rapid-0.1.1: Rapid.hs
-- Copyright 2016 Ertugrul Söylemez
--
-- Licensed under the Apache License, Version 2.0 (the "License");
-- you may not use this file except in compliance with the License.
-- You may obtain a copy of the License at
--
-- http://www.apache.org/licenses/LICENSE-2.0
--
-- Unless required by applicable law or agreed to in writing, software
-- distributed under the License is distributed on an "AS IS" BASIS,
-- WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-- See the License for the specific language governing permissions and
-- limitations under the License.
{-# LANGUAGE ScopedTypeVariables #-}
-- |
-- Module: Rapid
-- Copyright: Copyright 2016 Ertugrul Söylemez
-- License: Apache License 2.0
-- Maintainer: Ertugrul Söylemez <esz@posteo.de>
--
-- This library provides a safer and more convenient wrapper around the
-- <https://hackage.haskell.org/package/foreign-store foreign-store library>.
--
-- You can use it for background services within a GHCi session that
-- survive loading, reloading and unloading modules, which is
-- particularly useful when writing long-running programs like servers
-- and user interfaces.
--
-- __Please read the "Safety and securty" section below!__
module Rapid
( -- * Introduction
-- $intro
-- ** Communication
-- $communication
-- ** Reusing expensive resources
-- $reusing
-- ** Emacs integration
-- $emacs
-- ** Safety and security
-- $safety
-- * Hot code reloading
Rapid,
rapid,
-- * Threads
restart,
start,
stop,
-- * Communication
createRef,
deleteRef,
writeRef
)
where
import Control.Concurrent.Async
import Control.Concurrent.STM
import Control.Exception
import Data.Dynamic
import Data.Map.Strict (Map)
import qualified Data.Map.Strict as M
import Data.Word
import Foreign.Store
-- | Handle to the current Rapid state.
data Rapid k =
Rapid {
rLock :: TVar Bool, -- ^ Lock on the current state.
rRefs :: TVar (Map k Dynamic), -- ^ Mutable variables.
rThreads :: TVar (Map k (Async ())) -- ^ Active threads.
}
-- | Cancel the given thread and wait for it to finish.
cancelAndWait :: Async a -> IO ()
cancelAndWait tv = do
cancel tv
() <$ waitCatch tv
-- | Get the value of the mutable variable with the given name. If it
-- does not exist, it is created and initialised with the value returned
-- by the given action.
--
-- Mutable variables should only be used with values that can be
-- garbage-collected, for example communication primitives like
-- 'Control.Concurrent.MVar.MVar' and 'TVar', but also pure run-time
-- information that is expensive to generate, for example the parsed
-- contents of a file.
createRef
:: (Ord k, Typeable a)
=> Rapid k -- ^ Rapid state handle.
-> k -- ^ Name of the mutable variable.
-> IO a -- ^ Action to create.
-> IO a
createRef r k gen =
withRef r k $ \mxd ->
case mxd of
Nothing -> fmap (\x -> (Just (toDyn x), x)) gen
Just xd
| Just x <- fromDynamic xd -> pure (Just xd, x)
| otherwise -> throwIO (userError "createRef: Wrong reference type")
-- | Delete the mutable variable with the given name, if it exists.
deleteRef
:: (Ord k)
=> Rapid k -- ^ Rapid state handle.
-> k -- ^ Name of the mutable variable.
-> IO ()
deleteRef r k =
withRef r k (\_ -> pure (Nothing, ()))
-- | Retrieve the current Rapid state handle, and pass it to the given
-- continuation. If the state handle doesn't exist, it is created. The
-- key type @k@ is used for naming reloadable services like threads.
--
-- __Warning__: The key type must not change during a session. If you
-- need to change the key type, currently the safest option is to
-- restart GHCi.
--
-- This function uses the
-- <https://hackage.haskell.org/package/foreign-store foreign-store library>
-- to establish a state handle that survives GHCi reloads and is
-- suitable for hot reloading.
--
-- The first argument is the 'Store' index. If you do not use the
-- /foreign-store/ library in your development workflow, just use 0,
-- otherwise use any unused index.
rapid
:: forall k r.
Word32 -- ^ Store index (if in doubt, use 0).
-> (Rapid k -> IO r) -- ^ Action on the Rapid state.
-> IO r
rapid stNum k =
mask $ \unmask ->
lookupStore stNum >>=
maybe (storeAction store create)
(\_ -> readStore store) >>=
pass unmask
where
create =
pure Rapid
<*> newTVarIO False
<*> newTVarIO M.empty
<*> newTVarIO M.empty
pass unmask r = do
atomically $ do
readTVar (rLock r) >>= check . not
writeTVar (rLock r) True
unmask (k r) `finally` atomically (writeTVar (rLock r) False)
store :: Store (Rapid k)
store = Store stNum
-- | Create a thread with the given name that runs the given action.
--
-- The thread is restarted each time an update occurs.
restart
:: (Ord k)
=> Rapid k -- ^ Rapid state handle.
-> k -- ^ Name of the thread.
-> IO () -- ^ Action the thread runs.
-> IO ()
restart r k action =
withThread r k $ \mtv -> do
whenJust mtv cancelAndWait
Just <$> async action
-- | Create a thread with the given name that runs the given action.
--
-- When an update occurs and the thread is currently not running, it is
-- started.
start :: (Ord k) => Rapid k -> k -> IO () -> IO ()
start r k action =
withThread r k $
maybe (Just <$> async action)
(\tv -> poll tv >>=
maybe (pure (Just tv))
(\_ -> Just <$> async action))
-- | Delete the thread with the given name.
--
-- When an update occurs and the thread is currently running, it is
-- cancelled.
stop :: (Ord k) => Rapid k -> k -> x -> IO ()
stop r k _ =
withThread r k $ \mtv ->
Nothing <$ whenJust mtv cancelAndWait
-- | If 'Just', perform the given effect.
--
-- prop> whenJust Nothing _ = pure ()
-- prop> whenJust (Just x) f = () <$ f x
whenJust :: (Applicative m) => Maybe a -> (a -> m r) -> m ()
whenJust Nothing _ = pure ()
whenJust (Just x) f = () <$ f x
-- | Apply the given transform to the reference with the given name.
withRef
:: (Ord k)
=> Rapid k
-> k
-> (Maybe Dynamic -> IO (Maybe Dynamic, a))
-> IO a
withRef r k f = do
(mx, y) <- atomically (M.lookup k <$> readTVar (rRefs r)) >>=
f
atomically $ modifyTVar' (rRefs r) (maybe (M.delete k) (M.insert k) mx)
pure y
-- | Apply the given transform to the thread with the given name.
withThread
:: (Ord k)
=> Rapid k
-> k
-> (Maybe (Async ()) -> IO (Maybe (Async ())))
-> IO ()
withThread r k f =
atomically (M.lookup k <$> readTVar (rThreads r)) >>=
f >>=
atomically . modifyTVar' (rThreads r) . maybe (M.delete k) (M.insert k)
-- | Overwrite the mutable variable with the given name with the value
-- returned by the given action. If the mutable variable does not
-- exist, it is created.
--
-- This function may be used to change the value type of a mutable
-- variable.
writeRef
:: (Ord k, Typeable a)
=> Rapid k -- ^ Rapid state handle.
-> k -- ^ Name of the mutable variable.
-> IO a -- ^ Value action.
-> IO a
writeRef r k gen =
withRef r k $ \_ ->
fmap (\x -> (Just (toDyn x), x)) gen
{- $communication
If you need your background threads to communicate with each other, for
example by using concurrency primitives, some additional support is
required. You cannot just create a 'TVar' within your @update@ action.
It would be a different one for every invocation, so threads that are
restarted would not communicate with already running threads, because
they would use a fresh @TVar@, while the old threads would still use the
old one.
To solve this, you need to wrap your 'newTVar' action with 'createRef'.
The @TVar@ created this way will survive reloads in the same way as
background threads do. In particular, if there is already one from an
older invocation of @update@, it will be reused:
> import Control.Concurrent.STM
> import Control.Monad
> import Rapid
>
> update =
> rapid 0 $ \r -> do
> mv1 <- createRef r "var1" newEmptyTMVarIO
> mv2 <- createRef r "var2" newEmptyTMVarIO
>
> start r "producer" $
> mapM_ (atomically . putTMVar mv1) [0 :: Integer ..]
>
> restart r "consumer" $
> forever . atomically $ do
> x <- takeTMVar mv1
> putTMVar mv2 (x, "blah")
>
> -- For debugging the update action:
> replicateM_ 3 $
> atomically (takeTMVar mv2) >>= print
You can now change the string @"blah"@ in the consumer thread and then
run @update@. You will notice that the numbers in the left component of
the tuples keep increasing even after a reload, while the string in the
right component changes. That means the producer thread was not
restarted, but the consumer thread was. Yet the restarted consumer
thread still refers to the same @TVar@ as before, so it still receives
from the producer.
-}
{- $emacs
This library integrates well with
<https://haskell.github.io/haskell-mode/manual/latest/Interactive-Haskell.html haskell-interactive-mode>,
particularly with its somewhat hidden
@haskell-process-reload-devel-main@ function.
This function finds your @DevelMain@ module by looking for a buffer
named @DevelMain.hs@, loads or reloads it in your current project's
interactive session and then runs @update@. Assuming that you are
already using /haskell-interactive-mode/ all you need to do to use it is
to keep your @DevelMain@ module open in a buffer and type @M-x
haskell-process-reload-devel-main RET@ when you want to hot-reload. You
may want to bind it to a key:
> (define-key haskell-mode-map (kbd "C-c m") 'haskell-process-reload-devel-main)
Since you will likely always reload the current module before running
@update@, you can save a few keystrokes by defining a small function
that does both and bind that one to a key instead:
> (defun my-haskell-run-devel ()
> "Reloads the current module and then hot-reloads code via DevelMain.update."
> (interactive)
> (haskell-process-load-file)
> (haskell-process-reload-devel-main))
>
> (define-key haskell-mode-map (kbd "C-c m") 'my-haskell-run-devel)
-}
{- $intro
To use this library in your project create a module conventionally named
@DevelMain@ that exports an action conventionally named @update@:
> module DevelMain (update) where
>
> import Rapid
>
> update :: IO ()
> update =
> rapid 0 $ \r ->
> -- Your service management goes here.
> pure ()
The idea is that within a GHCi session this @update@ action is run
whenever you want to reload your project during development. In the
simplest case, like in a web application, your project consists of a
single service that is just restarted each time you reload. Here is an
example using the Snap Framework:
> import qualified Data.Text as T
> import Rapid
> import Snap.Core
> import Snap.Http.Server
>
> update =
> rapid 0 $ \r ->
> restart r "webserver" $
> quickHttpServe (writeText (T.pack "Hello world!"))
Once you run @update@ in a GHCi session, a server is started (port 8000)
that keeps running in the background, even when you reload modules. The
REPL is fully responsive, so you can continue working. When you want to
apply the changes you have made, you run @update@ again. To see this in
action, change the text string in the example, reload the module and
then run @update@. Also observe that nothing is changed until you
actually run @update@.
When you want to stop a running background thread, replace 'restart'
within the @update@ action by 'stop' and run @update@. The action given
to 'stop' is actually ignored. It only takes the action argument for
your convenience.
You can run multiple threads at the same time and also have threads that
are not restarted during a reload, but are only started and then kept
running:
> update =
> rapid 0 $ \r ->
> start r "database" myDatabase
> start r "worker" myBackgroundWorker
> restart r "webserver" myWebServer
Usually you would put 'restart' in front of the component that you are
currently working on, while using 'start' with all others.
-}
{- $reusing
Mutable references as introduced in the previous section can also be
used to shorten the development cycle in the case when an expensive
resource has to be created. As an example imagine that you need to
parse a huge file into a data structure. You can keep the result of
that in memory across reloads. Example with parsing JSON:
> import Control.Exception
> import Data.Aeson
> import qualified Data.ByteString as B
>
> update =
> rapid 0 $ \r ->
> value <- createRef r "file" $
> B.readFile "blah.json" >>=
> either (throwIO . userError) pure . eitherDecode
>
> -- You can now reuse 'value' across reloads.
If you want to recreate the value at some point, you can just change
'createRef' to 'writeRef' and then run @update@. Keep in mind to change
it back @createRef@ afterward. Use 'deleteRef' to remove values you no
longer need, so they can be garbage-collected.
-}
{- $safety
It's easy to crash your GHCi session with this library. In order to
prevent that, you must follow these rules:
* Do not change your service name type (the type argument of 'Rapid',
i.e. the second argument to 'restart', 'start' and 'stop') within a
session. The simplest way to do that is to resist the temptation to
define a custom name type, and just use strings instead. If you do
change the name type, you should restart GHCi.
* Be careful with mutable variable created with 'createRef': If the
value type changes (e.g. constructors or fields were changed), the
variable must be recreated, for example by using 'writeRef' once. This
most likely entails restarting all threads that were using the variable.
Again the safest option is to just restart GHCi.
* If any package in the current environment changes (especially this
library itself), for example by updating a package via @cabal@ or
@stack@, the @update@ action is likely to crash or go wrong in subtle
ways due to binary incompatibility. If packages change, restart GHCi.
* __This library is a development tool! Do not even think of using it to hot-reload in a productive environment!__
There are much safer and more appropriate ways to hot-reload code in
production, for example by using a plugin system.
The reason for this unsafety is that the underlying /foreign-store/
library is itself very unsafe in nature and requires that we maintain
binary compatibility. This library hides most of that unsafety, but
still requires that you follow the rules above.
Please take the last rule seriously and never ever use this library in
production! If something goes wrong during a reload, we do not get a
convenient run-time exception; we get a memory violation, which can
cause anything from a segfault to a remotely exploitable security hole.
-}