Зачем нужен
Haskell:

В начале было…

λ-исчисление

Киты

1

Как можно позже

main =
  let result = 2 `div` 0
  in print result
                    

main =
  let result = 2 `div` 0
  in putStr "Bye!"
                    

2

Как можно меньше

main =
  let cx     = 2 / 6.054
      nk     = 4 * 12.003
      coeffs = [cx, nk]
  in putStr "Nothing..."
                    

main =
  let cx     = 2 / 6.054
      nk     = 4 * 12.003
      coeffs = [cx, nk]
  in print $ length coeffs
                    

main =
  let cx     = 2 / 6.054
      nk     = 4 * 12.003
      coeffs = [cx, nk]
  in print $ coeffs !! 1
                    

main =
  let cx     = 2 / 6.054
      nk     = 4 * 12.003
      coeffs = [cx, nk]
  in print coeffs
                    

Зачем это нам?

1

Рациональность

main =
  let evens = [2, 4 .. 100]
  in print $ take 10 evens
                    

2

Бесконечность

main =
  let evens = [2, 4 ..]
  in print $ take 10 evens
                    

3

EDSL

(?) :: Bool -> (a, a) -> a
True  ? (f, _) = f
False ? (_, s) = s
                    

main = do
  putStr "Input URL prefix: "
  prefix <- getLine
  (prefix == "https") ?
    (putStr "Secure web!",
     exitFailure)
                    

Но…

Space leak

main =
  let result = 2 `div` 0
  in putStr "Bye!"
                    

bad :: Num b => [a] -> b -> b
bad []     c = c
bad (_:xs) c = bad xs $ c + 1
                    

bad [1,2,3] 0
                    

bad 1:[2,3] 0 + 1
                    

bad 1:2:[3] (0 + 1) + 1
                    

bad 1:2:3:[] ((0 + 1) + 1) + 1
                    

((0 + 1) + 1) + 1 = 3
                    

main =
  print $ bad [1..50000000] 0
                    

Что же делать?

1

Оптимизация

2

Ручками

ok :: Num b => [a] -> b -> b
ok []     c = c
ok (_:xs) c = ok xs $! c + 1
                    

Итак…