How to do it...

  1. Create a new project shopping-cart using the simple Stack template:
        stack new shopping-cart simple
  1. Open shopping-cart.cabal and add a dependency on the containers library in the build-depends subsection of the executable subsection:
        executable shopping-cart
         hs-source-dirs:      src
         main-is:             Main.lhs
         default-language:    Haskell2010
         build-depends:       base >= 4.7 && < 5
                       , containers
  1. Open src/Main.hs; we will add our code here. Import the module Data.Set:
        module Main where

        import Data.Set as Set
  1. Create a type to represent a book. The book contains the ISBN number, title of the book, and name of the author:
        data Book = Book { isbn :: String
                 , title :: String
                 , author :: String }
                   deriving Show
  1. Create the equality and ordering ...

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