I am having a lot of trouble understanding this compilation error. The code is the following:
#!/usr/bin/env stack
-- stack script --resolver lts-8.22
{-# LANGUAGE OverloadedStrings #-}
import Network.HTTP.Simple
import qualified Data.ByteString as BS
import Control.Monad.Reader
import Control.Monad.Except
import Control.Monad.Trans.Class
import Data.Conduit
type Cmd i o = ReaderT (BS.ByteString) (ExceptT HttpException (ConduitM i o IO))
runCmd :: Cmd i o a -> BS.ByteString -> ConduitM i o IO (Either HttpException a)
runCmd cmdTS host = runExceptT $ runReaderT cmdTS host
readHost :: Cmd () BS.ByteString ()
readHost = do
host <- ask
let req = setRequestMethod "GET"
$ setRequestHost host
$ defaultRequest
lift . lift $ httpSource req getResponseBody
Basically I have a MT stack of 4 layers and in readHost I am trying to lift an object from the second to bottom layer to the top layer so I have two lift composed together. But sadly the code will not compile and I got the following error:
[-Wdeferred-type-errors]
• No instance for (Control.Monad.Trans.Resource.Internal.MonadResource
IO)
arising from a use of ‘httpSource’
• In the second argument of ‘($)’, namely
‘httpSource req getResponseBody’
In a stmt of a 'do' block:
lift . lift $ httpSource req getResponseBody
In the expression:
do host <- ask
let req
= setRequestMethod "GET" $ setRequestHost host $ defaultRequest
lift . lift $ httpSource req getResponseBody
Please help me understand what this error message means. On a side note, I have not seen people building a stack like this with ConduiT sitting at the bottom, so I guess this is bad practice? What will be a good "design pattern" here? I know that the word "design patter" does not exist in haskell but I cannot think of a better word. Thanks a lot!
I started by reducing the layers in the monad transformer stack to shrink your code to this:
#!/usr/bin/env stack
-- stack script --resolver lts-8.22
{-# LANGUAGE OverloadedStrings #-}
import Network.HTTP.Simple
import qualified Data.ByteString as BS
import Control.Monad.Reader
import Control.Monad.Except
import Control.Monad.Trans.Class
import Data.Conduit
type Cmd i o = ConduitM i o IO
readHost :: BS.ByteString -> Cmd () BS.ByteString ()
readHost host = do
let req = setRequestMethod "GET" . setRequestHost host $ defaultRequest
httpSource req getResponseBody
I then ran stack {scriptname} and got an error very similar to the one you got
for the larger stack:
{scriptname}:17:3: error:
• No instance for (resourcet-1.1.9:Control.Monad.Trans.Resource.Internal.MonadResource
IO)
arising from a use of ‘httpSource’
• In a stmt of a 'do' block: httpSource req getResponseBody
In the expression:
do { let req
= setRequestMethod "GET" . setRequestHost host $ defaultRequest;
httpSource req getResponseBody }
In an equation for ‘readHost’:
readHost host
= do { let req = ...;
httpSource req getResponseBody }
Figuring out this smaller case may very well make the more complicated version easier to tackle.
Let's look at the type for httpSource:
httpSource :: (MonadResource m, MonadIO n) =>
Request
-> (Response (ConduitM i ByteString n ()) -> ConduitM i o m r)
-> ConduitM i o m r
Its return type is ConduitM i o m r where m must be an instance of
MonadResource.
Now let's have a second glance at the final return type of readHost.
It's Cmd () BS.ByteString () which is nothing but
ConduitM () BS.ByteString IO () in disguise.
Compare Conduit () BS.ByteString IO () with ConduitM i o m r to see that
m corresponds with IO.
Is IO an instance of MonadResource? According to GHC, it isn't.
A little looking around using Hoogle lets us know that ResourceT IO is an
instance of MonadResource. Let's try it out in a program.
-- stack script --resolver lts-8.22
{-# LANGUAGE OverloadedStrings #-}
import Network.HTTP.Simple
import qualified Data.ByteString as BS
import Control.Monad.Reader
import Control.Monad.Except
import Control.Monad.Trans.Class
import Control.Monad.Trans.Resource
import Data.Conduit
main :: IO ()
main = putStrLn "hello"
type Cmd i o = ConduitM i o (ResourceT IO)
readHost :: BS.ByteString -> Cmd () BS.ByteString ()
readHost host = do
let req = setRequestMethod "GET" . setRequestHost host $ defaultRequest
httpSource req getResponseBody
This compiles without errors and also prints to standard output.
I believe you can use this fix for the larger program. At the very least it explains the error.