A program defined using 'Free'

free.hs

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

data Program f a =
    Running (f (Program f a))
  | Terminated a

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

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

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

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

terminate :: Program Instruction a
terminate = Running Terminate

sigkill :: Program f Int
sigkill = Terminated 1

program :: Program Instruction Int
program =
    increment
  . increment
  . decrement
  $ sigkill