From 0f1146cc4cabd9fda4d270e0f91bfb98a5d75554 Mon Sep 17 00:00:00 2001
From: William Carroll <wpcarro@gmail.com>
Date: Thu, 16 Jul 2020 23:50:00 +0100
Subject: [PATCH] Partially complete the "Basic Libraries" chapter exercises

I was instructed to benchmark these functions, but I couldn't get the
benchmarking library to run using Nix -- although I'm *sure* it's
possible. Unfortunately the book recommends using `stack`, which I couldn't
reproduce.
---
 .../basic-libraries.hs                        | 60 +++++++++++++++++++
 1 file changed, 60 insertions(+)
 create mode 100644 scratch/haskell-programming-from-first-principles/basic-libraries.hs

diff --git a/scratch/haskell-programming-from-first-principles/basic-libraries.hs b/scratch/haskell-programming-from-first-principles/basic-libraries.hs
new file mode 100644
index 000000000..bb1f89987
--- /dev/null
+++ b/scratch/haskell-programming-from-first-principles/basic-libraries.hs
@@ -0,0 +1,60 @@
+module BasicLibrariesScratch where
+
+import Data.Function ((&))
+
+--------------------------------------------------------------------------------
+newtype DList a = DL { unDL :: [a] -> [a] }
+
+instance (Show a) => Show (DList a) where
+  show (DL x) = "DL " ++ show (x [])
+
+-- | Create an empty difference list.
+emptyDList :: DList a
+emptyDList = DL $ \xs -> xs
+{-# INLINE emptyDList #-}
+
+-- | Create a difference list with `x` as the only member.
+singleton :: a -> DList a
+singleton x =  DL $ \xs -> x : xs
+{-# INLINE singleton #-}
+
+-- | Convert the DList into a list.
+toList :: DList a -> [a]
+toList (DL unDL) = unDL mempty
+{-# INLINE toList #-}
+
+-- | Add an element to the end of a DList.
+infixr `snoc`
+snoc :: a -> DList a -> DList a
+snoc x (DL xs) = DL $ \ys -> xs (x : ys)
+{-# INLINE snoc #-}
+
+-- | Add an element to the beginning of a DList.
+infixr `cons`
+cons :: a -> DList a -> DList a
+cons x (DL xs) = DL $ \ys -> x : xs ys
+{-# INLINE cons #-}
+
+-- | Combine two DLists together.
+append :: DList a -> DList a -> DList a
+append (DL xs) (DL ys) = DL $ \zs -> zs & ys & xs
+{-# INLINE append #-}
+
+--------------------------------------------------------------------------------
+data Queue a =
+  Queue { one :: [a]
+        , two :: [a]
+        } deriving (Show, Eq)
+
+emptyQueue :: Queue a
+emptyQueue = Queue mempty mempty
+
+enqueue :: a -> Queue a -> Queue a
+enqueue x (Queue en de) = Queue (x:en) de
+
+dequeue :: Queue a -> Maybe (a, Queue a)
+dequeue (Queue [] []) = Nothing
+dequeue (Queue en []) =
+  let (d:de) = reverse en
+  in Just (d, Queue de [])
+dequeue (Queue en (d:de)) = Just (d, Queue en de)