examples.hs

import Data.Maybe

-- Lazy evaluation
natNums = [1..]
odds = [1,3..]
evens = [2,4..]

addXAndY :: Integer
addXAndY = sum
  where x = 1
        y = 2
        sum = x + y

-- Type inference
-- ghci repl:
-- Prelude> :t natNums
-- Prelude> natNums :: [Integer]

-- Function composition, partial application, currying
inc :: Integer -> Integer
-- inc i = i + 1
-- inc = \i -> i + 1
inc = (+1)

addOneToSomeOdds :: [Integer]
-- map (\x -> inc x) (take 10 odds)
-- map (\x -> inc x) $ take 10 odds
-- map (inc) $ take 10 odds
addOneToSomeOdds = map inc $ take 10 odds

-- Pattern Matching
greeting :: [Char] -> [Char]
greeting name
  | name == "Bryan" = ("Sup " ++ name)
  | name == "Abel" = ("Feel like some karoke, " ++ name)
  | otherwise = "Hey...you"

-- Pattern matching, lazy evaluation, recursion
quicksort :: Ord a => [a] -> [a]
quicksort [] = []
quicksort (x:xs) = quicksort lessThanOrEqual ++ [x] ++ quicksort greaterThan
  where lessThanOrEqual = filter (<= x) xs
        greaterThan = filter (> x) xs

-- TYPES!!!!
data Day = Monday | Tuesday | Wednesday | Thursday | Friday | Saturday | Sunday
  deriving (Eq, Ord, Show)

-- Main> show Monday
-- "Monday"

-- Main> Friday > Monday
-- True

-- Main> Friday == Monday
-- False

-- Main> :t Wednesday
-- Wednesday :: Day <-- Types are functions!

-- Goodbye null pointers!
-- Prelude> tail []
-- *** Exception: Prelude.tail: empty list

safeTail :: [a] -> Maybe [a]
safeTail []     = Nothing
safeTail (_:xs) = Just xs

-- *Main Data.Maybe> safeTail []
-- Nothing
-- *Main Data.Maybe> take 10 (safeTail odds)

fiveFromTail :: Maybe [a]
fiveFromTail = case safeTail [] of
  Just x -> Just (take 5 x)