From c98a7aa70df22f1fb5264dc0a1981de40281c5a7 Mon Sep 17 00:00:00 2001
From: Alp Mestanogullari <alpmestan@gmail.com>
Date: Tue, 27 Mar 2018 19:54:49 +0200
Subject: [PATCH] hash-fragment links

---
 servant/src/Servant/Utils/Links.hs      | 57 +++++++++++++++++++++----
 servant/test/Servant/Utils/LinksSpec.hs | 13 ++++++
 2 files changed, 61 insertions(+), 9 deletions(-)

diff --git a/servant/src/Servant/Utils/Links.hs b/servant/src/Servant/Utils/Links.hs
index 27c8ce97..bcce4ac8 100644
--- a/servant/src/Servant/Utils/Links.hs
+++ b/servant/src/Servant/Utils/Links.hs
@@ -80,7 +80,8 @@
 --  bad_link under api after trying the open (but empty) type family
 --  `IsElem'` as a last resort.
 module Servant.Utils.Links (
-  module Servant.API.TypeLevel,
+  module Servant.Utils.Links, URI(..), safeLink, module Servant.API.TypeLevel
+{-  module Servant.API.TypeLevel,
 
   -- * Building and using safe links
   --
@@ -97,7 +98,7 @@ module Servant.Utils.Links (
   -- ** Link accessors
   , Param (..)
   , linkSegments
-  , linkQueryParams
+  , linkQueryParams -}
 ) where
 
 import           Data.List
@@ -112,7 +113,7 @@ import qualified Data.Text.Encoding            as TE
 import           Data.Type.Bool
                  (If)
 import           GHC.TypeLits
-                 (KnownSymbol, symbolVal)
+                 (Symbol, KnownSymbol, symbolVal)
 import           Network.URI
                  (URI (..), escapeURIString, isUnreserved)
 import           Prelude ()
@@ -163,8 +164,9 @@ import           Web.HttpApiData
 -- The only way of constructing a 'Link' is using 'safeLink', which means any
 -- 'Link' is guaranteed to be part of the mentioned API.
 data Link = Link
-  { _segments    :: [Escaped]
-  , _queryParams :: [Param]
+  { _segments      :: [Escaped]
+  , _queryParams   :: [Param]
+  , _hashFragments :: [Escaped] -- what comes after #
   } deriving Show
 
 newtype Escaped = Escaped String
@@ -205,6 +207,9 @@ addQueryParam :: Param -> Link -> Link
 addQueryParam qp l =
     l { _queryParams = _queryParams l <> [qp] }
 
+addHashFragment :: Escaped -> Link -> Link
+addHashFragment seg l = l { _hashFragments = _hashFragments l <> [seg] }
+
 -- | Transform 'Link' into 'URI'.
 --
 -- >>> type API = "something" :> Get '[JSON] Int
@@ -246,11 +251,12 @@ data LinkArrayElementStyle
 -- sum?x=1&x=2&x=3
 --
 linkURI' :: LinkArrayElementStyle -> Link -> URI
-linkURI' addBrackets (Link segments q_params) =
+linkURI' addBrackets (Link segments q_params fragments) =
     URI mempty  -- No scheme (relative)
         Nothing -- Or authority (relative)
         (intercalate "/" $ map getEscaped segments)
-        (makeQueries q_params) mempty
+        (makeQueries q_params)
+        (intercalate "/" $ map getEscaped fragments)
   where
     makeQueries :: [Param] -> String
     makeQueries [] = ""
@@ -277,7 +283,7 @@ safeLink
     => Proxy api      -- ^ The whole API that this endpoint is a part of
     -> Proxy endpoint -- ^ The API endpoint you would like to point to
     -> MkLink endpoint
-safeLink _ endpoint = toLink endpoint (Link mempty mempty)
+safeLink _ endpoint = toLink endpoint (Link mempty mempty mempty)
 
 -- | Create all links in an API.
 --
@@ -301,7 +307,7 @@ allLinks
     :: forall api. HasLink api
     => Proxy api
     -> MkLink api
-allLinks api = toLink api (Link mempty mempty)
+allLinks api = toLink api (Link mempty mempty mempty)
 
 -- | Construct a toLink for an endpoint.
 class HasLink endpoint where
@@ -432,6 +438,39 @@ instance HasLink sub => HasLink (AuthProtect tag :> sub) where
   type MkLink (AuthProtect tag :> sub) = MkLink sub
   toLink _ = toLink (Proxy :: Proxy sub)
 
+-- #-prefixed URL fragments
+
+-- | A combinator for specifying what we expect to see
+--   in the URL fragments that come after the hash in URLs.
+--   The type @a@ contains such a description.
+data Hash (fragments :: [*])
+
+-- | a wrapper for static path, which allows us to store
+--   (wrapped) type-level strings in a type-level list whose
+--   elements (which are types) have kind *.
+data P (sym :: Symbol)
+
+instance HasLink sub => HasLink (Hash '[] :> sub) where
+  type MkLink (Hash '[] :> sub) = MkLink sub
+  toLink _ = toLink (Proxy :: Proxy sub)
+
+instance (KnownSymbol (sym :: Symbol), HasLink (Hash xs :> sub))
+      => HasLink (Hash (P sym ': xs) :> sub) where
+  type MkLink (Hash (P sym ': xs) :> sub) = MkLink (Hash xs :> sub)
+  toLink _ l = toLink (Proxy :: Proxy (Hash xs :> sub)) $
+    addHashFragment str l
+
+    where str = escaped (symbolVal (Proxy :: Proxy sym))
+
+instance (ToHttpApiData a, HasLink (Hash xs :> sub))
+      => HasLink (Hash (Capture' s mods a ': xs) :> sub) where
+  type MkLink (Hash (Capture' s mods a ': xs) :> sub)
+    = a -> MkLink (Hash xs :> sub)
+  toLink _ l a = toLink (Proxy :: Proxy (Hash xs :> sub)) $
+    addHashFragment str l
+
+    where str = escaped $ Text.unpack (toUrlPiece a)
+
 -- $setup
 -- >>> import Servant.API
 -- >>> import Data.Text (Text)
diff --git a/servant/test/Servant/Utils/LinksSpec.hs b/servant/test/Servant/Utils/LinksSpec.hs
index 1d30d578..9d0716c0 100644
--- a/servant/test/Servant/Utils/LinksSpec.hs
+++ b/servant/test/Servant/Utils/LinksSpec.hs
@@ -93,6 +93,9 @@ spec = describe "Servant.Utils.Links" $ do
         let (firstLink :<|> _) = allLinks comprehensiveAPIWithoutRaw
         firstLink `shouldBeLink` ""
 
+    it "generates correct links for hash-fragments" $ do
+        testLinkFragments 12 'c' "hello" True `shouldBe` "bar/c/hello"
+
 -- |
 -- Before https://github.com/CRogers/should-not-typecheck/issues/5 is fixed,
 -- we'll just use doctest
@@ -139,3 +142,13 @@ type WrongMethod = "get" :> Post '[JSON] NoContent
 type NotALink = "hello" :> ReqBody '[JSON] Bool :> Get '[JSON] Bool
 type AllGood = "get" :> Get '[JSON] NoContent
 type NoEndpoint = "empty" :> EmptyAPI
+
+type FooAPI
+  = "foo" :> Capture "fooid" Int
+          :> Hash '[P "bar", Capture "barid" Char, Capture "str" String]
+          :> Capture "baz" Bool
+          :> Get '[JSON] Int
+
+testLinkFragments :: Int -> Char -> String -> Bool -> String
+testLinkFragments a b c d = uriFragment . linkURI $
+  safeLink (Proxy :: Proxy FooAPI) (Proxy :: Proxy FooAPI) a b c d