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

Jon Fairbairn jon.fairbairn at cl.cam.ac.uk
Fri Jun 29 09:41:02 UTC 2018

Michael Snoyman <michael at snoyman.com> 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 at cl.cam.ac.uk

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