Ch07a2.purs

module Ch07a2 where

import Prelude           (Unit, discard, show, (==), (<=), (<), (>), (>=), ($))
import Data.Eq           (class Eq)
import Data.Ord          (class Ord)
import Data.Show         (class Show)
import Data.Generic.Rep  (class Generic)
import Data.Show.Generic (genericShow)
import Effect            (Effect)
import Effect.Console    (log)

-------------------- Data Types -----------------------------------------------------------

data Maybe  a   = Nothing | Just a
data Either a b = Left a  | Right b

-------------------- (Derived) Instances --------------------------------------------------

derive instance eqMaybe  :: Eq  a => Eq  (Maybe a)
derive instance ordMaybe :: Ord a => Ord (Maybe a)

derive instance genericEither :: Generic (Either a b) _
instance showEither :: (Show a, Show b) => Show (Either a b) where
  show = genericShow

derive instance genericMaybe :: Generic (Maybe a) _
instance showMaybe :: Show a => Show (Maybe a) where
  show = genericShow

-------------------- Tests ----------------------------------------------------------------

test :: Effect Unit
test = do
  log "Uncomment each line. DERIVE missing functions !!!"
  log $ show $ Just 5  == Just 5                           -- true
  log $ show $ Just 5  == Just 2                           -- false
  log $ show $ Just 5  == Nothing                          -- false
  log $ show $ Nothing == Just 5                           -- false
  log $ show $ Nothing == (Nothing :: Maybe Unit)          -- true
  log $ show $ (Left "left" :: Either _ Unit)              -- (Left "left")
  log $ show $ (Right (Just 42) :: Either Unit _)          -- (Right (Just 42))
  log $ show $ Just 1  <  Just 5                           -- true
  log $ show $ Just 5  <= Just 5                           -- true
  log $ show $ Just 5  >  Just 10                          -- false
  log $ show $ Just 10 >= Just 10                          -- true
  log $ show $ Just 99 >  Nothing                          -- true
  log $ show $ Just 99 <  Nothing                          -- false
  log $ show $ Just "abc"                                  -- (Just "abc")
  log $ show $ (Nothing :: Maybe Unit)                     -- Nothing