Partially complete some of the exercises for Composing Types
I believe there are two exercises sets in the "Composing Types" chapter. Here are *some* of my answers so far... I'm having trouble implementing Foldable for Compose. I was able to implement a version of it by adding the (Functor f) constraint to the instance signature, but I think I cheated. I will revisit these problems as well as the earlier exercises later.
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module ComposingTypesScratch where
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import Data.Function ((&))
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import Data.Bifunctor
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import qualified Data.Foldable as F
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--------------------------------------------------------------------------------
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newtype Identity a =
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Identity { getIdentity :: a }
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deriving (Eq, Show)
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newtype Compose f g a =
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Compose { getCompose :: f (g a) }
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deriving (Eq, Show)
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--------------------------------------------------------------------------------
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instance (Functor f, Functor g) => Functor (Compose f g) where
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fmap f (Compose getCompose) = Compose $ (fmap . fmap) f getCompose
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instance (Applicative f, Applicative g) => Applicative (Compose f g) where
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pure x = x & pure & pure & Compose
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fgf <*> fga = undefined
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--------------------------------------------------------------------------------
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instance (Foldable f, Foldable g) => Foldable (Compose f g) where
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foldMap toMonoid x = undefined
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instance (Traversable f, Traversable g) => Traversable (Compose f g) where
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traverse = undefined
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--------------------------------------------------------------------------------
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data Deux a b = Deux a b deriving (Show, Eq)
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instance Bifunctor Deux where
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bimap f g (Deux x y) = Deux (f x) (g y)
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data Const a b = Const a deriving (Show, Eq)
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instance Bifunctor Const where
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bimap f _ (Const x) = Const (f x)
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data Drei a b c = Drei a b c deriving (Show, Eq)
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instance Bifunctor (Drei a) where
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bimap f g (Drei x y z) = Drei x (f y) (g z)
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data SuperDrei a b c = SuperDrei a b deriving (Show, Eq)
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instance Bifunctor (SuperDrei a) where
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bimap f g (SuperDrei x y) = SuperDrei x (f y)
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data SemiDrei a b c = SemiDrei a deriving (Show, Eq)
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instance Bifunctor (SemiDrei a) where
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bimap _ _ (SemiDrei x) = SemiDrei x
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data Quadriceps a b c d = Quadzzz a b c d
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instance Bifunctor (Quadriceps a b) where
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bimap f g (Quadzzz w x y z) = Quadzzz w x (f y) (g z)
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-- | Analogue for Either
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data LeftRight a b
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= Failure a
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| Success b
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deriving (Show, Eq)
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instance Bifunctor LeftRight where
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bimap f _ (Failure x) = Failure (f x)
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bimap _ g (Success y) = Success (g y)
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