Add File clock

rhine/rhine.cabal

@@ -85,6 +85,7 @@ library
   exposed-modules:
     FRP.Rhine
     FRP.Rhine.Clock
+    FRP.Rhine.Clock.File
     FRP.Rhine.Clock.FixedStep
     FRP.Rhine.Clock.Periodic
     FRP.Rhine.Clock.Proxy

rhine/src/FRP/Rhine/Clock/File.hs

@@ -0,0 +1,75 @@
+module FRP.Rhine.Clock.File where
+
+-- base
+import Data.Bifunctor (first)
+import System.IO
+import System.IO.Error
+
+-- transformers
+import Control.Monad.Trans.Class
+import Control.Monad.Trans.Except
+
+-- rhine
+
+import Data.Void
+import FRP.Rhine.Clock
+import FRP.Rhine.Clock.Proxy
+
+-- | A clock that opens a file in read mode and extracts data of type @a@ from it.
+data File e a = File
+  { filename :: FilePath
+  -- ^ The path of the file to be opened
+  , action :: Handle -> IO (Either e a)
+  -- ^ The action to be performed on the file handle,
+  --   e.g. a line being read
+  }
+
+{- | Read a line of text from a text file.
+
+For higher performance, you will typically want to use a 'Text' or 'ByteString' version,
+see https://github.com/turion/rhine/issues/257.
+-}
+type TextFile = File Void String
+
+{- | Create a 'TextFile' from a file path.
+
+It ticks at every line of the file.
+Its 'Tag' will be 'String', the current line.
+-}
+textFile :: FilePath -> TextFile
+textFile filename =
+  File
+    { filename
+    , action = fmap Right . hGetLine
+    }
+
+instance GetClockProxy (File e a)
+
+{- | The only non-error exception that the 'File' clock can throw.
+
+It is thrown when the file reaches its end.
+
+To handle this exception outside of @rhine@,
+lift all other signal components to the 'ExceptT' transformer,
+call 'flow' on the whole 'Rhine',
+and then 'runExceptT'.
+
+To handle this exception inside of @rhine@,
+you will probably want to use 'eraseClock' on the 'Rhine' containing the 'File',
+and then add the result to another signal network.
+-}
+data FileException = EndOfFile
+  deriving (Show)
+
+instance Clock (ExceptT (Either e FileException) IO) (File e a) where
+  type Time (File e a) = Integer
+  type Tag (File e a) = a
+  initClock File {filename, action} = lift $ do
+    handle <- openFile filename ReadMode
+    let getLineHandle = arrM $ const $ ExceptT $ do
+          catchIOError (Data.Bifunctor.first Left <$> action handle) $ \e -> do
+            hClose handle
+            if isEOFError e
+              then return $ Left $ Right EndOfFile
+              else ioError e
+    return (count &&& getLineHandle, 0)