Add remaining haskell book exercises

Haskell-book/28/DifferenceList/.gitignore

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+.stack-work/
+DifferenceList.cabal
+*~

Haskell-book/28/DifferenceList/Setup.hs

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+import Distribution.Simple
+main = defaultMain

Haskell-book/28/DifferenceList/app/Main.hs

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+module Main where
+
+import Criterion.Main
+import Data.DList
+import qualified Data.Queue as Q
+import qualified Data.Sequence as S
+
+schlemiel :: Int -> [Int]
+schlemiel i = go i []
+    where go 0 xs = xs
+          go n xs = go (n - 1) ([n] ++ xs)
+
+constructDlist :: Int -> [Int]
+constructDlist i = toList $ go i empty
+    where go 0 xs = xs
+          go n xs = go (n - 1) (singleton n `append` xs)
+
+processQueue :: Int -> Q.Queue Int
+processQueue i = clear $ Q.pop $ fill i Q.empty
+    where fill 0 xs = xs
+          fill n xs = fill (n - 1) (Q.push n xs)
+          clear Nothing = Q.empty
+          clear (Just xs) = clear $ Q.pop $ snd xs
+
+processList :: Int -> [Int]
+processList i = go (schlemiel i)
+    where go [] = []
+          go (x:xs) = xs
+
+processSeq :: Int -> S.Seq Int
+processSeq i = go $ S.fromList $ schlemiel i
+    where go xs = if S.null xs then xs else go (S.deleteAt 0 xs)
+
+main :: IO ()
+main = defaultMain
+    [ bench "concat list" $
+      whnf schlemiel 123456
+    , bench "concat dlist" $
+      whnf constructDlist 123456
+    , bench "process queue" $
+      whnf processQueue 12345
+    , bench "process list" $
+      whnf processList 12345
+    , bench "process sequence" $
+      whnf processSeq 12345
+    ]

Haskell-book/28/DifferenceList/package.yaml

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+name:                DifferenceList
+version:             0.1.0.0
+license:             BSD3
+author:              "Eugen Wissner"
+maintainer:          "belka@caraus.de"
+copyright:           "2018 Eugen Wissner"
+
+dependencies:
+- base >= 4.7 && < 5
+- containers
+
+library:
+  source-dirs: src
+
+tests:
+  DifferenceList-test:
+    main:                Spec.hs
+    source-dirs:         test
+    ghc-options:
+    - -threaded
+    - -rtsopts
+    - -with-rtsopts=-N
+    dependencies:
+    - DifferenceList
+    - hspec
+
+executables:
+  benchmark:
+    main:                Main.hs
+    source-dirs:         app
+    ghc-options:
+    - -threaded
+    - -rtsopts
+    - -with-rtsopts=-N
+    dependencies:
+    - DifferenceList
+    - criterion

Haskell-book/28/DifferenceList/src/Data/DList.hs

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+module Data.DList
+    ( DList(..)
+    , empty
+    , singleton
+    , toList
+    , cons
+    , snoc
+    , append
+    ) where
+
+newtype DList a = DL { unDL :: [a] -> [a] }
+
+empty :: DList a
+empty = DL ([] ++)
+{-# INLINE empty #-}
+
+singleton :: a -> DList a
+singleton x = DL ([x] ++)
+{-# INLINE singleton #-}
+
+toList :: DList a -> [a]
+toList xsf = unDL xsf []
+{-# INLINE toList #-}
+
+-- Prepend a single element to a dlist.
+infixr `cons`
+cons :: a -> DList a -> DList a
+cons x xs = DL ((x:) . unDL xs)
+{-# INLINE cons #-}
+
+-- Append a single element to a dlist.
+infixl `snoc`
+snoc :: DList a -> a -> DList a
+snoc xs x = append xs $ singleton x
+{-# INLINE snoc #-}
+
+-- Append dlists.
+append :: DList a -> DList a -> DList a
+append xsf ysf = DL $ (unDL xsf) . (unDL ysf)
+{-# INLINE append #-}

Haskell-book/28/DifferenceList/src/Data/Queue.hs

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+module Data.Queue
+    ( Queue(..)
+    , empty
+    , isEmpty
+    , push
+    , pop
+    ) where
+
+-- From Okasaki's Purely Functional Data Structures
+data Queue a =
+    Queue { enqueue :: [a]
+          , dequeue :: [a]
+          } deriving (Eq, Show)
+
+isEmpty :: Queue a -> Bool
+isEmpty xs = length (enqueue xs) == 0
+          && length (dequeue xs) == 0
+
+empty :: Queue a
+empty = Queue [] []
+
+-- adds an item
+push :: a -> Queue a -> Queue a
+push x xs = Queue { enqueue = x : (enqueue xs)
+                  , dequeue = dequeue xs }
+
+pop :: Queue a -> Maybe (a, Queue a)
+pop xs = popFromLists (enqueue xs) (dequeue xs)
+    where popFromLists [] [] = Nothing
+          popFromLists en (d:de) = Just (d, Queue en de)
+          popFromLists en [] = popFromLists [] (reverse en)
+
+-- We’re going to give you less code this time, but your task is to
+-- implement the above and write a benchmark comparing it against
+-- performing alternating pushes and pops from a queue based on a
+-- single list. Alternating so that you can’t take advantage of reversing
+-- the list after a long series of pushes in order to perform a long series
+-- of pops efficiently.
+--
+-- Don’t forget to handle the case where the dequeue is empty and
+-- you need to shift items from the enqueue to the dequeue. You need
+-- to do so without violating “first come, first served”.
+-- Lastly, benchmark it against Sequence. Come up with a variety of
+-- tests. Add additional operations for your Queue type if you want.

Haskell-book/28/DifferenceList/stack.yaml

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+# This file was automatically generated by 'stack init'
+#
+# Some commonly used options have been documented as comments in this file.
+# For advanced use and comprehensive documentation of the format, please see:
+# https://docs.haskellstack.org/en/stable/yaml_configuration/
+
+# Resolver to choose a 'specific' stackage snapshot or a compiler version.
+# A snapshot resolver dictates the compiler version and the set of packages
+# to be used for project dependencies. For example:
+#
+# resolver: lts-3.5
+# resolver: nightly-2015-09-21
+# resolver: ghc-7.10.2
+# resolver: ghcjs-0.1.0_ghc-7.10.2
+#
+# The location of a snapshot can be provided as a file or url. Stack assumes
+# a snapshot provided as a file might change, whereas a url resource does not.
+#
+# resolver: ./custom-snapshot.yaml
+# resolver: https://example.com/snapshots/2018-01-01.yaml
+resolver: lts-11.10
+
+# User packages to be built.
+# Various formats can be used as shown in the example below.
+#
+# packages:
+# - some-directory
+# - https://example.com/foo/bar/baz-0.0.2.tar.gz
+# - location:
+#    git: https://github.com/commercialhaskell/stack.git
+#    commit: e7b331f14bcffb8367cd58fbfc8b40ec7642100a
+# - location: https://github.com/commercialhaskell/stack/commit/e7b331f14bcffb8367cd58fbfc8b40ec7642100a
+#  subdirs:
+#  - auto-update
+#  - wai
+packages:
+- .
+# Dependency packages to be pulled from upstream that are not in the resolver
+# using the same syntax as the packages field.
+# (e.g., acme-missiles-0.3)
+# extra-deps: []
+
+# Override default flag values for local packages and extra-deps
+# flags: {}
+
+# Extra package databases containing global packages
+# extra-package-dbs: []
+
+# Control whether we use the GHC we find on the path
+# system-ghc: true
+#
+# Require a specific version of stack, using version ranges
+# require-stack-version: -any # Default
+# require-stack-version: ">=1.7"
+#
+# Override the architecture used by stack, especially useful on Windows
+# arch: i386
+# arch: x86_64
+#
+# Extra directories used by stack for building
+# extra-include-dirs: [/path/to/dir]
+# extra-lib-dirs: [/path/to/dir]
+#
+# Allow a newer minor version of GHC than the snapshot specifies
+# compiler-check: newer-minor

Haskell-book/28/DifferenceList/test/Spec.hs

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+module Main where
+
+import Test.Hspec
+import Data.Queue
+
+main :: IO ()
+main = hspec $ do
+    describe "empty" $ do
+        it "returns an empty queue" $ do
+            (empty :: Queue Int) `shouldBe` (Queue [] [])
+
+    describe "push" $ do
+        it "puts an element into an empty queue" $ do
+            (push 5 empty) `shouldBe` (Queue [5] [])
+
+    describe "pop" $ do
+        it "takes the only element from the queue" $ do
+            (pop (Queue [5] [])) `shouldBe` (Just (5, Queue [] []))
+        it "returns nothing if the queue is empty" $ do
+            (pop ((Queue [] [])::Queue Int)) `shouldBe` Nothing
+        it "takes elements in the FIFO order" $ do
+            let queue = push 3 (push 5 empty)
+             in pop queue `shouldBe` Just (5, Queue [] [3])
+
+    describe "isEmpty" $ do
+        it "tells when the queue is empty" $ do
+            (isEmpty (empty :: Queue Int)) `shouldBe` True
+        it "tells when the enqueue part isn't empty" $ do
+            let queue = push 3 empty
+             in isEmpty queue `shouldBe` False
+        it "tells when the dequeue part isn't empty" $ do
+            let queue = fmap snd (pop $ push 3 (push 5 empty))
+             in fmap isEmpty queue `shouldBe` Just False