Frege - consequently functional programming for the JVM
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The document discusses functional programming concepts in Frege, a purely functional programming language for the JVM. It provides examples of defining and composing functions, pattern matching, lazy evaluation, pure functions, and Java interoperability. It also discusses Frege's type system and use of monads to encapsulate effects like mutation and I/O.
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Frege - consequently functional programming for the JVM
- 4. Why do we care? a = 1 b = 2 c = b b = a a = c
- 5. Why do we care? a = 1 b = 2 c = b b = a a = c 1
- 6. Why do we care? a = 1 b = 2 c = b b = a a = c 1 1 2
- 7. Why do we care? a = 1 b = 2 c = b b = a a = c 1 1 2 1 2 2
- 8. Why do we care? a = 1 b = 2 c = b b = a a = c 1 1 2 1 2 2
- 9. Why do we care? a = 1 b = 2 c = b b = a a = c 1 1 2 1 2 2 1 1 2
- 10. Why do we care? a = 1 b = 2 c = b b = a a = c 1 1 2 1 2 2 1 1 2
- 11. Why do we care? a = 1 b = 2 c = b b = a a = c 1 1 2 1 2 2 1 1 22 1 2
- 12. Why do we care? a = 1 b = 2 c = b b = a a = c 1 1 2 1 2 2 1 1 2 2 1 2
- 13. Why do we care? a = 1 b = 2 c = b b = a a = c 1 1 2 1 2 2 1 1 2 2 1 2 state1 state2 state3
- 14. Why do we care? a = 1 b = 2 c = b b = a a = c 1 1 2 1 2 2 1 1 2 2 1 2 time1 time2 time3 state1 state2 state3
- 15. Operational Reasoning a = 1 b = 2 c = b b = a a = c 1 1 2 1 2 2 1 1 2 2 1 2 time1 time2 time3 state1 state2 state3
- 16. Operational Reasoning a = 1 b = 2 c = b b = a a = c 1 1 2 1 2 2 1 1 2 2 1 2 time1 time2 time3 state1 state2 state3
- 17. Using functions a = 1 b = 2 1 1 2
- 18. Using functions a = 1 b = 2 1 1 2 2 1 swap(a,b) = (b,a)
- 19. The Key Idea Assignments change state and introduce time. Statements do side effects. Let’s just remove them!
- 22. You would not believe just how different it is!
- 24. Define a Function frege> times a b = a * b frege> times 3 4 12 frege> :type times Num α => α -‐> α -‐> α
- 25. Define a Function frege> times a b = a * b frege> (times 3) 4 12 frege> :type times Num α => α -‐>(α -‐> α) no types declared function appl. left associative tell the inferred type typeclass only 1 parameter! return type is a function! thumb: „two params of same numeric type returning that type“ no comma
- 26. Reference a Function frege> twotimes = times 2 frege> twotimes 3 6 frege> :t twotimes Int -‐> Int
- 27. Reference a Function frege> twotimes = times 2 frege> twotimes 3 6 frege> :t twotimes Int -‐> Int No second argument! „Currying“, „schönfinkeling“, or „partial function application“. Concept invented by Gottlob Frege. inferred types are more specific
- 28. Function Composition frege> twotimes (threetimes 2) 12 frege> sixtimes = twotimes . threetimes frege> sixtimes 2 frege> :t sixtimes Int -‐> Int
- 29. Function Composition frege> twotimes (threetimes 2) 12 frege> sixtimes = twotimes . threetimes frege> sixtimes 2 frege> :t sixtimes Int -‐> Int f(g(x)) more about this later (f ° g) (x)
- 30. Pattern Matching frege> times 0 (threetimes 2) 0 frege> times 0 b = 0
- 31. Pattern Matching frege> times 0 (threetimes 2) 0 frege> times 0 b = 0 unnecessarily evaluated shortcuttingpattern matching
- 32. Lazy Evaluation frege> times 0 (length [1..]) 0 endless sequence evaluation would never stop Pattern matching and non-strict evaluation to the rescue!
- 33. Pure Functions Java T foo(Pair<T,U> p) {…} Frege foo :: (α,β) -‐> α What could possibly happen? What could possibly happen?
- 34. Pure Functions Java T foo(Pair<T,U> p) {…} Frege foo :: (α,β) -‐> α Everything! State changes, file or db access, missile launch,… a is returned
- 35. Pure Functions can be cached (memoized) can be evaluated lazily can be evaluated in advance can be evaluated concurrently can be eliminated in common subexpressions can be optimized
- 36. Java Interoperability do not mix OO and FP combine them
- 37. Java -> Frege Scripting: JSR 223 Service: compile *.fr file call static method simple
- 38. pure native encode java.net.URLEncoder.encode :: String -> String encode “Dierk König“ native millis java.lang.System.currentTimeMillis :: () -> IO Long millis () millis () past = millis () - 1000 Does not compile! Frege -> Java This is a key distinction between Frege and other JVM languages! even Java can be pure
- 39. Frege allows calling Java but never unprotected! is explicit about effects just like Haskell
- 40. Type System Ubiquitous static typing allows global type inference and thus more safety and less work for the programmer You don’t need to specify any types at all! But sometimes you do anyway…
- 41. Mutable I/O Mutable Mutable Keep the mess out! Pure Computation Pure Computation Pure Computation
- 42. Mutable I/O Mutable Mutable Keep the mess out! Pure Computation Pure Computation Pure Computation Ok, these are Monads. Be brave. Think of them as contexts that the type system propagates and makes un-escapable. Thread- safe by design! Checked by compiler
- 43. Some Cool Stuff
- 44. Zipping addzip [] _ = [] addzip _ [] = [] addzip (x:xs) (y:ys) = (x + y : addzip xs ys )
- 45. Zipping addzip [] _ = [] addzip _ [] = [] addzip (x:xs) (y:ys) = (x + y : addzip xs ys ) use as addzip [1,2,3] [1,2,3] == [2,4,6] Pattern matching feels like Prolog Why only for the (+) function? We could be more general…
- 46. High Order Functions zipWith f [] _ = [] zipWith f _ [] = [] zipWith f (x:xs) (y:ys) = (f x y : zipWith xs ys )
- 47. High Order Functions zipWith f [] _ = [] zipWith f _ [] = [] zipWith f (x:xs) (y:ys) = (f x y : zipWith xs ys ) use as zipWith (+) [1,2,3] [1,2,3] == [2,4,6] and, yes we can now define addzip = zipWith (+) invented by Gottlob Frege
- 49. Fizzbuzz Imperative public class FizzBuzz{ public static void main(String[] args){ for(int i= 1; i <= 100; i++){ if(i % 15 == 0{ System.out.println(„FizzBuzz"); }else if(i % 3 == 0){ System.out.println("Fizz"); }else if(i % 5 == 0){ System.out.println("Buzz"); }else{ System.out.println(i); } } } }
- 50. Fizzbuzz Logical fizzes = cycle ["", "", "fizz"] buzzes = cycle ["", "", "", "", "buzz"] pattern = zipWith (++) fizzes buzzes fizzbuzz = zipWith (max . show) pattern [1..] main _ = do for (take 100 fizzbuzz) println
- 51. Fizzbuzz Comparison Imperative Logical Conditionals 4 0 Operators 7 1 Nesting level 3 0 Sequencing sensitive transparent Maintainability - - - +
- 52. History Java promise: „No more pointers!“ But NullPointerExceptions (?)
- 53. Frege is different No More But state no state (unless declared) statements expressions (+ „do“ notation) assignments definitions variables ST monad as „agent“ interfaces type classes classes & objects algebraic data types inheritance parametric polymorphism null references Maybe NullPointerExceptions Bottom, error
- 55. Frege in comparison safe Java Groovy Frege Haskell concept by Simon Peyton-Jones Frege makes the Haskell spirit accessible to the Java programmer and provides a new level of safety. apply logic run computers practical
- 56. Why Frege Robustness under parallel execution Robustness under composition Robustness under increments Robustness under refactoring Enables local and equational reasoning Best way to learn FP
- 57. How? http://www.frege-‐lang.org @fregelang stackoverflow „frege“ tag edX FP101 MOOC (start Oct 15th!)
- 58. Gottlob Frege "As I think about acts of integrity and grace, I realise that there is nothing in my knowledge that compares with Frege’s dedication to truth… It was almost superhuman.“ —Bertrand Russel "Not many people managed to create a revolution in thought. Frege did.“ —Graham Priest Lecture on Gottlob Frege: http://www.youtube.com/watch?v=foITiYYu2bc
- 60. Dierk König Canoo mittie Please fill the feedback forms