Is DeepQuery missing a HasLink instance?

[saurabhnanda]

Upgraded to servant-server-0.20.2 to specifically use the new DeepQuery feature, however, got the following compilation error upon using it:

/home/vl/vacationlabs/haskell/src/Plugin/ItinerarySegments/Links.hs:13:14-38: error:
    * There is no instance for HasLink (DeepQuery
                                          "search" Models.ItinerarySegment.SearchOpts :> ...)
    * In the expression:
          allFieldLinks' linkToText :: Routes (AsLink Text)
      In a pattern binding:
        Routes {..} = allFieldLinks' linkToText :: Routes (AsLink Text)
   |
13 | Routes{..} = allFieldLinks' linkToText :: Routes (AsLink Text)
   |              ^^^^^^^^^^^^^^^^^^^^^^^^^

[tchoutri]

cc @divarvel

[saurabhnanda]

@tchoutri @divarvel does this look good to raise a PR (except the toS stuff, of course):

instance (KnownSymbol sym, ToDeepQuery record, HasLink sub) => HasLink (DeepQuery sym record :> sub) where
  type MkLink (DeepQuery sym record :> sub) a =
    record -> MkLink sub a

  -- toLink :: (Link -> a) -> Proxy endpoint -> Link -> MkLink endpoint a
  toLink toA _ lnk record =
    toLink toA (Proxy @sub) $ addParams lnk
    where
      k :: Text
      k = toS $ symbolVal (Proxy @sym)

      mkSingleParam x = 
        let (a, b) = generateDeepParam k x
        in SingleParam (toS a) (toS $ escapeURIString isUnreserved $ toS $ fromMaybe "" b)

      addParams lnk = 
        DL.foldl' (\memo x -> addQueryParam (mkSingleParam x) memo) lnk $ 
        toDeepQuery record

Also, why does ToDeepQuery deal with Text when inserting the results into the actual Link would force one to convert them to String?

[divarvel]

ah yes, the missing HasLink instance is an overview indeed.

The PR makes the choice to use Text everywhere instead of String in alignment with Network.HTTP.Types which provides QueryText, and more generally to avoid using strings as much as possible since it’s good haskell practice.

I forgot a bit about the original PR, but the HasLink instance should look a lot like the HasClient instance.

A PR adding HasLink support for DeepQuery should also add it to QueryString (the subtle difference is that DeepQuery qs params should be added to the query string, while QueryString qs params should replace the query string (see the difference between the two HasClient impls).