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An Introduction to Functional Programming - DeveloperUG - 20140311

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Andreas Pauley

The document provides an introduction to functional programming concepts. It defines functional programming as consisting entirely of functions. It discusses key functional programming concepts like immutable variables, recursion, higher-order functions, and common functional patterns like map, filter and fold. It provides examples of functions in Haskell and Python and illustrates how these concepts can be applied to problems like processing bank account data.

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Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now An Introduction to Functional Programming Andreas Pauley – @apauley Lambda Luminaries DeveloperUG March 11, 2014
Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now @lambdaluminary We meet once a month, on the second Monday of the month. http://www.meetup.com/lambda-luminaries/
Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now Jemstep Retirement portfolio analysis in Scala. http://www.jemstep.com/
Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now
Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now
Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now “ No matter what language you work in, programming in a functional style provides benefits. You should do it whenever it is convenient, and you should think hard about the decision when it isn’t convenient. ” — John Carmack, ID Software [2]
Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now Quake
Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now But what exactly is “Functional Programming”?
Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now Functional Programming, noun: Functional Programming is a list of things you CAN’T do. [7]
Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now You can’t vary your variables 1> X = 42. 42 2> X = X + 1. ** exception error: no match of right hand side value 43
Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now No while/for loops. Sorry :-( int i; for (i=1; i<=3; i++) { System.out.println(i); }
Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now You can’t mutate/change your data structures Python >>> list1 = [1,2,3] >>> list2 = list1 >>> print list1.reverse() None >>> list1 [3, 2, 1] >>> list2 [3, 2, 1] Haskell > let list1 = [1,2,3] > let list2 = list1 > reverse(list1) [3,2,1] > list1 [1,2,3] > list2 [1,2,3]
Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now You can’t have any side effects
Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now Are you kidding me? How can anyone program like this???
Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now GOTO 10 This sounds like “You can’t have GOTO statements” See Hughes and Dijkstra [1, 3]
Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now It’s not about what we cannot do.
Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now We need a better definition of Functional Programming.
Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now Programming Paradigms (Very Simplified) Imperative Declarative
Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now Functional Programming, noun: “ Functional programming is so called because a program consists entirely of functions. ” — John Hughes, Why Functional Programming Matters [1, p. 1]
Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now OK... so what exactly is a function?
Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now An example function f(x) = 2x2 − 2x + 3 −1 1 2 3 4 6 8 10 12 14 x y
Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now Variables in functions f(x) = 2x2 − 2x + 3 When we evaluate the function: f(2) = 8 − 4 + 3 = 7 • The value of x will not change inside the function body.
Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now Variables in functions f(x) = 2x2 − 2x + 3 When we evaluate the function: f(2) = 8 − 4 + 3 = 7 • The value of x will not change inside the function body. • Same input, same output. Every time. (Referential Transparency)
Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now Variables in functions f(x) = 2x2 − 2x + 3 When we evaluate the function: f(2) = 8 − 4 + 3 = 7 • The value of x will not change inside the function body. • Same input, same output. Every time. (Referential Transparency) • We can call f multiple times without any side effects (Idempotence).
Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now Variables in functions f(x) = 2x2 − 2x + 3 When we evaluate the function: f(2) = 8 − 4 + 3 = 7 • The value of x will not change inside the function body. • Same input, same output. Every time. (Referential Transparency) • We can call f multiple times without any side effects (Idempotence). • We don’t have to recalculate f(2), we can replace any occurrence of f(2) with 7 (Memoization).
Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now Varying variables does not make sense x = x + 1 x − x = 1 0 = 1 ∴ x = x + 1
Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now Functions can call other functions g(x) = f(x) + 1
Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now Values are functions Constant values are just functions with no input parameters x = 42 Python function definition: def x(): return 42 Haskell function definition: x = 42
Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now Functions can be composed h(x) = (f ◦ g)(x) = f(g(x)) or h = f ◦ g
Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now Higher-order Functions Functions can take functions as input. Functions can return functions as the result. h(f, g, x) = f(x) + g(2)
Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now Higher-order Functions The derivative of f(x) returns another function. f(x) = 2x2 − 2x + 3 d dxf(x) = 4x − 2 −1 1 2 3 5 10 15 x y
Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now A functional program consists entirely of functions def main(): time = datetime.now() args = sys.argv[1:] print outputString(time, args) def outputString(time, args): return str(time) + " " + joinArgs(args) def joinArgs(args): return "-".join(args) $ ./justfunctions.py Hello from Python 2014-02-15 10:36:42.062697 Hello-from-Python
Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now A functional program consists entirely of functions main :: IO() main = do time <- getCurrentTime args <- getArgs putStrLn (outputString time args) outputString :: UTCTime -> [String] -> String outputString time args = show(time) ++ " " ++ joinArgs(args) joinArgs :: [String] -> String joinArgs = intercalate "-" $ ./justfunctions Hello from Haskell 2014-02-15 08:36:50.822728 UTC Hello-from-Haskell
Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now Some Haskell Syntax Python: def outputString(time, args): return str(time) + " " + joinArgs(args) Haskell: outputString :: UTCTime -> [String] -> String outputString time args = show(time) ++ " " ++ joinArgs(args)
Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now Recursive function: Haskell doubleAll :: [Int] -> [Int] doubleAll [] = [] doubleAll (x:xs) = x*2 : doubleAll xs Example use in the interactive interpreter: Prelude Main> doubleAll [8,2,3] [16,4,6]
Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now Recursive function expanded doubleAll [] = [] doubleAll (x:xs) = x*2 : doubleAll xs doubleAll [8,2,3] 16 : (doubleAll [2,3]) 16 : 4 : (doubleAll [3]) 16 : 4 : 6 : (doubleAll []) 16 : 4 : 6 : [] 16 : 4 : [6] 16 : [4,6] [16,4,6]
Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now Recursive function: Python def doubleAll(numbers): if numbers == []: return [] else: first = numbers[0] rest = numbers[1:] return [first * 2] + doubleAll(rest) Example use in the interactive interpreter: >>> doubleAll([8,2,3]) [16, 4, 6]
Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now Pattern Matching: Haskell doubleAll :: [Int] -> [Int] doubleAll [] = [] doubleAll (x:xs) = x*2 : doubleAll xs
Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now Higher-order Functions 3 Basic List Operations Map Convert each element of a list into some other value.
Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now Higher-order Functions 3 Basic List Operations Map Convert each element of a list into some other value. Filter Get a subset of a list based on some condition.
Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now Higher-order Functions 3 Basic List Operations Map Convert each element of a list into some other value. Filter Get a subset of a list based on some condition. Fold Reduce a list of items to a single value.
Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now Map Apply a function to each element of a list, and you get a new list. a1 a2 a3 ... an b1 b2 b3 ... bn f(a1) f(an)
Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now Map The built-in Haskell map function: map :: (a -> b) -> [a] -> [b] map _ [] = [] map f (x:xs) = f x : map f xs
Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now Map The builtin Haskell map function: map :: (a -> b) -> [a] -> [b] map _ [] = [] map f (x:xs) = f x : map f xs Hmmm, looks very similar to our previous doubleAll function: doubleAll :: [Int] -> [Int] doubleAll [] = [] doubleAll (x:xs) = x*2 : doubleAll xs
Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now Map So our doubleAll can actually be simplified. In Haskell: doubleAll = map (*2) In Python: def doubleAll(numbers): return map(lambda x: x * 2, numbers)
Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now Map doubleAll :: Num a => [a] -> [a] doubleAll = map (*2) 8 2 3 16 4 6 ∗2 ∗2 ∗2
Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now Account Data data Bank = ABSA | Capitec | FNB | Nedbank | SBSA data Account = Account {bank :: Bank, accNum :: String, owner :: String, balance :: Amount}
Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now Account Data [Account {accNum="4076814233", owner="J. Doe", balance=(Amount 123000.23), bank=ABSA}, Account {accNum="6868773585", owner="J. Black", balance=(Amount 5782347.99), bank=FNB}, Account {accNum="4055892156", owner="A. Kay", balance=(Amount 100), bank=ABSA}, Account {accNum="6584539813", owner="S. Jones", balance=(Amount 2937361.45), bank=FNB}]
Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now Map Map on account data: balances :: [Account] -> [Amount] balances accounts = map balance accounts acc1 acc2 acc3 acc4 acc5 bal1 bal2 bal3 bal4 bal5 balance(acc1) balance(acc5)
Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now Filter acc1 acc2 acc3 acc4 acc5 acc1 acc3 acc5 in? in? in? in? in?
Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now Filter Filter on account data: topAccounts :: [Account] -> [Account] topAccounts accounts = filter isRich accounts isRich :: Account -> Bool isRich acc = balance acc >= (Amount 1000000) Output: *Main> topAccounts accounts [FNB 6868773585 (J. Black) R5782347.99, FNB 6584539813 (S. Jones) R2937361.45]
Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now Fold/Reduce/Inject foldl (+) 0 [8,2,3] 13 0 8 2 3 8 2 3 10 3 13
Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now Fold/Reduce/Inject balanceSum :: [Account] -> Amount balanceSum accounts = foldl (+) 0 (balances accounts) 0 bal1 bal2 bal3 sum bal2 bal3 sum bal3 sum
Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now Function Composition balanceSum :: [Account] -> Amount balanceSum accounts = foldl (+) 0 (balances accounts) 0 bal1 bal2 bal3 sum bal2 bal3 sum bal3 sum s2 :: [Account] -> Amount s2 = balances |> (foldl(+)0) h(x) = (f ◦ g)(x) = f(g(x)) h = f ◦ g s3 = f1 ◦ balances s3 :: [Account] -> Amount s3 = (foldl(+)0) . balances
Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now balancesPerBank foldl insertBalance Map.empty accounts {} acc1 acc2 acc3 map acc2 acc3 map acc3 map
Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now Fold type BankMap = Map Bank Amount balancesPerBank :: [Account] -> BankMap balancesPerBank = foldl insertBalance Map.empty Output: *Main> balancesPerBank accounts fromList [(ABSA,R123100.23),(FNB,R8719709.44)]
Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now Fold type BankMap = Map Bank Amount balancesPerBank :: [Account] -> BankMap balancesPerBank = foldl insertBalance Map.empty insertBalance :: BankMap -> Account -> BankMap insertBalance bankmap account = Map.insert key value bankmap where key = bank account value = addBalance bankmap account addBalance :: BankMap -> Account -> Amount addBalance bankmap account = case (Map.lookup (bank account) bankmap) of Nothing -> balance account Just bal -> (balance account) + bal
Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now Programming Paradigms (Very Simplified) Imperative Declarative
Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now In imperative programming: • Your variables can vary any time!
Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now In imperative programming: • Your variables can vary any time! • You have to use locks to be thread-safe!
Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now In imperative programming: • Your variables can vary any time! • You have to use locks to be thread-safe! • You have to write your own loops for the most basic list operations!
Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now In imperative programming: • Your variables can vary any time! • You have to use locks to be thread-safe! • You have to write your own loops for the most basic list operations! • Your data structures are mutable!
Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now In imperative programming: • Your variables can vary any time! • You have to use locks to be thread-safe! • You have to write your own loops for the most basic list operations! • Your data structures are mutable! • You have to defensively make copies of data to prevent bugs!
Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now In imperative programming: • Your variables can vary any time! • You have to use locks to be thread-safe! • You have to write your own loops for the most basic list operations! • Your data structures are mutable! • You have to defensively make copies of data to prevent bugs! • You have to defensively check for null values!
Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now In imperative programming: • Your variables can vary any time! • You have to use locks to be thread-safe! • You have to write your own loops for the most basic list operations! • Your data structures are mutable! • You have to defensively make copies of data to prevent bugs! • You have to defensively check for null values! • You have to think about implicit state! (this, self)
Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now In imperative programming: • Your variables can vary any time! • You have to use locks to be thread-safe! • You have to write your own loops for the most basic list operations! • Your data structures are mutable! • You have to defensively make copies of data to prevent bugs! • You have to defensively check for null values! • You have to think about implicit state! (this, self) • Code is generally riddled with side effects!
Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now Are you kidding me? How can anyone program like this???
Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now
Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now Join the anti-for campaign Less loops, more map/filter/fold http://weblogs.asp.net/podwysocki/archive/2009/06/26/ the-anti-for-campaign.aspx
Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now Treat side effects as a first-class concern
Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now Learn a functional language “ A language that doesn’t affect the way you think about programming, is not worth knowing. ” — Alan Perlis[5]
Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now Disclaim your inheritance Write non-trivial code without using objects and inheritance. Get re-usability with higher-order functions. Try to minimise moving parts instead of encapsulating moving parts. [4]
Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now Join our group @lambdaluminary We meet once a month, on the second Monday of the month. http://www.meetup.com/lambda-luminaries/
Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now Get out of your comfort zone Functional Programming is unfamiliar territory for most. “ If you want everything to be familiar you will never learn anything new. ” — Rich Hickey, author of Clojure[6]
Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now Companies In South Africa Jemstep, Sandton Using Scala for Fund Analysis Allan Gray, Cape Town Using Scala for backend logic and system integration. Yuppiechef, Cape Town Using Clojure for their Warehouse Management System. Cognician, Cape Town Using Clojure to create coaching/learning modules. Eldo Energy, Johannesburg Using Clojure for automated meter reading and intelligent monitoring of consumer energy. Rheo Systems, Pretoria Using Clojure for supply chain integration.
Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now Companies In South Africa Pattern Matched Technologies, Midrand Using Erlang for all systems, eg. processing high volumes of financial transactions. Effective Control Systems, Kyalami Using Erlang for printer management. Mira Networks, Somerset West Using Erlang for billing administration and mobile development. Kotive Using Scala for designing workflow processes.
Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now Online Courses Functional Thinking by Neal Ford O’ Reilly http://shop.oreilly.com/product/0636920030393.do Functional Programming Principles in Scala EPFL University https://www.coursera.org/course/progfun School of Haskell FP Complete https://www.fpcomplete.com/school Programming Languages University of Washington https://www.coursera.org/course/proglang
Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now Books Miran Lipovaˇca Learn You a Haskell for Great Good! http://learnyouahaskell.com/ Fred H´ebert Learn You Some Erlang for Great Good! http://learnyousomeerlang.com/ Yaron Minski, Anil Madhavapeddy, Jason Hickey Real World OCaml https://realworldocaml.org/ Paul Chiusano, R´unar Bjarnason Functional Programming in Scala http://www.manning.com/bjarnason/
Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now References I John Hughes Why Functional Programming Matters http://www.cs.kent.ac.uk/people/staff/dat/miranda/ whyfp90.pdf John Carmack Functional Programming in C++ http://www.altdevblogaday.com/2012/04/26/ functional-programming-in-c/ Edsger W. Dijkstra Go To Statement Considered Harmful http://www.u.arizona.edu/~rubinson/copyright_ violations/Go_To_Considered_Harmful.html
Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now References II Tweet by Michael Feathers https://twitter.com/mfeathers/status/29581296216 Alan Jay Perlis http://www.cs.yale.edu/quotes.html Rich Hickey http: //www.infoq.com/presentations/Simple-Made-Easy Andreas Pauley An Introduction to Functional Programming https://github.com/apauley/fp_presentation

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An Introduction to Functional Programming - DeveloperUG - 20140311

  • 1. Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now An Introduction to Functional Programming Andreas Pauley – @apauley Lambda Luminaries DeveloperUG March 11, 2014
  • 2. Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now @lambdaluminary We meet once a month, on the second Monday of the month. http://www.meetup.com/lambda-luminaries/
  • 3. Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now Jemstep Retirement portfolio analysis in Scala. http://www.jemstep.com/
  • 4. Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now
  • 5. Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now
  • 6. Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now “ No matter what language you work in, programming in a functional style provides benefits. You should do it whenever it is convenient, and you should think hard about the decision when it isn’t convenient. ” — John Carmack, ID Software [2]
  • 7. Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now Quake
  • 8. Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now But what exactly is “Functional Programming”?
  • 9. Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now Functional Programming, noun: Functional Programming is a list of things you CAN’T do. [7]
  • 10. Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now You can’t vary your variables 1> X = 42. 42 2> X = X + 1. ** exception error: no match of right hand side value 43
  • 11. Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now No while/for loops. Sorry :-( int i; for (i=1; i<=3; i++) { System.out.println(i); }
  • 12. Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now You can’t mutate/change your data structures Python >>> list1 = [1,2,3] >>> list2 = list1 >>> print list1.reverse() None >>> list1 [3, 2, 1] >>> list2 [3, 2, 1] Haskell > let list1 = [1,2,3] > let list2 = list1 > reverse(list1) [3,2,1] > list1 [1,2,3] > list2 [1,2,3]
  • 13. Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now You can’t have any side effects
  • 14. Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now Are you kidding me? How can anyone program like this???
  • 15. Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now GOTO 10 This sounds like “You can’t have GOTO statements” See Hughes and Dijkstra [1, 3]
  • 16. Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now It’s not about what we cannot do.
  • 17. Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now We need a better definition of Functional Programming.
  • 18. Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now Programming Paradigms (Very Simplified) Imperative Declarative
  • 19. Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now Functional Programming, noun: “ Functional programming is so called because a program consists entirely of functions. ” — John Hughes, Why Functional Programming Matters [1, p. 1]
  • 20. Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now OK... so what exactly is a function?
  • 21. Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now An example function f(x) = 2x2 − 2x + 3 −1 1 2 3 4 6 8 10 12 14 x y
  • 22. Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now Variables in functions f(x) = 2x2 − 2x + 3 When we evaluate the function: f(2) = 8 − 4 + 3 = 7 • The value of x will not change inside the function body.
  • 23. Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now Variables in functions f(x) = 2x2 − 2x + 3 When we evaluate the function: f(2) = 8 − 4 + 3 = 7 • The value of x will not change inside the function body. • Same input, same output. Every time. (Referential Transparency)
  • 24. Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now Variables in functions f(x) = 2x2 − 2x + 3 When we evaluate the function: f(2) = 8 − 4 + 3 = 7 • The value of x will not change inside the function body. • Same input, same output. Every time. (Referential Transparency) • We can call f multiple times without any side effects (Idempotence).
  • 25. Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now Variables in functions f(x) = 2x2 − 2x + 3 When we evaluate the function: f(2) = 8 − 4 + 3 = 7 • The value of x will not change inside the function body. • Same input, same output. Every time. (Referential Transparency) • We can call f multiple times without any side effects (Idempotence). • We don’t have to recalculate f(2), we can replace any occurrence of f(2) with 7 (Memoization).
  • 26. Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now Varying variables does not make sense x = x + 1 x − x = 1 0 = 1 ∴ x = x + 1
  • 27. Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now Functions can call other functions g(x) = f(x) + 1
  • 28. Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now Values are functions Constant values are just functions with no input parameters x = 42 Python function definition: def x(): return 42 Haskell function definition: x = 42
  • 29. Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now Functions can be composed h(x) = (f ◦ g)(x) = f(g(x)) or h = f ◦ g
  • 30. Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now Higher-order Functions Functions can take functions as input. Functions can return functions as the result. h(f, g, x) = f(x) + g(2)
  • 31. Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now Higher-order Functions The derivative of f(x) returns another function. f(x) = 2x2 − 2x + 3 d dxf(x) = 4x − 2 −1 1 2 3 5 10 15 x y
  • 32. Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now A functional program consists entirely of functions def main(): time = datetime.now() args = sys.argv[1:] print outputString(time, args) def outputString(time, args): return str(time) + " " + joinArgs(args) def joinArgs(args): return "-".join(args) $ ./justfunctions.py Hello from Python 2014-02-15 10:36:42.062697 Hello-from-Python
  • 33. Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now A functional program consists entirely of functions main :: IO() main = do time <- getCurrentTime args <- getArgs putStrLn (outputString time args) outputString :: UTCTime -> [String] -> String outputString time args = show(time) ++ " " ++ joinArgs(args) joinArgs :: [String] -> String joinArgs = intercalate "-" $ ./justfunctions Hello from Haskell 2014-02-15 08:36:50.822728 UTC Hello-from-Haskell
  • 34. Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now Some Haskell Syntax Python: def outputString(time, args): return str(time) + " " + joinArgs(args) Haskell: outputString :: UTCTime -> [String] -> String outputString time args = show(time) ++ " " ++ joinArgs(args)
  • 35. Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now Recursive function: Haskell doubleAll :: [Int] -> [Int] doubleAll [] = [] doubleAll (x:xs) = x*2 : doubleAll xs Example use in the interactive interpreter: Prelude Main> doubleAll [8,2,3] [16,4,6]
  • 36. Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now Recursive function expanded doubleAll [] = [] doubleAll (x:xs) = x*2 : doubleAll xs doubleAll [8,2,3] 16 : (doubleAll [2,3]) 16 : 4 : (doubleAll [3]) 16 : 4 : 6 : (doubleAll []) 16 : 4 : 6 : [] 16 : 4 : [6] 16 : [4,6] [16,4,6]
  • 37. Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now Recursive function: Python def doubleAll(numbers): if numbers == []: return [] else: first = numbers[0] rest = numbers[1:] return [first * 2] + doubleAll(rest) Example use in the interactive interpreter: >>> doubleAll([8,2,3]) [16, 4, 6]
  • 38. Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now Pattern Matching: Haskell doubleAll :: [Int] -> [Int] doubleAll [] = [] doubleAll (x:xs) = x*2 : doubleAll xs
  • 39. Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now Higher-order Functions 3 Basic List Operations Map Convert each element of a list into some other value.
  • 40. Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now Higher-order Functions 3 Basic List Operations Map Convert each element of a list into some other value. Filter Get a subset of a list based on some condition.
  • 41. Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now Higher-order Functions 3 Basic List Operations Map Convert each element of a list into some other value. Filter Get a subset of a list based on some condition. Fold Reduce a list of items to a single value.
  • 42. Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now Map Apply a function to each element of a list, and you get a new list. a1 a2 a3 ... an b1 b2 b3 ... bn f(a1) f(an)
  • 43. Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now Map The built-in Haskell map function: map :: (a -> b) -> [a] -> [b] map _ [] = [] map f (x:xs) = f x : map f xs
  • 44. Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now Map The builtin Haskell map function: map :: (a -> b) -> [a] -> [b] map _ [] = [] map f (x:xs) = f x : map f xs Hmmm, looks very similar to our previous doubleAll function: doubleAll :: [Int] -> [Int] doubleAll [] = [] doubleAll (x:xs) = x*2 : doubleAll xs
  • 45. Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now Map So our doubleAll can actually be simplified. In Haskell: doubleAll = map (*2) In Python: def doubleAll(numbers): return map(lambda x: x * 2, numbers)
  • 46. Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now Map doubleAll :: Num a => [a] -> [a] doubleAll = map (*2) 8 2 3 16 4 6 ∗2 ∗2 ∗2
  • 47. Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now Account Data data Bank = ABSA | Capitec | FNB | Nedbank | SBSA data Account = Account {bank :: Bank, accNum :: String, owner :: String, balance :: Amount}
  • 48. Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now Account Data [Account {accNum="4076814233", owner="J. Doe", balance=(Amount 123000.23), bank=ABSA}, Account {accNum="6868773585", owner="J. Black", balance=(Amount 5782347.99), bank=FNB}, Account {accNum="4055892156", owner="A. Kay", balance=(Amount 100), bank=ABSA}, Account {accNum="6584539813", owner="S. Jones", balance=(Amount 2937361.45), bank=FNB}]
  • 49. Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now Map Map on account data: balances :: [Account] -> [Amount] balances accounts = map balance accounts acc1 acc2 acc3 acc4 acc5 bal1 bal2 bal3 bal4 bal5 balance(acc1) balance(acc5)
  • 50. Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now Filter acc1 acc2 acc3 acc4 acc5 acc1 acc3 acc5 in? in? in? in? in?
  • 51. Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now Filter Filter on account data: topAccounts :: [Account] -> [Account] topAccounts accounts = filter isRich accounts isRich :: Account -> Bool isRich acc = balance acc >= (Amount 1000000) Output: *Main> topAccounts accounts [FNB 6868773585 (J. Black) R5782347.99, FNB 6584539813 (S. Jones) R2937361.45]
  • 52. Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now Fold/Reduce/Inject foldl (+) 0 [8,2,3] 13 0 8 2 3 8 2 3 10 3 13
  • 53. Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now Fold/Reduce/Inject balanceSum :: [Account] -> Amount balanceSum accounts = foldl (+) 0 (balances accounts) 0 bal1 bal2 bal3 sum bal2 bal3 sum bal3 sum
  • 54. Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now Function Composition balanceSum :: [Account] -> Amount balanceSum accounts = foldl (+) 0 (balances accounts) 0 bal1 bal2 bal3 sum bal2 bal3 sum bal3 sum s2 :: [Account] -> Amount s2 = balances |> (foldl(+)0) h(x) = (f ◦ g)(x) = f(g(x)) h = f ◦ g s3 = f1 ◦ balances s3 :: [Account] -> Amount s3 = (foldl(+)0) . balances
  • 55. Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now balancesPerBank foldl insertBalance Map.empty accounts {} acc1 acc2 acc3 map acc2 acc3 map acc3 map
  • 56. Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now Fold type BankMap = Map Bank Amount balancesPerBank :: [Account] -> BankMap balancesPerBank = foldl insertBalance Map.empty Output: *Main> balancesPerBank accounts fromList [(ABSA,R123100.23),(FNB,R8719709.44)]
  • 57. Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now Fold type BankMap = Map Bank Amount balancesPerBank :: [Account] -> BankMap balancesPerBank = foldl insertBalance Map.empty insertBalance :: BankMap -> Account -> BankMap insertBalance bankmap account = Map.insert key value bankmap where key = bank account value = addBalance bankmap account addBalance :: BankMap -> Account -> Amount addBalance bankmap account = case (Map.lookup (bank account) bankmap) of Nothing -> balance account Just bal -> (balance account) + bal
  • 58. Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now Programming Paradigms (Very Simplified) Imperative Declarative
  • 59. Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now In imperative programming: • Your variables can vary any time!
  • 60. Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now In imperative programming: • Your variables can vary any time! • You have to use locks to be thread-safe!
  • 61. Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now In imperative programming: • Your variables can vary any time! • You have to use locks to be thread-safe! • You have to write your own loops for the most basic list operations!
  • 62. Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now In imperative programming: • Your variables can vary any time! • You have to use locks to be thread-safe! • You have to write your own loops for the most basic list operations! • Your data structures are mutable!
  • 63. Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now In imperative programming: • Your variables can vary any time! • You have to use locks to be thread-safe! • You have to write your own loops for the most basic list operations! • Your data structures are mutable! • You have to defensively make copies of data to prevent bugs!
  • 64. Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now In imperative programming: • Your variables can vary any time! • You have to use locks to be thread-safe! • You have to write your own loops for the most basic list operations! • Your data structures are mutable! • You have to defensively make copies of data to prevent bugs! • You have to defensively check for null values!
  • 65. Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now In imperative programming: • Your variables can vary any time! • You have to use locks to be thread-safe! • You have to write your own loops for the most basic list operations! • Your data structures are mutable! • You have to defensively make copies of data to prevent bugs! • You have to defensively check for null values! • You have to think about implicit state! (this, self)
  • 66. Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now In imperative programming: • Your variables can vary any time! • You have to use locks to be thread-safe! • You have to write your own loops for the most basic list operations! • Your data structures are mutable! • You have to defensively make copies of data to prevent bugs! • You have to defensively check for null values! • You have to think about implicit state! (this, self) • Code is generally riddled with side effects!
  • 67. Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now Are you kidding me? How can anyone program like this???
  • 68. Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now
  • 69. Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now Join the anti-for campaign Less loops, more map/filter/fold http://weblogs.asp.net/podwysocki/archive/2009/06/26/ the-anti-for-campaign.aspx
  • 70. Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now Treat side effects as a first-class concern
  • 71. Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now Learn a functional language “ A language that doesn’t affect the way you think about programming, is not worth knowing. ” — Alan Perlis[5]
  • 72. Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now Disclaim your inheritance Write non-trivial code without using objects and inheritance. Get re-usability with higher-order functions. Try to minimise moving parts instead of encapsulating moving parts. [4]
  • 73. Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now Join our group @lambdaluminary We meet once a month, on the second Monday of the month. http://www.meetup.com/lambda-luminaries/
  • 74. Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now Get out of your comfort zone Functional Programming is unfamiliar territory for most. “ If you want everything to be familiar you will never learn anything new. ” — Rich Hickey, author of Clojure[6]
  • 75. Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now Companies In South Africa Jemstep, Sandton Using Scala for Fund Analysis Allan Gray, Cape Town Using Scala for backend logic and system integration. Yuppiechef, Cape Town Using Clojure for their Warehouse Management System. Cognician, Cape Town Using Clojure to create coaching/learning modules. Eldo Energy, Johannesburg Using Clojure for automated meter reading and intelligent monitoring of consumer energy. Rheo Systems, Pretoria Using Clojure for supply chain integration.
  • 76. Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now Companies In South Africa Pattern Matched Technologies, Midrand Using Erlang for all systems, eg. processing high volumes of financial transactions. Effective Control Systems, Kyalami Using Erlang for printer management. Mira Networks, Somerset West Using Erlang for billing administration and mobile development. Kotive Using Scala for designing workflow processes.
  • 77. Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now Online Courses Functional Thinking by Neal Ford O’ Reilly http://shop.oreilly.com/product/0636920030393.do Functional Programming Principles in Scala EPFL University https://www.coursera.org/course/progfun School of Haskell FP Complete https://www.fpcomplete.com/school Programming Languages University of Washington https://www.coursera.org/course/proglang
  • 78. Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now Books Miran Lipovaˇca Learn You a Haskell for Great Good! http://learnyouahaskell.com/ Fred H´ebert Learn You Some Erlang for Great Good! http://learnyousomeerlang.com/ Yaron Minski, Anil Madhavapeddy, Jason Hickey Real World OCaml https://realworldocaml.org/ Paul Chiusano, R´unar Bjarnason Functional Programming in Scala http://www.manning.com/bjarnason/
  • 79. Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now References I John Hughes Why Functional Programming Matters http://www.cs.kent.ac.uk/people/staff/dat/miranda/ whyfp90.pdf John Carmack Functional Programming in C++ http://www.altdevblogaday.com/2012/04/26/ functional-programming-in-c/ Edsger W. Dijkstra Go To Statement Considered Harmful http://www.u.arizona.edu/~rubinson/copyright_ violations/Go_To_Considered_Harmful.html
  • 80. Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now References II Tweet by Michael Feathers https://twitter.com/mfeathers/status/29581296216 Alan Jay Perlis http://www.cs.yale.edu/quotes.html Rich Hickey http: //www.infoq.com/presentations/Simple-Made-Easy Andreas Pauley An Introduction to Functional Programming https://github.com/apauley/fp_presentation