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{-# LANGUAGE DeriveFunctor #-}
module Main where
-- Derive functor for a simple data type
data List a = Nil | Cons a (List a)
deriving (Functor,Show)
someList = Cons 1 (Cons 1 (Cons 2 (Cons 3 Nil)))
doubleList = fmap (*2) someList
test1 = do
putStr "normal: " >> print someList
putStr "double: " >> print doubleList
-- Derive functor for a data type with functions and tuples
data ReaderWriter r w a = RW { runRW :: r -> (a,w) }
deriving (Functor)
data Cont r a = Cont { runCont :: (a -> r) -> r }
deriving (Functor)
test2 = do
let rw = RW (\r -> ("something",r*3))
putStr "normal: " >> print (runRW rw 123)
putStr "reverse: " >> print (runRW (fmap reverse rw) 456)
let five = Cont ($ 5)
putStr "normal: " >> runCont five print
putStr "double: " >> runCont (fmap (*2) five) print
-- Derive functor in such a way that we need a constraint
newtype Compose f g a = Compose (f (g a))
deriving (Functor,Show)
listOfLists = Compose [[1,2,3],[7,8,9]]
test3 = do
putStr "normal: " >> print listOfLists
putStr "double: " >> print (fmap (*2) listOfLists)
-- All tests
main = do
test1
test2
test3
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