Frege - a Haskell for the JVM

Purely functional programming for the JVM.
Presented by Dierk König, Canoo, at JAX, Mainz, 2015

Transcript

1. Frege ein Haskell für die JVM
2. Dierk König Canoo mittie
3. different Frege is
4. different Frege is very
5. You would not believe just how different it is!
6. Online REPL http://try.frege-lang.org
7. Define a Function frege>    times  a  b  =  a  *  b   frege>    times  3  4   12   frege>  :type  times   Num  α  =>  α  -­‐>  α  -­‐>  α
8. 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
9. Reference a Function frege>  twotimes  =  times  2   frege>  twotimes  3   6     frege>  :t  twotimes   Int  -­‐>  Int
10. 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
11. Function Composition frege>  twotimes  (threetimes  2)   12   frege>  sixtimes  =  twotimes  .  threetimes   frege>  sixtimes  2   frege>  :t  sixtimes   Int  -­‐>  Int
12. 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)
13. Pattern Matching frege>  times  0  (threetimes  2)   0   frege>  times  0  b  =  0
14. Pattern Matching frege>  times  0  (threetimes  2)   0   frege>  times  0  b  =  0 unnecessarily evaluated shortcutting pattern matching
15. Lazy Evaluation frege>  times  0  (length  [1..])   0 endless sequence evaluation would never stop Pattern matching and non-strict evaluation to the rescue!
16. Pure Functions Java   T  foo(Pair<T,U>  p)  {…}   Frege   foo  ::  (α,β)  -­‐>  α What could possibly happen? What could possibly happen?
17. 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
18. 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
19. Java Interoperability do not mix OO and FP combine them
20. Java -> Frege Scripting:  JSR  223   Service:
       compile  *.fr  file
       call  static  method simple
21. Frege -> Java data Date = native java.util.Date where
 native new :: () -> IO (MutableIO Date) -- new Date()
 native toString :: Mutable s Date -> ST s String -- d.toString() This is a key distinction between Frege and
 other JVM languages! Declaring the messiness.
22. Frege allows  calling  Java
      but  never  unprotected!   is  explicit  about  effects
      just  like  Haskell  
23. Some Cool Stuff
24. Zipping addzip  []  _    =  []
 addzip  _    []  =  []     addzip  (x:xs)  (y:ys)  =  
              (x  +  y  :  addzip  xs  ys  )
25. 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…
26. High Order Functions zipWith  f  []  _    =  []
 zipWith  f  _    []  =  []     zipWith  f  (x:xs)  (y:ys)  =  
              (f  x  y  :  zipWith  xs  ys  )  
27. 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
28. Fizzbuzz http://c2.com/cgi/wiki?FizzBuzzTest http://dierk.github.io/FregeGoodness/
 html/index.html#fizzbuzz
29. 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);
 }  }  }  }
30. Fizzbuzz Logical fizzes      =  cycle      ["",  "",  "fizz"]
 buzzes      =  cycle      ["",  "",  "",  "",  "buzz"]
 pattern    =  zipWith  (++)  fizzes  buzzes   fizzbuzz  =  zipWith  combine  pattern  [1..]  where
      combine  word  number  =  if  word  ==  ""
                                                        then  show  number
                                                        else  word   main  _      =  do
                      for  (take  100  fizzbuzz)  println
31. Fizzbuzz Comparison Imperative Logical Conditionals 4 1 Operators 7 1 Nesting level 3 0 Sequencing sensitive transparent Maintainability - - - +
32. List Comprehension Pythagorean triples: a2 + b2 = c2 pyth  n  =  [      (x,y,z)      |  x  <-­‐  [1..n],  y  <-­‐  [1..n],  z  <-­‐  [1..n],      x*x  +  y*y  ==  z*z    ]  
33. List Comprehension Pythagorean triples: a2 + b2 = c2 pyth  n  =  [      (x,y,z)      |  x  <-­‐  [1..n],  y  <-­‐  [1..n],  z  <-­‐  [1..n],      x*x  +  y*y  ==  z*z    ]   select from where „brute force“ or „executable specification“. A more efficient solution:
34. List Comprehension Pythagorean triples: a2 + b2 = c2 [  (m*m-­‐n*n,  2*m*n,  m*m+n*n)     |  m  <-­‐  [2..],  n  <-­‐  [1..m-­‐1]     ]   endless production think „nested loop“ „select“ functions dynamic „from“ empty „where“
35. QuickCheck -­‐-­‐  An  AVL  tree  is  balanced  so   that  the  height  of  the  left  and   right  subtree  differ  by  at  most  1   p_balance  =  forAll  aTree  
    (\tree  -­‐>  abs  tree.balance  <  2)   QuickCheck will create 500 different trees covering all corner cases in creation and validate the invariant. (from Frege source code)
36. Type System Ultra-strong static typing allows
 extremely good type inference and thus more safety and less work for the programmer You don’t need to specify any types at all! But you sometimes do anyway…
37. Mutable
 I/O Mutable Mutable Keep the mess out! Pure Computation Pure Computation Pure Computation
38. 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
39. History Java promise: „No more pointers!“ But NullPointerExceptions (?)
40. 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
41. Frege in comparison practical safe Java Groovy Frege Haskell
42. 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
43. Why Frege Explicit effects for safety & concurrency
 Pure = no effects (implicit or explicit)
 Guarantees extend into Java calls
 Laziness enforces purity and
 immutable values (no assignments)
 Type inference for the above requires a 
 purely functional language
 (only expressions, no statements)
44. Why Frege and then it is just a pleasure to work with
45. How? http://www.frege-­‐lang.org   stackoverflow  „frege“  tag   https://github.com/Dierk/Real_World_Frege Join the effort!
46. 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
47. Dierk König Canoo Please fill the feedback forms mittie