CS168 - Notes week of 24 Sep

Rough notes from our REPL session.

square x = x*x

-- Type classes:
--  Num : supports (+), (-), (*)
--  Eq : supports (==), (/=)
-- Ord : supports (<), (<=), (>), (>=), compare

--     ASCIIbetical
--     Lexicographic : roughly means "dictionary order"

-- "Data type" declaration.
data Color = Red | Green | Blue
           | Cyan | Magenta | Yellow
  deriving (Show, Eq, Enum, Bounded)

-- Enum : can enumerate (list) values
-- Bounded : has a start and end (min,max)

combine Red Green = Yellow
combine Red Blue = Magenta
combine Blue Green = Cyan

-- Can use the 'type' keyword for
-- a type "synonym"
type Year = Int

currentYear = 2018 :: Year

-- The Maybe type:
--data Maybe a
--  = Nothing | Just a

count :: Maybe Int
count = Just 10

-- Maybe is a type constructor for saying
-- that a value of that type may be present
-- or absent (Just or Nothing)

-- maybe is a function that allows us
-- to process a Maybe value by specifying
-- a default to use in case it is Nothing.

fact1 :: Integer -> Integer
fact1 n
  | n < 0 = error "Negative"
  | n == 0 = 1
  | otherwise = n * fact1 (n-1)

fact2 :: Integer -> Maybe Integer
fact2 n
  | n < 0 = Nothing
  | n == 0 = Just 1
  | otherwise = fmap (n *) (fact2 (n-1))

main = return ()

Source