1. Who's More Functional
Groovy, Kotlin, Scala or
Java?
Andrey Breslav
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2. About Me
• Project lead of Kotlin
➡ at JetBrains since 2010
Andrey Breslav
• EG member of JSR-335
➡ Project Lambda
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3. Prologue
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4. Prologue
– Dad (or Mom), is Java a
functional language?
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5. Prologue
– Dad (or Mom), is Java a
functional language?
– Don't you know your dad
from your mom?!
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6. What is FP like?
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7. Alonzo Church 𝛌
1936
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8. Alonzo Church 𝛌
1936 1946
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9. Alonzo Church 𝛌
1936 1946
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10. Alonzo Church 𝛌
1936 1946 1956
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11. John Backus
Alonzo Church 𝛌
1936 1946 1956
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12. Can Programming
be Liberated from
the von Neumann
Style?
John Backus
Alonzo Church 𝛌
1936 1946 1956 1976
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13. 6
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14. – Dad, what's
good about FP?
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15. – Dad, what's
good about FP?
– It makes you
look smart
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16. – Dad, what's
good about FP?
– It makes you
look smart
– Like wearing
glasses?
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17. Effects (Mutability)
𝛌
John von Neumann
Inputs
commands
State VS f(g(x), y)
Outputs
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18. 1, 1, 2, 3, 5, 8, 13, 21, 34, ...
Leonardo Fibonacci
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19. 1, 1, 2, 3, 5, 8, 13, 21, 34, ...
Leonardo Fibonacci
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20. 1, 1, 2, 3, 5, 8, 13, 21, 34, ...
Are you functional?
Kotlin V
Groovy V
Leonardo Fibonacci
Scala V
Java 8 V
Recursion
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21. 1, 1, 2, 3, 5, 8, 13, 21, 34, ...
Leonardo Fibonacci
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22. 1, 1, 2, 3, 5, 8, 13, 21, 34, ...
Leonardo Fibonacci
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23. 1, 1, 2, 3, 5, 8, 13, 21, 34, ...
Leonardo Fibonacci
commands
State
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24. Effect-Free
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25. Effect-Free
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26. Effect-Free
– Dad, how do I
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27. Effect-Free
– Dad, how do I
- print("Hello")?
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28. Effect-Free
– Dad, how do I
- print("Hello")?
- write to a file?
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29. Effect-Free
– Dad, how do I
- print("Hello")?
- write to a file?
- do both?
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30. Effect-Free
Are you functional?
Kotlin V X
– Dad, how do I
- print("Hello")?
Groovy V X
- write to a file?
Scala V X
- do both?
Java 8 V X
Recursion Pure
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31. Summary
• FP makes things simpler
• Sometimes at a huge price
• Our languages are
not purely functional
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32. Higher Order
FP brings order :)
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33. FibonacciTest
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34. FibonacciTest
– How do I test both
implementations?
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35. test(f)
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36. test(f)
Are you functional?
Kotlin V X V
Groovy V X V
Scala V X V
Java 8 V X V
Recursion Pure HO
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37. GoF and FP rd er
r-O n
he io
ig ct
H n
Strategy = Fu
Command Observer Abstract State Visitor
Factory
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38. Internal Iteration
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39. Internal Iteration
users: ArrayList
user 1 Hello
user 2 Hello
user 3 Hello
user 4 Hello
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40. Internal Iteration
users: ArrayList users: ParallelCollection
user 1 Hello
user 2 Hello
user 3 Hello
user 4 Hello
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41. Internal Iteration
users: ArrayList users: ParallelCollection
user 1 Hello
user 2 Hello
user 3 Hello
user 4 Hello
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42. Internal Iteration
users: ArrayList users: ParallelCollection
user 1 Hello
user 2 Hello
user 3 Hello
user 4 Hello
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43. Internal Iteration
users: ArrayList users: ParallelCollection
user 1 Hello
user 2 Hello
user 3 Hello
user 4 Hello
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44. Internal Iteration
users: ArrayList users: ParallelCollection
user 1 Hello
user 2 Hello
user 3 Hello
user 4 Hello
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45. Summary
• Good old callbacks/strategies
• Very important abstraction
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46. ADT
What is your "A" for?
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47. Example: Messages
Server Client
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48. Example: Messages
Server Client
search for "lambda"
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49. Example: Messages
Server Client
search for "lambda"
exact match at ...
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50. Example: Messages
Server Client
search for "lambda"
exact match at ...
similarity at ...
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51. Example: Messages
Server Client
search for "lambda"
exact match at ...
similarity at ...
message ::= search for term
| exact item
| similar item
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52. ADT
Abstract Algebraic
data type data type
~I ~D
nte ata
rfa cla
ce ss
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53. Are you functional?
ADT
Kotlin V X V V
Groovy V X V V
Abstract Algebraic
Scala typeV
data X V data type
V
~I V ~D
Java 8 nt X V V ata
erf cla
ace ss
Recursion Pure HO ADT
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54. ADT in Kotlin
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55. Utilization
You may have your cake, but can you eat it too?
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56. Up to 5 HO-functions
for free! g rad
U p for
e
m ore
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57. Static Utility Methods
backwards!
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58. Static Utility Methods
backwards!
as opposed to
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59. Extension Functions
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60. Extension Functions
Receiver Type
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61. Extension Functions
Receiver Type
receiver
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62. Summary
• Extending existing types
• Without changing the classes
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63. Epilogue
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64. – Dad, should I use FP?
Epilogue
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65. – Dad, should I use FP?
Epilogue
– What is your problem?!
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66. – Dad, should I use FP?
Epilogue
– What is your problem?!
P.S. Kotlin is cool :) http://kotlin.jetbrains.org
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67. Kotlin Resources
• Docs: http://kotlin.jetbrains.org
• Demo: http://kotlin-demo.jetbrains.com
• Code: http://github.com/jetbrains/kotlin
• Twitter:
➡ @project_kotlin
➡ @abreslav
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