Making Our Own Types and Typeclasses

Making Our Own Types and Typeclasses

1. Algebraic data types examples

   • data Bool = False | True
   • data Int = -2147483648 | -2147483647 | ... | -1 | 0 | 1 | 2 | ... | 2147483647

2. Create type

   • data
     • To add functionalities
       • deriving
     • simple implementation
       • data Car = Car String String Int deriving (Show)
     • best implementation
       • data Car = Car {company :: String, model :: String, year :: Int} deriving (Show)
     • multiple defining
       • |

3. type parameters

   • constructors
     • data Maybe a = Nothing | Just a
       • rename parameters into functions
         • tellCar :: Car -> String tellCar (Car {company = c, model = m, year = y}) = "This " ++ c ++ " " ++ m ++ " was made in " ++ show y
   • derived instances
     • deriving (Eq)
       • compare types
     • add functionality of class
     • deriving (Eq, Show, Read)

4. Type synonyms

   • type String = [Char]
   • types are equivalent and interchangeable
   • type AssocList k v = [(k,v)]

5. Recursive data structures

   • value of some type contains values of that type
     • 3:(4:(5:6:[]))
   • data Tree a = EmptyTree | Node a (Tree a) (Tree a) deriving (Show, Read, Eq)
     • a -> Tree a - > Tree a

6. Typeclasses 102

   • interfaces
     • equality
     • comparing
     • ordering
     • enumeration
   • class Eq a where
   • instance Eq TrafficLight where
   • class (Eq a) => Num a where

7. Functor typeclass

   • mapping
     • read class
   • (==) :: (Eq a) => a -> a -> Bool
   • map :: (a -> b) -> [a] -> [b]

8. Kinds and some type-foo

   • Family type
     • :k <type>
       • :k Int
         • Int :: *
   • data Barry t k p = Barry { yabba :: p, dabba :: t k }
     • Barry :: (* -> *) -> * -> * -> *