subtree(users/wpcarro): docking briefcase at '24f5a642'

git-subtree-dir: users/wpcarro
git-subtree-mainline: 464bbcb15c
git-subtree-split: 24f5a642af
Change-Id: I6105b3762b79126b3488359c95978cadb3efa789

users/wpcarro/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)