Bluefin bracket fun

gistfile1.txt

-- A stateful funciton with a counter and a boolean saying if it has terminated
-- We use bracket to always set the termination boolean, even if there is an exception
-- in the initial action.
ghci> let f st act = 
  bracket 
    (pure ()) 
    (\_ -> modify st (\(c,fin) -> (c, True))) 
    (\_ -> do { act; modify st (\(c, fin) -> ((c + 1), fin)) })
ghci> :t f
f :: (st :> es, Num a1) =>
     State (a1, Bool) st -> Eff es a2 -> Eff es ()
-- Normal execution, the counter is incremented and the termination bool is set
ghci> runPureEff $ runState (0, False) $ \st -> f st (pure ())
((),(1,True))
-- Exceptional execution, the counter is not incremented but the termination bool is still set
ghci> runPureEff $ runState (0, False) $ \st -> try $ \ex -> f st (throw ex 42)
(Left 42,(0,True))