Prepare for http

FeedMonad/src/Control/Monad/HTTP.hs

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+{-# LANGUAGE DeriveFunctor #-}
+{-# LANGUAGE LambdaCase #-}
+module Control.Monad.HTTP where
+import Data.ByteString.Lazy (ByteString)
+import Data.URL (URL (URL))
+import Control.Monad.Free.Church (F, liftF, foldF)
+import Network.HTTP.Client (Manager, httpLbs, parseRequest, responseBody)
+import System.IO.Unsafe (unsafeInterleaveIO)
+
+data OpsF f = Fetch URL (ByteString -> f)
+  deriving Functor
+
+-- | A restricted HTTP monad
+--
+-- While the "regular" code could access http whenever and it would be
+-- fine, the middleware functions shouldn't probably have full IO
+-- access. So restricting the access and providing manager and user
+-- agents should prove useful
+type FetchM = F OpsF
+
+-- | Execute a http request and return the bytestring
+--
+-- Possible HTTP exceptions are thrown (how?)
+fetch :: URL -> FetchM ByteString
+fetch u = liftF (Fetch u id)
+
+-- Run the http monad
+--
+-- The requests are executed lazily, both as an atomic request and as
+-- a lazy bytestring. Be careful.
+execute :: Manager -> FetchM a -> IO a
+execute manager = foldF $ \case
+  Fetch (URL u) k -> k <$> unsafeInterleaveIO (fetchR u)
+  where
+    fetchR u = do
+      req <- parseRequest u
+      responseBody <$> httpLbs req manager