integer
complexCalc :: Integer -> (Integer -> (Integer -> Integer))
> complexCalc 10 20 30
answerToEverything = 42
complexCalc x y z = x * y * z * answerToEverything

153. FunWithFunctions
> :t complexCalc
complexCalc :: Integer -> Integer -> Integer -> Integer
complexCalc :: Integer -> (Integer -> (Integer -> Integer))
> complexCalc 10 20 30
> ((complexCalc 10) 20) 30
answerToEverything = 42
complexCalc x y z = x * y * z * answerToEverything

154. FunWithTypes

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

156. FunWithTypes
❏ We’ve already seen: Char, Integers & Bool
❏ Other build-in types:

157. FunWithTypes
❏ We’ve already seen: Char, Integers & Bool
❏ Other build-in types:
❏ Int, Float, Doubles

158. FunWithTypes
❏ We’ve already seen: Char, Integers & Bool
❏ Other build-in types:
❏ Int, Float, Doubles
❏ Tuples

159. FunWithTypes
❏ We’ve already seen: Char, Integers & Bool
❏ Other build-in types:
❏ Int, Float, Doubles
❏ Tuples
> (False, "Is it?")
it :: (Bool, [Char])
> (10.4, True, "Values", 'Y')
it :: (Double, Bool, [Char], Char)

160. FunWithTypes
❏ We’ve already seen: Char, Integers & Bool
❏ Other build-in types:
❏ Int, Float, Doubles
❏ Tuples

161. FunWithTypes
❏ We’ve already seen: Char, Integers & Bool
❏ Other build-in types:
❏ Int, Float, Doubles
❏ Tuples
❏ Lists
> [False, False, True]
it :: [Bool]
> [10, 20, 30]
it :: [Integer]
> ["one", "two", "three"]
it :: [[Char]]

162. Polymorphic types

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

164. Polymorphic types
> length [False, False, True]
3
> length [10, 20, 30, 40]
4

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

166. Polymorphic types
> length [False, False, True]
3
> length [10, 20, 30, 40]
4
> length ["one", "two", "three"]
3
length :: [a] -> Int

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

168. List101

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

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

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

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

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

174. List101
numbers = [1, 3, 5, 7]
numbers !! 2 -> 5
null numbers -> False
head numbers -> 1
tail numbers -> [3, 5, 7]
init numbers -> [1, 3, 5]

175. List101
numbers = [1, 3, 5, 7]
numbers !! 2 -> 5
null numbers -> False
head numbers -> 1
tail numbers -> [3, 5, 7]
init numbers -> [1, 3, 5]
last numbers -> 7

176. List101
numbers = [1, 3, 5, 7]
numbers !! 2 -> 5
null numbers -> False
head numbers -> 1
tail numbers -> [3, 5, 7]
init numbers -> [1, 3, 5]
last numbers -> 7
drop 2 numbers -> [5, 7]

177. List101
numbers = [1, 3, 5, 7]
numbers !! 2 -> 5
null numbers -> False
head numbers -> 1
tail numbers -> [3, 5, 7]
init numbers -> [1, 3, 5]
last numbers -> 7
drop 2 numbers -> [5, 7]
take 2 numbers -> [1, 3]

178. List101
numbers = [1, 3, 5, 7]
numbers !! 2 -> 5
null numbers -> False
head numbers -> 1
tail numbers -> [3, 5, 7]
init numbers -> [1, 3, 5]
last numbers -> 7
drop 2 numbers -> [5, 7]
take 2 numbers -> [1, 3]
elem 5 numbers -> True

179. List101

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

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

182. 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, ...

183. 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]

184. 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,..

185. 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]

186. 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]

187. List101

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

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

190. 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), ...

191. 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)]

192. 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]

193. 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)]

194. 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)]

195. 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]

196. Classes

197. Classes
caseclassFoo(...)???

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

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

200. TypeClasses
❏ Eq

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

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

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

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

205. TypeClasses
❏ Eq
(==) :: a -> a -> Bool; (/=) :: a -> a -> Bool
❏ Ord
< , <= , > , >= :: a -> a -> Bool; min , max :: a -> a -> a
❏ Show
show :: a -> String

206. TypeClasses
❏ Eq
(==) :: a -> a -> Bool; (/=) :: a -> a -> Bool
❏ Ord
< , <= , > , >= :: a -> a -> Bool; min , max :: a -> a -> a
❏ Show
show :: a -> String
❏ Read

207. TypeClasses
❏ Eq
(==) :: a -> a -> Bool; (/=) :: a -> a -> Bool
❏ Ord
< , <= , > , >= :: a -> a -> Bool; min , max :: a -> a -> a
❏ Show
show :: a -> String
❏ Read
read :: String -> a

208. TypeClasses
> 10 == 20
False

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

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

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

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

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

214. Typeclasses&classconstraints
add a b = a + b

215. Typeclasses&classconstraints
add :: Num a => a -> a -> a
add a b = a + b

216. Typeclasses&classconstraints
add :: Num a => a -> a -> a
add a b = a + b
add 10 20

217. Typeclasses&classconstraints
add :: Num a => a -> a -> a
add a b = a + b
add 10 20
add 5.0 7.0

218. Typeclasses&classconstraints
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 ++ "!!")

219. Typeclasses&classconstraints
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])
weirdo n d str = (n + 2, d / 2.0, str ++ "!!")

220. Typeclasses&classconstraints
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])
weirdo n d str = (n + 2, d / 2.0, str ++ "!!")
weirdo 7 7.0 “kuku”

221. Typeclasses&classconstraints
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])
weirdo n d str = (n + 2, d / 2.0, str ++ "!!")
weirdo 7 7.0 “kuku”
weirdo 7.0 5.0 “mono”

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

223. FunWithFunctions,Part2
sum :: Num a => [a] -> a
sum list = if null list then 0
else 1 + sum (tail list)

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

225. FunWithFunctions,Part2
sum :: Num a => [a] -> a
sum [] = 0

226. FunWithFunctions,Part2
sum :: Num a => [a] -> a
sum [] = 0
sum x:xs = 1 + sum xs

227. FunWithFunctions,Part2
programmingLanguage :: [Char] -> [Char]

228. FunWithFunctions,Part2
programmingLanguage :: [Char] -> [Char]
programmingLanguage "Scala" = "Good"

229. FunWithFunctions,Part2
programmingLanguage :: [Char] -> [Char]
programmingLanguage "Scala" = "Good"
programmingLanguage "Haskell" = "Even better!"

230. FunWithFunctions,Part2
programmingLanguage :: [Char] -> [Char]
programmingLanguage "Scala" = "Good"
programmingLanguage "Haskell" = "Even better!"
programmingLanguage other = "Meh..." ++ other ++ ", pf.."

231. FunWithFunctions,Part2
first :: (a, b, c) -> a
first (x, _, _) = x
second :: (a, b, c) -> b
second (_, y, _) = y
third :: (a, b, c) -> c
third (_, _, z) = z

232. FunWithFunctions,Part2
rankMe :: (Num a, Ord a) => a -> [Char]

233. FunWithFunctions,Part2
rankMe :: (Num a, Ord a) => a -> [Char]
rankMe lines

234. FunWithFunctions,Part2
rankMe :: (Num a, Ord a) => a -> [Char]
rankMe lines
| lines > 100 = "Senior"

235. FunWithFunctions,Part2
rankMe :: (Num a, Ord a) => a -> [Char]
rankMe lines
| lines > 100 = "Senior"
| lines > 30 = "Regular"

236. FunWithFunctions,Part2
rankMe :: (Num a, Ord a) => a -> [Char]
rankMe lines
| lines > 100 = "Senior"
| lines > 30 = "Regular"
| lines >= 0 = "Junior"

237. FunWithFunctions,Part2
rankMe :: (Num a, Ord a) => a -> [Char]
rankMe lines
| lines > 100 = "Senior"
| lines > 30 = "Regular"
| lines >= 0 = "Junior"
| lines < 0 = "Congrats! You are a true Hacker!"

238. FunWithFunctions,Part2
rankMe :: (Num a, Ord a) => a -> a -> a -> [Char]
rankMe added deleted modified
| lines > 100 = "Senior"
| lines > 30 = "Regular"
| lines >= 0 = "Junior"
| lines < 0 = "Congrats! You are a true Hacker!"

239. FunWithFunctions,Part2
rankMe :: (Num a, Ord a) => a -> a -> a -> [Char]
rankMe added deleted modified
| lines > 100 = "Senior"
| lines > 30 = "Regular"
| lines >= 0 = "Junior"
| lines < 0 = "Congrats! You are a true Hacker!"
where lines = added - deleted

240. Creatingnewtypes

241. Creatingnewtypes-aliasing

242. Creatingnewtypes-aliasing
type String = [Char]

243. Creatingnewtypes-aliasing
type String = [Char]
type Point = (Int, Int)

244. Creatingnewtypes-aliasing
type String = [Char]
type Point = (Int, Int)
type Parser a = String -> (a, String)

245. Creatingnewtypes

246. Creatingnewtypes-Datadeclarations

247. Creatingnewtypes-Datadeclarations
data OS = Windows | OSX | Linux | Unix

248. Creatingnewtypes-Datadeclarations
data OS = Windows | OSX | Linux | Unix
bootup :: OS -> [Char]

249. Creatingnewtypes-Datadeclarations
data OS = Windows | OSX | Linux | Unix
bootup :: OS -> [Char]
bootup Unix = "Up and running"

250. Creatingnewtypes-Datadeclarations
data OS = Windows | OSX | Linux | Unix
bootup :: OS -> [Char]
bootup Unix = "Up and running"
bootup Linux = "Well, I was actually never down..."

251. 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"

252. 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 Windows = "Upgrading to Windows 10, this should take a
second.."

253. Creatingnewtypes-Datadeclarations
data OS = Windows | OSX | Linux | Unix

254. Creatingnewtypes-Datadeclarations
data OS = Windows | OSX | Linux | Unix
class Bootable a where
boot :: a -> [Char]

255. Creatingnewtypes-Datadeclarations
data OS = Windows | OSX | Linux | Unix
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.."

256. Creatingnewtypes-Datadeclarations
data OS = Windows | OSX | Linux | Unix

257. Creatingnewtypes-Datadeclarations
data OS = Windows | OSX | Linux | Unix
instance Eq OS where
Unix == Unix = True
Linux == Linux = True
OSX == OSX = True
Windows == Windows = True
_ == _ = False

258. Creatingnewtypes-Datadeclarations
data OS = Windows | OSX | Linux | Unix
instance Eq OS where
Unix == Unix = True
Linux == Linux = True
OSX == OSX = True
Windows == Windows = True
_ == _ = False
> Unix == Windows
False

259. Creatingnewtypes-Datadeclarations
data OS = Windows | OSX | Linux | Unix

260. Creatingnewtypes-Datadeclarations
data OS = Windows | OSX | Linux | Unix
instance Show OS where
show Unix = "Unix"
show Linux = "Linux"
show OSX = "OSX"
show Windows = "Windows"

261. Creatingnewtypes-Datadeclarations
data OS = Windows | OSX | Linux | Unix
instance Show OS where
show Unix = "Unix"
show Linux = "Linux"
show OSX = "OSX"
show Windows = "Windows"
> show Unix
"Unix"

262. Creatingnewtypes-Datadeclarations
data OS = Windows | OSX | Linux | Unix

263. Creatingnewtypes-Datadeclarations
data OS = Windows | OSX | Linux | Unix
deriving (Eq, Ord, Show, Read)

264. Creatingnewtypes-Datadeclarations
data OS = Windows | OSX | Linux | Unix
deriving (Eq, Ord, Show, Read)
> Windows == Unix
False

265. Creatingnewtypes-Datadeclarations
data OS = Windows | OSX | Linux | Unix
deriving (Eq, Ord, Show, Read)
> Windows == Unix
False
> show Windows
"Windows"
> read "Windows"::OS
Windows
> Linux > Windows
True

266. Creatingnewtypes-Datadeclarations
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”

267. Creatingnewtypes-Datadeclarations
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”

268. Expressiveness

269. Quicksort

270. Quicksort
qs [] = []

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

272. Quicksort
qs [] = []
qs (x : xs) = qs smaller ++ [x] ++ qs larger
where smaller = [a | a <- xs, a <= x]
larger = [b | b <- xs, b > x]

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

274. FizzBuzz

275. FizzBuzz
1,

276. FizzBuzz
1,2

277. FizzBuzz
1,2,Fizz,

278. FizzBuzz
1,2,Fizz,4,Buzz,

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

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

281. FizzBuzz

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

283. FizzBuzz
fizzes = cycle ["", "", "fizz"]
buzzes = cycle ["", "", "", "", "buzz"]
pattern = zipWith (++) fizzes buzzes

284. FizzBuzz
fizzes = cycle ["", "", "fizz"]
buzzes = cycle ["", "", "", "", "buzz"]
pattern = zipWith (++) fizzes buzzes
> take 16 pattern
["","","fizz","","buzz","fizz","","","fizz","buzz","","
fizz","","","fizzbuzz",""]

285. FizzBuzz
fizzes = cycle ["", "", "fizz"]
buzzes = cycle ["", "", "", "", "buzz"]
pattern = zipWith (++) fizzes buzzes

286. FizzBuzz
fizzes = cycle ["", "", "fizz"]
buzzes = cycle ["", "", "", "", "buzz"]
pattern = zipWith (++) fizzes buzzes
fizzbuzz = zipWith combine pattern [1..]
where combine word number =
if null word then show number else word

287. FizzBuzz
fizzes = cycle ["", "", "fizz"]
buzzes = cycle ["", "", "", "", "buzz"]
pattern = zipWith (++) fizzes buzzes
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"]

288. Whatnext?

289. Whatnext?

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

291. Whatnext?
1. Read “Why Functional Programming Matters paper”
2. Try to implement examples from that paper using Haskell

292. Whatnext?
1. Read “Why Functional Programming Matters paper”
2. Try to implement examples from that paper using Haskell
3. Sign for FP101x

293. Whatnext?
1. Read “Why Functional Programming Matters paper”
2. Try to implement examples from that paper using Haskell
3. Sign for FP101x
https://courses.edx.org/courses/course-v1:
DelftX+FP101x+3T2015/courseware/79633017711e44c9878df4f33701
4766/

294. Whatnext?
1. Read “Why Functional Programming Matters paper”
2. Try to implement examples from that paper using Haskell
3. Sign for FP101x
4. Try out Frege (Haskell on the JVM)

295. Whatnext?
1. Read “Why Functional Programming Matters paper”
2. Try to implement examples from that paper using Haskell
3. Sign for FP101x
4. Try out Frege (Haskell on the JVM)
https://github.com/Frege/frege

296. PawelSzulc

297. PawelSzulc
twitter: @rabbitonweb,

298. PawelSzulc
twitter: @rabbitonweb,
github: github.com/rabbitonweb

299. PawelSzulc
twitter: @rabbitonweb,
github: github.com/rabbitonweb
blog: rabbitonweb.com

300. PawelSzulc
THANKYOUVERYMUCH
twitter: @rabbitonweb,
github: github.com/rabbitonweb
blog: rabbitonweb.com