A program defined using 'Fix'

fix.hs

{-# LANGUAGE DeriveFunctor #-}
{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE StandaloneDeriving #-}
{-# LANGUAGE UndecidableInstances #-}

data Program f = Running (f (Program f))

deriving instance (Show (f (Program f))) => Show (Program f)

data Instruction r =
    Increment r
  | Decrement r
  | Terminate
  deriving (Functor, Show)

increment :: Program Instruction -> Program Instruction
increment = Running . Increment

decrement :: Program Instruction -> Program Instruction
decrement = Running . Decrement

terminate :: Program Instruction
terminate = Running Terminate

program :: Program Instruction
program =
    increment
  . increment
  . decrement
  $ terminate