Re: [Haskell-cafe] Using Wai with Conduit and ResourceT (was Re: (New to Conduits) mixing lazy lists and Conduits?)

I'm not set up to run a FastCGI script easily on my system, but replacing your example with Warp as the handler works just fine on my machine. On Fri, Jun 29, 2018 at 12:41 PM Jon Fairbairn wrote:

Michael Snoyman writes:

...

I'd have to see a complete repro to know why the program in question doesn't stream.

Thanks. Here’s a fairly small example

``` module Main where

import Prelude hiding (mapM_) import Conduit import Data.Conduit.List (mapM_) import System.FilePath import Data.ByteString.UTF8 import Data.Binary.Builder import GHC.IO.Exception (IOException)

import Network.Wai.Handler.FastCGI (run) import Network.Wai.Conduit (Application, responseStream) import Network.HTTP.Types.Status import Network.HTTP.Types.Header

data_directory = "./test-data/"

main = run $ app

app:: Application app request respond = do respond $ responseSourceRes status200 [(hContentType, fromString "text/plain; charset=UTF-8")] $ do yieldCBS "\nBEGIN\n" yield Flush wrapSourceFile $ data_directory "file1" wrapSourceFile $ data_directory "a_pipe" yieldCBS "END\n" yield Flush

wrapSourceFile:: (MonadUnliftIO m, MonadResource m) => FilePath -> ConduitM a (Flush Builder) m () wrapSourceFile path = do yieldCBS ("\n" ++ path ++ ":\n") catchC (sourceFile path .| mapC (Chunk . fromByteString)) (\e -> yieldCBS $ "Error: " ++ show (e::IOException) ++ "\n") yieldCBS "\n" yield Flush

yieldCBS:: Monad m => String -> ConduitT i (Flush Builder) m () yieldCBS = yield . Chunk . fromByteString . fromString

responseSourceRes status headers res_conduit = responseStream status200 headers (\send flush -> runConduitRes $ res_conduit .| mapM_ (\e->liftIO $ case e of Chunk c -> send c Flush -> flush ))

```

The various flushes in there were attempts to make something come out.

...

But I _can_ explain how best to do something like this.

...

To frame this: why is something like ResourceT needed here? The issue is we want to ensure exception safety around the open file handle, and guarantee that the handle is closed regardless of any exceptions being thrown. ResourceT solves this problem by letting you register cleanup actions. This allows for solving some really complicated dynamic allocation problems, but for most cases it's overkill. Instead, a simple use of the bracket pattern is sufficient. You can do that with `withSourceFile`:

``` #!/usr/bin/env stack -- stack --resolver lts-11.10 script import Network.Wai import Network.Wai.Handler.Warp import Network.Wai.Conduit import Network.HTTP.Types import Conduit import Data.ByteString.Builder (byteString)

main :: IO () main = run 3000 app

app :: Application app _req respond = withSourceFile "Main.hs" $ \src -> respond $ responseSource status200 [] $ src .| mapC (Chunk . byteString)

I don’t think that will work for what I’m trying to do as the decision to open which file is made within the conduit.

...

You can also do this by sticking with ResourceT, which requires jumping through some hoops with monad transformers to ensure the original ResourceT context is used. I don't recommend this approach unless you really need it: it's complicated, and slightly slower than the above. But in case you're curious:

Thanks. I think that may be what I want, but it’ll take a while to digest

-- Jón Fairbairn Jon.Fairbairn@cl.cam.ac.uk

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