Computing a fibonacci number with State Monad

In this recipe, we will use the State Monad as defined by the mtl library. The mtl library defines many useful, well-articulated monads. We will use the State Monad to calculate a fibonacci number. 

How to do it...

1. Create a new project using the simple Stack template:

        stack new fibonacci-state simple

2. Open fibonacci-state.cabal and add packages mtl and containers to the build-depends subsection of the executable section:

         executable fibonacci-state
         hs-source-dirs: src
         main-is: Main.hs
         default-language: Haskell2010
         build-depends: base >= 4.7 && < 5
                 , mtl
                 , containers

3. Build the project so that the dependent packages will be pulled by stack:

        stack build

4. Open src/Main.hs. We will be editing this file.
5. Import the following modules after initial module declaration:

        import Control.Applicative
        import Control.Monad.State
        import Data...