From 3665ea457bf30e4a0de7733608e3e16c8e18090c Mon Sep 17 00:00:00 2001
From: William Carroll <wpcarro@gmail.com>
Date: Sat, 11 Jul 2020 11:25:07 +0100
Subject: 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.
---
 .../composing-types.hs                             | 75 ++++++++++++++++++++++
 1 file changed, 75 insertions(+)
 create mode 100644 scratch/haskell-programming-from-first-principles/composing-types.hs

diff --git a/scratch/haskell-programming-from-first-principles/composing-types.hs b/scratch/haskell-programming-from-first-principles/composing-types.hs
new file mode 100644
index 000000000000..378cfb7ceae6
--- /dev/null
+++ b/scratch/haskell-programming-from-first-principles/composing-types.hs
@@ -0,0 +1,75 @@
+module ComposingTypesScratch where
+
+import Data.Function ((&))
+import Data.Bifunctor
+
+import qualified Data.Foldable as F
+
+--------------------------------------------------------------------------------
+
+newtype Identity a =
+  Identity { getIdentity :: a }
+  deriving (Eq, Show)
+
+newtype Compose f g a =
+  Compose { getCompose :: f (g a) }
+  deriving (Eq, Show)
+
+--------------------------------------------------------------------------------
+
+instance (Functor f, Functor g) => Functor (Compose f g) where
+  fmap f (Compose getCompose) = Compose $ (fmap . fmap) f getCompose
+
+instance (Applicative f, Applicative g) => Applicative (Compose f g) where
+  pure x = x & pure & pure & Compose
+  fgf <*> fga = undefined
+
+--------------------------------------------------------------------------------
+
+instance (Foldable f, Foldable g) => Foldable (Compose f g) where
+  foldMap toMonoid x = undefined
+
+instance (Traversable f, Traversable g) => Traversable (Compose f g) where
+  traverse = undefined
+
+--------------------------------------------------------------------------------
+
+data Deux a b = Deux a b deriving (Show, Eq)
+
+instance Bifunctor Deux where
+  bimap f g (Deux x y) = Deux (f x) (g y)
+
+data Const a b = Const a deriving (Show, Eq)
+
+instance Bifunctor Const where
+  bimap f _ (Const x) = Const (f x)
+
+data Drei a b c = Drei a b c deriving (Show, Eq)
+
+instance Bifunctor (Drei a) where
+  bimap f g (Drei x y z) = Drei x (f y) (g z)
+
+data SuperDrei a b c = SuperDrei a b deriving (Show, Eq)
+
+instance Bifunctor (SuperDrei a) where
+  bimap f g (SuperDrei x y) = SuperDrei x (f y)
+
+data SemiDrei a b c = SemiDrei a deriving (Show, Eq)
+
+instance Bifunctor (SemiDrei a) where
+  bimap _ _ (SemiDrei x) = SemiDrei x
+
+data Quadriceps a b c d = Quadzzz a b c d
+
+instance Bifunctor (Quadriceps a b) where
+  bimap f g (Quadzzz w x y z) = Quadzzz w x (f y) (g z)
+
+-- | Analogue for Either
+data LeftRight a b
+  = Failure a
+  | Success b
+  deriving (Show, Eq)
+
+instance Bifunctor LeftRight where
+  bimap f _ (Failure x) = Failure (f x)
+  bimap _ g (Success y) = Success (g y)
-- 
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