Status

{-# LANGUAGE DataKinds #-}
{-# LANGUAGE GADTs #-}
{-# LANGUAGE KindSignatures #-}
{-# LANGUAGE TypeFamilies #-}

data IsStatus = IsRunning | IsFailed

data Status :: IsStatus -> * where
  Running :: Status 'IsRunning
  Failed :: Status 'IsFailed

data FailureReason = Unknown | PEBKAC deriving (Show)

type family GarbageCollect s :: * where
  GarbageCollect 'IsRunning = IO ()
  GarbageCollect 'IsFailed = [FailureReason] -> IO ()

garbageCollect :: Status s -> GarbageCollect s
garbageCollect Running = putStrLn "running"
garbageCollect Failed = \reasons -> putStrLn ("failed " ++ show reasons)

main :: IO ()
main = do
  garbageCollect Running
  garbageCollect Failed [Unknown, PEBKAC]