ef886d7 · typeable/wai

5-- second, and write the current time to a shared 'IORef', than it is for each

6-- request to make its own call to 'getCurrentTime'.

7--

8−-- But for a low-volume server, whose request frequency is less than once per

8+-- But for a low-volume server, whose request frequency is less than once per

9−-- second, that approach will result in /more/ calls to 'getCurrentTime' than

9+-- second, that approach will result in /more/ calls to 'getCurrentTime' than

10-- necessary, and worse, kills idle GC.

11--

12-- This library solves that problem by allowing you to define actions which will

13−-- either be performed by a dedicated thread, or, in times of low volume, will

13+-- either be performed by a dedicated thread, or, in times of low volume, will

14-- be executed by the calling thread.

15--

16-- Example usage:

42import Control.Concurrent (forkIO, threadDelay)

43import Control.Concurrent.MVar (newEmptyMVar, putMVar, readMVar,

44 takeMVar, tryPutMVar)

45−import Control.Exception (SomeException, catch, throw, mask_, try)

45+import Control.Exception (SomeException, catch, mask_, throw,

46+ try)

46import Control.Monad (void)

47import Control.Monad (void)

47import Data.IORef (newIORef, readIORef, writeIORef)

48import Data.IORef (newIORef, readIORef, writeIORef)

48

49

54 { updateFreq = 1000000

55 { updateFreq = 1000000

55 , updateSpawnThreshold = 3

56 , updateSpawnThreshold = 3

56 , updateAction = return ()

57 , updateAction = return ()

58+ , updateActionModify = Nothing

57 }

59 }

58

60

59-- | Settings to control how values are updated.

61-- | Settings to control how values are updated.

90 -- Default: does nothing.

92 -- Default: does nothing.

91 --

93 --

92 -- @since 0.1.0

94 -- @since 0.1.0

95+ , updateActionModify :: Maybe (a -> IO a)

96+ -- ^ Optional action to be performed to get the current value

97+ -- and updating it if necessary.

98+ --

99+ -- Default: does nothing.

93 }

100 }

94

101

95-- | Generate an action which will either read from an automatically

102-- | Generate an action which will either read from an automatically

140 -- This infinite loop makes up out worker thread. It takes an a

147 -- This infinite loop makes up out worker thread. It takes an a

141 -- responseVar value where the next value should be putMVar'ed to for

148 -- responseVar value where the next value should be putMVar'ed to for

142 -- the benefit of any requesters currently blocked on it.

149 -- the benefit of any requesters currently blocked on it.

143− let loop responseVar = do

150+ let loop responseVar maybea = do

144 -- block until a value is actually needed

151 -- block until a value is actually needed

145 takeMVar needsRunning

152 takeMVar needsRunning

146

153

147 -- new value requested, so run the updateAction

154 -- new value requested, so run the updateAction

148− a <- catchSome $ updateAction us

155+ a <- catchSome $ maybe (updateAction us) id (updateActionModify us <*> maybea)

149

156

150 -- we got a new value, update currRef and lastValue

157 -- we got a new value, update currRef and lastValue

151 writeIORef currRef $ Right a

158 writeIORef currRef $ Right a

160 -- variable.

167 -- variable.

161 responseVar' <- newEmptyMVar

168 responseVar' <- newEmptyMVar

162 writeIORef currRef $ Left responseVar'

169 writeIORef currRef $ Left responseVar'

163− loop responseVar'

170+ loop responseVar' (Just a)

164

171

165 -- Kick off the loop, with the initial responseVar0 variable.

172 -- Kick off the loop, with the initial responseVar0 variable.

166− loop responseVar0

173+ loop responseVar0 Nothing

167

174

168 return $ do

175 return $ do

169 mval <- readIORef currRef

176 mval <- readIORef currRef