Working with the Reader monad transformer

In the previous recipe, we implemented our own State Monad transformer. In this recipe, we will revisit the mtl library and use the Reader monad transformer. The Reader monad transformer is a restricted version of the State Monad transformer in which we are allowed only to get the state (but not modify it).

How to do it...

1. Create a new project read-trans using the simple Stack template.
2. Open read-trans.cabal and add the following dependencies in the build-depends subsection of the executable section:

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

3. Open src/Main.hs. We will add our example over here. Import the following module after the initial module declaration:

       import Control.Monad.Reader

4. Write an example in which we keep read from an integer state, and check if our state meets certain criteria:

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