Add remaining haskell book exercises

Haskell-book/15/optional/.gitignore

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

Haskell-book/15/optional/Setup.hs

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+import Distribution.Simple
+main = defaultMain

Haskell-book/15/optional/app/First.hs

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+module Main where
+
+import Data.Monoid
+import Optional
+import Test.QuickCheck
+
+newtype First' a =
+    First' { getFirst' :: Optional a }
+    deriving (Eq, Show)
+
+instance Monoid (First' a) where
+    mempty = First' Nada
+    mappend (First' Nada) x = x
+    mappend x _ = x
+
+instance Arbitrary a => Arbitrary (First' a) where
+    arbitrary = frequency [ (1, return $ First' Nada)
+                          , (1, fmap (First' . Only) arbitrary) ]
+
+firstMappend :: First' a
+             -> First' a
+             -> First' a
+firstMappend = mappend
+
+type FirstMappend =
+    First' String
+  -> First' String
+  -> First' String
+  -> Bool
+
+type FstId =
+    First' String -> Bool
+
+monoidAssoc :: (Eq m, Monoid m) => m -> m -> m -> Bool
+monoidAssoc a b c =
+    (a <> (b <> c)) == ((a <> b) <> c)
+
+monoidLeftIdentity :: (Eq m, Monoid m) => m -> Bool
+monoidLeftIdentity a = (mempty <> a) == a
+
+monoidRightIdentity :: (Eq m, Monoid m) => m -> Bool
+monoidRightIdentity a = (a <> mempty) == a
+
+main :: IO ()
+main = do
+    quickCheck (monoidAssoc :: FirstMappend)
+    quickCheck (monoidLeftIdentity :: FstId)
+    quickCheck (monoidRightIdentity :: FstId)

Haskell-book/15/optional/package.yaml

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

Haskell-book/15/optional/src/Optional.hs

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+module Optional where
+
+data Optional a =
+    Nada
+  | Only a
+  deriving (Eq, Show)
+
+instance Monoid a => Monoid (Optional a) where
+    mempty = Nada
+    mappend x Nada = x
+    mappend Nada x = x
+    mappend (Only x) (Only y) = Only (mappend x y)

Haskell-book/15/optional/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
+# resolver:
+#  name: custom-snapshot
+#  location: "./custom-snapshot.yaml"
+resolver: lts-10.2
+
+# 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
+#   extra-dep: true
+#  subdirs:
+#  - auto-update
+#  - wai
+#
+# A package marked 'extra-dep: true' will only be built if demanded by a
+# non-dependency (i.e. a user package), and its test suites and benchmarks
+# will not be run. This is useful for tweaking upstream packages.
+packages:
+- .
+# Dependency packages to be pulled from upstream that are not in the resolver
+# (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: ">=1.6"
+#
+# 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/15/optional/test/Spec.hs

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+import Data.Monoid
+import Optional
+import Test.Hspec
+
+main :: IO ()
+main = hspec $ do
+    describe "Sum" $ do
+        it "1 + 1 is 2" $ do
+            Only (Sum 1) `mappend` Only (Sum 1) `shouldBe` Only (Sum {getSum = 2})
+    describe "Product" $ do
+        it "4 * 2 is 8" $ do
+            Only (Product 4) `mappend` Only (Product 2) `shouldBe` Only (Product {getProduct = 8})
+    describe "Sum with Nada" $ do
+        it "1 + Nada is 1" $ do
+            Only (Sum 1) `mappend` Nada `shouldBe` Only (Sum {getSum = 1})
+    describe "List" $ do
+        it "[1] <> Nada is [1]" $ do
+            Only [1] `mappend` Nada `shouldBe` Only [1]
+    describe "Nada with sum" $ do
+        it "Nada + 1 is 1" $ do
+            Nada `mappend` Only (Sum 1) `shouldBe` Only (Sum {getSum = 1})