Haskell Cookbook

How to do it...

1. Create a new project working-with-existentials with a simple stack template:

       stack new working-with-existentials simple

2. Open src/Main.hs. We will be adding our source here.
3. Enable the existential support, and define the Main module. Also add StandaloneDeriving extension.

        {-# LANGUAGE ExistentialQuantification, StandaloneDeriving #-}

        module Main where

4. Define an existential type:

        data Display = forall a . Show a => Display a

5. Create a standalone instance of Show Display:

        deriving instance Show Display 

6. Create a list of Displays, and, call it in the main function:

 displayList :: [Display]
 displayList = [ Display 10
              , Display ["One","Two","Three"]
              , Display 10.0
              , Display (Just "Something")
              , Display True ] ...