Add remaining haskell book exercises

Haskell-book/15/optional.cabal

@@ -0,0 +1,45 @@
+name:           optional
+version:        0.1.0.0
+author:         Eugen Wissner
+maintainer:     belka@caraus.de
+copyright:      2018 Eugen Wissner
+license:        BSD3
+build-type:     Simple
+cabal-version:  >= 1.10
+
+library
+  hs-source-dirs:
+      src
+  build-depends:
+      base >=4.7 && <5
+  exposed-modules:
+      Optional
+  other-modules:
+      Paths_optional
+  default-language: Haskell2010
+
+test-suite optional-test
+  type: exitcode-stdio-1.0
+  main-is: Spec.hs
+  hs-source-dirs:
+      test
+  ghc-options: -threaded -rtsopts -with-rtsopts=-N
+  build-depends:
+      base >=4.7 && <5
+    , hspec
+    , optional
+  other-modules:
+      Paths_optional
+  default-language: Haskell2010
+
+test-suite first-test
+  type: exitcode-stdio-1.0
+  main-is: First.hs
+  hs-source-dirs:
+      app
+  ghc-options: -threaded -rtsopts -with-rtsopts=-N
+  build-depends:
+      base >=4.7 && <5
+    , QuickCheck
+    , optional
+  default-language: Haskell2010