MinimalExample.hs

#!/usr/bin/env cabal
{- cabal:
build-depends: base
               , dependent-sum
               , mtl
-}

{-# language GADTs #-}
{-# language LambdaCase #-}

import Data.Dependent.Sum (DSum(..))
import Data.Functor.Identity

data MinimalTag a where
  MinimalTag_String :: MinimalTag String
  MinimalTag_Int :: MinimalTag Int
  MinimalTag_Double :: MinimalTag Double
  MinimalTag_Integer :: MinimalTag Integer
  MinimalTag_Float :: MinimalTag Float
  
val :: DSum MinimalTag Identity
val = MinimalTag_Int :=> Identity 42

main :: IO ()
main = putStrLn $ function val
 
function :: DSum MinimalTag Identity -> String
function = \case
  MinimalTag_String :=> Identity string -> show string
  MinimalTag_Int :=> Identity int -> show int
  MinimalTag_Double :=> Identity double -> show double
  MinimalTag_Integer :=> Identity integer -> show integer
  MinimalTag_Float :=> Identity float -> show float