{-# LANGUAGE CPP #-}
{-# LANGUAGE RankNTypes #-}
{-| An example Haskell program to copy data from one handle to another might
look like this:
> main =
> withFile "inFile.txt" ReadMode $ \inHandle ->
> withFile "outFile.txt" WriteMode $ \outHandle ->
> copy inHandle outHandle
>
> -- A hypothetical function that copies data from one handle to another
> copy :: Handle -> Handle -> IO ()
`System.IO.withFile` is one of many functions that acquire some resource in
an exception-safe way. These functions take a callback function as an
argument and they invoke the callback on the resource when it becomes
available, guaranteeing that the resource is properly disposed if the
callback throws an exception.
These functions usually have a type that ends with the following pattern:
> Callback
> -- -----------
> withXXX :: ... -> (a -> IO r) -> IO r
Here are some examples of this pattern from the @base@ libraries:
> withArray :: Storable a => [a] -> (Ptr a -> IO r) -> IO r
> withBuffer :: Buffer e -> (Ptr e -> IO r) -> IO r
> withCAString :: String -> (CString -> IO r) -> IO r
> withForeignPtr :: ForeignPtr a -> (Ptr a -> IO r) -> IO r
> withMVar :: Mvar a -> (a -> IO r) -> IO r
> withPool :: (Pool -> IO r) -> IO r
Acquiring multiple resources in this way requires nesting callbacks.
However, you can wrap anything of the form @((a -> IO r) -> IO r)@ in the
`Managed` monad, which translates binds to callbacks for you:
> import Control.Monad.Managed
> import System.IO
>
> inFile :: FilePath -> Managed Handle
> inFile filePath = managed (withFile filePath ReadMode)
>
> outFile :: FilePath -> Managed Handle
> outFile filePath = managed (withFile filePath WriteMode)
>
> main = runManaged $ do
> inHandle <- inFile "inFile.txt"
> outHandle <- outFile "outFile.txt"
> liftIO (copy inHandle outHandle)
... or you can just wrap things inline:
> main = runManaged $ do
> inHandle <- managed (withFile "inFile.txt" ReadMode)
> outHandle <- managed (withFile "outFile.txt" WriteMode)
> liftIO (copy inHandle outHandle)
Additionally, since `Managed` is a `Monad`, you can take advantage of all
your favorite combinators from "Control.Monad". For example, the
`Foreign.Marshal.Utils.withMany` function from "Foreign.Marshal.Utils"
becomes a trivial wrapper around `mapM`:
> withMany :: (a -> (b -> IO r) -> IO r) -> [a] -> ([b] -> IO r) -> IO r
> withMany f = with . mapM (Managed . f)
Another reason to use `Managed` is that if you wrap a `Monoid` value in
`Managed` you get back a new `Monoid`:
> instance Monoid a => Monoid (Managed a)
This lets you combine managed resources transparently. You can also lift
operations from some numeric type classes this way, too, such as the `Num`
type class.
NOTE: `Managed` may leak space if used in an infinite loop like this
example:
> import Control.Monad
> import Control.Monad.Managed
>
> main = runManaged (forever (liftIO (print 1)))
If you need to acquire a resource for a long-lived loop, you can instead
acquire the resource first and run the loop in `IO`, using either of the
following two equivalent idioms:
> with resource (\r -> forever (useThe r))
>
> do r <- resource
> liftIO (forever (useThe r))
-}
module Control.Monad.Managed (
-- * Managed
Managed,
MonadManaged(..),
managed,
managed_,
with,
runManaged,
-- * Re-exports
-- $reexports
module Control.Monad.IO.Class
) where
import Control.Monad.IO.Class (MonadIO(liftIO))
import Control.Monad.Trans.Class (lift)
#if MIN_VERSION_base(4,8,0)
import Control.Applicative (liftA2)
#else
import Control.Applicative
import Data.Monoid (Monoid(..))
#endif
#if !(MIN_VERSION_base(4,11,0))
import Data.Semigroup (Semigroup(..))
#endif
import qualified Control.Monad.Trans.Cont as Cont
#if MIN_VERSION_transformers(0,4,0)
import qualified Control.Monad.Trans.Except as Except
#endif
import qualified Control.Monad.Trans.Identity as Identity
import qualified Control.Monad.Trans.Maybe as Maybe
import qualified Control.Monad.Trans.Reader as Reader
import qualified Control.Monad.Trans.RWS.Lazy as RWS.Lazy
import qualified Control.Monad.Trans.RWS.Strict as RWS.Strict
import qualified Control.Monad.Trans.State.Lazy as State.Lazy
import qualified Control.Monad.Trans.State.Strict as State.Strict
import qualified Control.Monad.Trans.Writer.Lazy as Writer.Lazy
import qualified Control.Monad.Trans.Writer.Strict as Writer.Strict
-- | A managed resource that you acquire using `with`
newtype Managed a = Managed { (>>-) :: forall r . (a -> IO r) -> IO r }
instance Functor Managed where
fmap f mx = Managed (\return_ ->
mx >>- \x ->
return_ (f x) )
instance Applicative Managed where
pure r = Managed (\return_ ->
return_ r )
mf <*> mx = Managed (\return_ ->
mf >>- \f ->
mx >>- \x ->
return_ (f x) )
instance Monad Managed where
return r = Managed (\return_ ->
return_ r )
ma >>= f = Managed (\return_ ->
ma >>- \a ->
f a >>- \b ->
return_ b )
instance MonadIO Managed where
liftIO m = Managed (\return_ -> do
a <- m
return_ a )
instance Semigroup a => Semigroup (Managed a) where
(<>) = liftA2 (<>)
instance Monoid a => Monoid (Managed a) where
mempty = pure mempty
#if !(MIN_VERSION_base(4,11,0))
mappend = liftA2 mappend
#endif
instance Num a => Num (Managed a) where
fromInteger = pure . fromInteger
negate = fmap negate
abs = fmap abs
signum = fmap signum
(+) = liftA2 (+)
(*) = liftA2 (*)
(-) = liftA2 (-)
instance Fractional a => Fractional (Managed a) where
fromRational = pure . fromRational
recip = fmap recip
(/) = liftA2 (/)
instance Floating a => Floating (Managed a) where
pi = pure pi
exp = fmap exp
sqrt = fmap sqrt
log = fmap log
sin = fmap sin
tan = fmap tan
cos = fmap cos
asin = fmap sin
atan = fmap atan
acos = fmap acos
sinh = fmap sinh
tanh = fmap tanh
cosh = fmap cosh
asinh = fmap asinh
atanh = fmap atanh
acosh = fmap acosh
(**) = liftA2 (**)
logBase = liftA2 logBase
{-| You can embed a `Managed` action within any `Monad` that implements
`MonadManaged` by using the `using` function
All instances must obey the following two laws:
> using (return x) = return x
>
> using (m >>= f) = using m >>= \x -> using (f x)
-}
class MonadIO m => MonadManaged m where
using :: Managed a -> m a
instance MonadManaged Managed where
using = id
instance MonadManaged m => MonadManaged (Cont.ContT r m) where
using m = lift (using m)
#if MIN_VERSION_transformers(0,4,0)
instance MonadManaged m => MonadManaged (Except.ExceptT e m) where
using m = lift (using m)
#endif
instance MonadManaged m => MonadManaged (Identity.IdentityT m) where
using m = lift (using m)
instance MonadManaged m => MonadManaged (Maybe.MaybeT m) where
using m = lift (using m)
instance MonadManaged m => MonadManaged (Reader.ReaderT r m) where
using m = lift (using m)
instance (Monoid w, MonadManaged m) => MonadManaged (RWS.Lazy.RWST r w s m) where
using m = lift (using m)
instance (Monoid w, MonadManaged m) => MonadManaged (RWS.Strict.RWST r w s m) where
using m = lift (using m)
instance MonadManaged m => MonadManaged (State.Strict.StateT s m) where
using m = lift (using m)
instance MonadManaged m => MonadManaged (State.Lazy.StateT s m) where
using m = lift (using m)
instance (Monoid w, MonadManaged m) => MonadManaged (Writer.Strict.WriterT w m) where
using m = lift (using m)
instance (Monoid w, MonadManaged m) => MonadManaged (Writer.Lazy.WriterT w m) where
using m = lift (using m)
-- | Build a `Managed` value
managed :: (forall r . (a -> IO r) -> IO r) -> Managed a
managed = Managed
-- | Like 'managed' but for resource-less operations.
managed_ :: (forall r. IO r -> IO r) -> Managed ()
managed_ f = managed $ \g -> f $ g ()
{-| Acquire a `Managed` value
This is a potentially unsafe function since it allows a resource to escape
its scope. For example, you might use `Managed` to safely acquire a
file handle, like this:
> import qualified System.IO as IO
>
> example :: Managed Handle
> example = managed (IO.withFile "foo.txt" IO.ReadMode)
... and if you never used the `with` function then you would never run the
risk of accessing the `Handle` after the file was closed. However, if you
use `with` then you can incorrectly access the handle after the handle is
closed, like this:
> bad :: IO ()
> bad = do
> handle <- with example return
> IO.hPutStrLn handle "bar" -- This will fail because the handle is closed
... so only use `with` if you know what you are doing and you're returning
a value that is not a resource being managed.
-}
with :: Managed a -> (a -> IO r) -> IO r
with = (>>-)
-- | Run a `Managed` computation, enforcing that no acquired resources leak
runManaged :: Managed () -> IO ()
runManaged m = m >>- return
{- $reexports
"Control.Monad.IO.Class" re-exports 'MonadIO'
-}