Lightweight checked exceptions in Haskell

Checked.hs

{-# LANGUAGE CPP                 #-}
{-# LANGUAGE FlexibleContexts    #-}
{-# LANGUAGE RankNTypes          #-}
{-# LANGUAGE ScopedTypeVariables #-}

#if __GLASGOW_HASKELL__ >= 710
{-# LANGUAGE AllowAmbiguousTypes #-}
#endif

-- | Lightweight checked exceptions
--
-- See <http://www.well-typed.com/blog/2015/07/checked-exceptions/>.
-- Written by Edsko de Vries. Copyright 2015 Well-Typed LLP.
--
-- Tested with ghc 7.2, 7.4, 7.6, 7.8 and 7.10.
module Checked where

import Control.Exception (Exception, IOException)
import Unsafe.Coerce (unsafeCoerce)
import qualified Control.Exception as E

{-------------------------------------------------------------------------------
  Main definitions
-------------------------------------------------------------------------------}

-- | Checked exceptions
class Throws e where
  throwChecked :: e -> IO a

-- | Wrap an action that may throw a checked exception
--
-- This is used internally in 'rethrowUnchecked' to avoid impredicative
-- instantiation of the type of 'unsafeCoerce'.
newtype Wrap e a = Wrap (Throws e => IO a)

-- | Rethrow checked exceptions as unchecked (regular) exceptions
rethrowUnchecked :: forall e a. (Throws e => IO a) -> (Exception e => IO a)
rethrowUnchecked act = aux act E.throwIO
  where
    aux :: (Throws e => IO a) -> ((e -> IO a) -> IO a)
    aux = unsafeCoerce . Wrap

-- | Catch a checked exception
--
-- This is the only way to discharge a 'Throws' type class constraint.
catchChecked :: Exception e => (Throws e => IO a) -> (e -> IO a) -> IO a
catchChecked = E.catch . rethrowUnchecked

{-------------------------------------------------------------------------------
  Additional definitions
-------------------------------------------------------------------------------}

-- | 'catchChecked' with the arguments reversed
handleChecked :: Exception e => (e -> IO a) -> (Throws e => IO a) -> IO a
handleChecked act handler = catchChecked handler act

-- | Throw an unchecked exception
--
-- This is just an alias for 'throw', but makes it evident that this is a very
-- intentional use of an unchecked exception.
throwUnchecked :: Exception e => e -> IO a
throwUnchecked = E.throwIO

-- | Rethrow IO exceptions as checked exceptions
checkIO :: Throws IOException => IO a -> IO a
checkIO = E.handle $ \(ex :: IOException) -> throwChecked ex

{-------------------------------------------------------------------------------
  Example
-------------------------------------------------------------------------------}

readFile' :: Throws IOException => FilePath -> IO String
readFile' = checkIO . readFile

readEtcPasswd :: IO String
readEtcPasswd = catchChecked (readFile' "/etc/passwd") $ \(ex :: IOException) ->
                  return "Could not read"

See improved version at https://gist.github.com/edsko/f1f566f77422398fba7d.