Boulder Startup Week: What is functional programming?

Transcript

1. FUNCTIONAL PROGRAMMING
2. WHAT ARE FUNCTIONS?
3. ▸ A relation from a set to a set {(true, false), (false, true), (false, false)} ▸ One output object for every input {(true, false), (false, true)}
4. ACTUAL FUNCTIONS
5. boolean not(boolean b) { return !b; } String hello(String s) { return "Hello " + s; }
6. NOT FUNCTIONS
7. System.out.println("Hello world"); System.getTimeMillis(); x = x + 1; boolean update(int j) { i += j; return j + 1; }
8. ... functional programming is a restriction on how we write programs, but not on what programs we can express. — Rúnar Bjarnason and Paul Chiusano, Functional Programming in Scala, 2014
9. EQUATIONAL REASONING
10. main :: IO () main = print $ sum [1, 2, 3] sum :: [Int] -> Int sum = foldl (+) 0 foldl :: (Int -> Int -> Int) -> Int -> [Int] -> Int foldl f z xs = go z xs where go z [] = z go z (x:xs) = go (f z x) xs
11. main :: IO () main = print $ sum [1, 2, 3] sum :: [Int] -> Int sum xs = go 0 xs where go z [] = z go z (x:xs) = go (z + x) xs
12. main :: IO () main = print sum' sum' :: Int sum' = go 0 [1, 2, 3] where go z [] = z go z (x:xs) = go (z + x) xs
13. main :: IO () main = print sum' sum' :: Int sum' = go (((0 + 1) + 2) + 3) [] where go z [] = z
14. main :: IO () main = print sum' sum' :: Int sum' = (((0 + 1) + 2) + 3)
15. main :: IO () main = print sum' sum' :: Int sum' = 6
16. main :: IO () main = print 6
17. NOT EQUATIONAL REASONING
18. val x = { println("Hello"); 1 } x + x // Hello // 2
19. { println("Hello"); 1 } + { println("Hello"); 1 } // Hello // Hello // 2
20. PRINCIPLE OF COMPOSITIONALITY
21. Meaning of a complex statement is the combination of the meanings of its parts.
22. COMPROMISING
23. Can we write partially functional programs?
24. TOOLS
25. ▸ Haskell ▸ ML ▸ Scala ▸ F#
26. GO WRITE FUNCTIONS!