Serving Static Files With Servant / Wai

Question

I am following this tutorial http://www.parsonsmatt.org/programming/2015/06/07/servant-persistent.html to create APIs through servant. I want to customize the server to serve static files as well but couldn't find a way to do it.

I am using the stack build tool.

I modified the Main.hs file's run to include static (run port $ static $ logger $ app cfg) and I imported Network.Wai.Middleware.Static (static). I also added wai-middleware-static >=0.7.0 && < 0.71 to my cabal file.

When I run stack build I get: (Update: This part is totally my error. I added the the package to the wrong cabal file.. lame. Importing Network.Wai.Middleware.Static works and serves static files. Leaving the error below in case anyone searches for it and finds it useful.)

Could not find module ‘Network.Wai.Middleware.Static’
Perhaps you meant
  Network.Wai.Middleware.Gzip (from wai-extra-3.0.7.1@waiex_GpotceEdscHD6hq9p0wPOJ)
  Network.Wai.Middleware.Jsonp (from wai-extra-3.0.7.1@waiex_GpotceEdscHD6hq9p0wPOJ)
  Network.Wai.Middleware.Local (from wai-extra-3.0.7.1@waiex_GpotceEdscHD6hq9p0wPOJ)

Next I tried using servant's serveDirectory as follows (simplified):

type  API = "users" :> Get   '[JSON]   [Person]
            :<|> "static" :> Raw
server = createPerson :<|> serveDirectory "/static" 

I get this error:

Couldn't match type ‘IO’ with ‘EitherT ServantErr IO’
arising from a functional dependency between:
  constraint ‘Servant.Server.Internal.Enter.Enter
                (IO Network.Wai.Internal.ResponseReceived)
                (AppM :~> EitherT ServantErr IO)
                (IO Network.Wai.Internal.ResponseReceived)’
    arising from a use of ‘enter’
  instance ‘Servant.Server.Internal.Enter.Enter
              (m a) (m :~> n) (n a)’
    at <no location info>
In the expression: enter (readerToEither cfg) server
In an equation for ‘readerServer’:
    readerServer cfg = enter (readerToEither cfg) server

I am a Haskell beginner and I am not familiar with Wai so unsure where to even begin. What changes do I need to make the example code in the Blog post to serve static files?

Edit: Since the comments get hidden from the default view, I am pasting my last comment here:

Here is toned down version of Matt's code from his blog. I consolidated all his modules into a single file, removed all the database stuff but did not clean up the extensions/imports. When I run this code I get the above type mismatch error. Please note that this code does not use Network.Wai.Middleware.Static and I am using qualified import of Servant StaticFiles.

Answer 1

As described in the relevant section of servant's tutorial, the whole deal with enter is to have your request handlers use some monad m (in your case some ReaderT monad) and to provide a way to convert a computation in m to a computation in servant's standard EitherT ServantErr IO monad.

The problem here though is that you define a bunch of request handlers in ReaderT and an additional one to serve static files, and call enter on all of these. The ReaderT handlers are converted to EitherT ... handlers just fine, but enter tries to convert the serveDirectory call from ReaderT ... to EitherT .... This is of course not going to happen anytime soon, since serveDirectory isn't a computation in ReaderT ... to begin with!

servant could arguably just leave serveDirectory alone -- at this point I don't have a definite opinion on whether we should do that or not, or if it's better to just have the file-serving handler be glued separately, to the result of calling enter on all the other endpoints. Here's how this would look like (look for -- NEW to see the changes):

type PersonAPI = 
    "users" :> Capture "name" String :> Get '[JSON] Person
   -- NEW: removed Raw from here

-- NEW
type WholeAPI = PersonAPI :<|> Raw

type AppM = ReaderT Config (EitherT ServantErr IO)

userAPI :: Proxy PersonAPI
userAPI = Proxy

-- NEW
wholeAPI :: Proxy WholeAPI
wholeAPI = Proxy

-- NEW: changed 'userAPI' to 'wholeAPI'
app :: Config -> Application
app cfg = serve wholeAPI (readerServer cfg)

readerServer :: Config -> Server WholeAPI
readerServer cfg = enter (readerToEither cfg) server
              :<|> S.serveDirectory "/static" -- NEW

readerToEither :: Config -> AppM :~> EitherT ServantErr IO
readerToEither cfg = Nat $ \x -> runReaderT x cfg

server :: ServerT PersonAPI AppM
server = singlePerson

singlePerson :: String -> AppM Person
singlePerson str = do
    let person = Person { name = "Joe", email = "[email protected]" }
    return person

I have brought this topic to the attention of the other servant developers anyway, thanks! We hadn't really thought about the interaction between enter and serveDirectory so far (well, I did not).