GADT Example

test.hs

{-# LANGUAGE GADTs #-}

module Main where

data SpellHeal   = SpellHeal Int deriving (Show)
data SpellDamage = SpellDamage Int deriving (Show)
data SpellRitual = SpellRitual Float deriving (Show)

data Creature = Creature Int deriving (Show)

data Spell a where
  Heal   :: Int -> Spell SpellHeal
  Damage :: Int -> Spell SpellDamage
  Ritual :: Float -> Spell SpellRitual

useSpell :: Spell a -> Creature -> Creature
useSpell (Heal amount) (Creature hp) = Creature (hp + amount)
useSpell (Damage amount) (Creature hp) = Creature (hp - amount)

doRitual :: Spell a -> Bool
doRitual (Ritual duration) = True

main :: IO ()
main = do
  print $ useSpell (Heal 1) (Creature 10)
  print $ doRitual (Ritual 10.0)