How to capture a path segment optionally? (was: Any reason why CaptureHint is not exported?)

[saurabhnanda]

I am knee-deep into finding a solution for How to optionally capture a path segment in Haskell servant? and I think I have hit upon something that could work (given below) BUT it won't compile without access to Servant.Server.Internal.Router.CaptureHint. Any reason why it has not been exported? Without access to that type, is there any other way to use addCapture?

data HostWithLocale = HostWithLocale

instance (HasServer api context) => HasServer (HostWithLocale :> api) context where
  type ServerT (HostWithLocale :> api) m = (Hostname, Maybe LanguageCode) -> ServerT api m
  
  hoistServerWithContext _ pc nt s = hoistServerWithContext (Proxy :: Proxy api) pc nt . s  

  route _ context server = 
    CaptureRouter [hint] $ route (Proxy :: Proxy api) context subserver
    where
      hint = CaptureHint "language" (typeRep (Proxy :: Proxy (Hostname, Maybe LanguageCode)))
      subserver = addCapture server $ \pathSegment -> withRequest $ \req -> 
        case DL.lookup (fromString "Host") (requestHeaders req) of
          Nothing -> 
            delayedFail err406
          Just hname -> 
            if Prelude.not (isWhiteListedDomain hname)
              then delayedFail err406
              else if pathSegment `DL.elem` knownLanguageCodes
                then pure $ (toS hname, Just $ LanguageCode pathSegment)
                else pure $ (toS hname, Nothing)

isWhiteListedDomain :: C8.ByteString -> Bool
isWhiteListedDomain = undefined 
    
knownLanguageCodes :: [Text]
knownLanguageCodes = undefined

[fisx]

I don't understand. CaptureHint is exported from Servant.Server.Internal.Router, and internal modules are exposed in the cabal file. did you look in the right place? (did i...?)

[tchoutri]

@fisx I believe Saurabh is asking "why is this not part of the external API" (which would be presumed to be stable, or at least officially endorsed).

[fisx]

i see, that makes sense. i'm not sure, then. as an application developer i'm really bad at library interface hygene and would probably just import the internal module. :)

[tchoutri]

@saurabhnanda Could you please use this module and tell us if the constructs in it are mature enough? If they are, they I guess we can move them to an external module. Even making a guide for your usecase could be a fantastic way to help out the next person who needs i18n. :)

[saurabhnanda]

@tchoutri @fisx my bad. CaptureHint is indeed exported. I was on an older version of servant in this particular project and didn't realize it.

Now, even though I have access to CaptureRouter, addCapture and CaptureHint, my solution is not working. It expects the language fragment to always be present. It cannot capture optionally. I've updated the code snippet above (in the original post).