ch22

src/haskellbook/ch22/ch22.applicative.hs

@@ -0,0 +1,35 @@
+{-# LANGUAGE InstanceSigs #-}
+
+newtype Reader r a = Reader { runReader :: r -> a }
+
+
+instance Functor (Reader r) where
+ fmap :: (a -> b) -> Reader r a -> Reader r b
+ fmap f (Reader ra) = Reader $ \r -> f (ra r)
+
+
+instance Applicative (Reader r) where
+ pure :: a -> Reader r a
+ pure a = Reader $ const a
+
+ (<*>) :: Reader r (a -> b) -> Reader r a -> Reader r b
+ (Reader rab) <*> (Reader ra) = Reader $ \r -> rab r (ra r)
+
+
+newtype HumanName = HumanName String deriving (Eq, Show)
+
+newtype DogName = DogName String deriving (Eq, Show)
+
+newtype Address = Address String deriving (Eq, Show)
+
+data Person = Person { humanName :: HumanName, dogName :: DogName, address :: Address } deriving (Eq, Show)
+
+data Dog = Dog { dogsName :: DogName, dogsAddress :: Address } deriving (Eq, Show)
+
+getDogR :: Person -> Dog
+getDogR = Dog <$> dogName <*> address
+
+getDogR' :: Reader Person Dog
+--getDogR' = Reader $ \p -> Dog (dogName p) (address p) -- ooh, bad soln
+getDogR' = Dog <$> Reader dogName <*> Reader address
+

src/haskellbook/ch22/ch22.hs

@@ -0,0 +1,42 @@
+import Data.Char
+
+cap :: [Char] -> [Char]
+cap xs = map toUpper xs
+
+rev :: [Char] -> [Char]
+rev xs = reverse xs
+
+composed :: [Char] -> [Char]
+composed = cap . rev
+
+fmapped :: [Char] -> [Char]
+fmapped = fmap cap rev
+
+tupled :: [Char] -> ([Char], [Char])
+tupled = (,) <$> cap <*> rev
+
+tupled' :: [Char] -> ([Char], [Char])
+tupled' = do
+ a <- rev
+ b <- cap
+ return (a, b)
+
+tupled'' :: [Char] -> ([Char], [Char])
+tupled'' = (rev . cap) >>= (,)
+
+
+newtype Reader r a = Reader { runReader :: r -> a }
+
+instance Functor (Reader r) where
+-- fmap :: (a -> b) -> Reader r a -> Reader r b
+ fmap f (Reader ra) = Reader $ \r -> f (ra r)
+
+ask :: Reader a a
+ask = Reader id
+
+myLiftA2 :: Applicative f => (a -> b -> c) -> f a -> f b -> f c
+myLiftA2 f a b = f <$> a <*> b
+
+asks :: (r -> a) -> Reader r a
+asks f = Reader f
+