1 files changed, 66 insertions, 0 deletions
diff --git a/examples/Pipes.hs b/examples/Pipes.hs
new file mode 100644
index 0000000..7b50f93
--- /dev/null
+++ b/examples/Pipes.hs
@@ -0,0 +1,66 @@
+{-# LANGUAGE RankNTypes, ScopedTypeVariables, DeriveGeneric,
+  DeriveAnyClass #-}
+
+module Coalpit.IO (runMain, runMain', handleErrors) where
+
+import Data.Proxy (Proxy(..))
+import System.Environment (getProgName, getArgs)
+import Control.Monad.IO.Class (MonadIO, liftIO)
+import System.Exit (die)
+import System.IO (hPutStrLn, stderr)
+import Pipes ((>->), Pipe, yield, await, lift, runEffect)
+import Control.Monad (mapM_, forever)
+import qualified Pipes.Prelude as PP
+import Coalpit.Core (Coalpit, fromArgs, defOpt, usage)
+import Coalpit.DSV (readDSV, showDSV)
+import GHC.Generics
+
+-- | Runs a given action on each 'Left' value, embedding that action's
+-- result into the data stream.
+handleErrors :: MonadIO m => (e -> m [a]) -> Pipe (Either e a) a m ()
+handleErrors e = forever $ do
+  v <- await
+  case v of
+    Left err -> do
+      vs <- lift $ e err
+      mapM_ yield vs
+    Right x -> yield x
+
+-- | Runs a given 'Pipe' between input producer and output consumer.
+-- Prints an error and usage instructions if it fails to parse the
+-- arguments, and passes the input through 'handleErrors'.
+runMain :: forall m a i o. (MonadIO m, Coalpit a, Coalpit i, Coalpit o)
+        => (String -> m [i])
+        -- ^ An action to run on error (see 'handleErrors').
+        -> (a -> Pipe i o m ())
+        -- ^ Main function.
+        -> m ()
+runMain e f = do
+  pn <- liftIO getProgName
+  let u = Prelude.concat ["Usage: ", pn, " ", usage defOpt (Proxy :: Proxy a)]
+  args <- liftIO getArgs
+  a <- either (liftIO . die . (++ u)) pure $ fromArgs defOpt args
+  runEffect $
+    PP.stdinLn
+    >-> PP.map (readDSV defOpt)
+    >-> handleErrors e
+    >-> f a
+    >-> PP.map (showDSV defOpt)
+    >-> PP.stdoutLn
+
+-- | Same as 'runMain', but just prints errors into 'stderr'.
+runMain' :: forall m a i o. (MonadIO m, Coalpit a, Coalpit i, Coalpit o)
+         => (a -> Pipe i o m ())
+         -- ^ Main function.
+         -> m ()
+runMain' = runMain (\e -> liftIO $ hPutStrLn stderr e >> pure [])
+
+
+data Args = Args { arg1 :: Maybe Int, arg2 :: Double }
+  deriving (Generic, Coalpit)
+data Input = Input Double deriving (Generic, Coalpit)
+data Output = Foo Double | Bar deriving (Generic, Coalpit)
+
+main :: IO ()
+main = runMain' $ \a -> PP.mapM $ \(Input i) ->
+  pure $ Foo $ maybe (arg2 a) fromIntegral (arg1 a) + i