Adding support for lucid2

[TristanCacqueray]

Dear maintainers, would you mind adding an extra package to support lucid2 in servant, or would you prefer creating an extra repository for that?

[georgefst]

I don't know what the best solution here is. I personally think Chris' Immutable Publishing Policy, however well-meaning, is going to prove pretty painful in practice for downstream libraries, with current tooling.

[chreekat]

Well, here's one weird idea:

diff --git a/example/Main.hs b/example/Main.hs
index c73e24c..ece8b9f 100644
--- a/example/Main.hs
+++ b/example/Main.hs
@@ -3,11 +3,11 @@
 {-# LANGUAGE DataKinds         #-}
 {-# LANGUAGE OverloadedStrings #-}
 {-# LANGUAGE TypeOperators     #-}
+{-# LANGUAGE CPP #-}
 module Main (main) where
 
 import           Data.Maybe
                  (fromMaybe)
-import           Lucid
 import           Lucid.Servant
 import           Network.Wai
                  (Application)
@@ -20,6 +20,20 @@ import           Text.Read
 
 import qualified Network.Wai.Handler.Warp as Warp
 
+#if defined(LUCID2)
+import           Lucid
+import qualified Lucid
+#else
+import           Lucid hiding (Attribute)
+import qualified Lucid
+#endif
+
+#if defined(LUCID2)
+type Attribute = Lucid.Attributes
+#else
+type Attribute = Lucid.Attribute
+#endif
+
 type API = "string" :> Get '[HTML] String
     :<|> "nested" :> "html" :> Get '[HTML] (Html ())
 
diff --git a/servant-lucid.cabal b/servant-lucid.cabal
index 4c4d0bf..4594918 100644
--- a/servant-lucid.cabal
+++ b/servant-lucid.cabal
@@ -26,14 +26,22 @@ source-repository head
   type: git
   location: http://github.com/haskell-servant/servant-lucid.git
 
+flag lucid2
+  description: Use lucid2 instead of lucid1
+  default: False
+  manual: False
+
 library
   exposed-modules:     Servant.HTML.Lucid
                        Lucid.Servant
   build-depends:       base       >=4.9     && <5
                      , http-media >=0.6.4   && <0.9
-                     , lucid      >=2.9.8   && <2.12
                      , text       >=1.2.3.0 && <1.3 || >= 2 && < 2.1
                      , servant    >=0.17    && <0.21
+  if flag(lucid2)
+    build-depends:     lucid2
+  else
+    build-depends:     lucid      >=2.9.8   && <2.12
 
   if !impl(ghc >= 8.0)
     build-depends:     semigroups >=0.18.4  && <0.20
@@ -42,6 +50,9 @@ library
   default-language:    Haskell2010
   ghc-options: -Wall
 
+  if flag(lucid2)
+    cpp-options: -DLUCID2
+
 test-suite example
   type: exitcode-stdio-1.0
   main-is: Main.hs
@@ -50,9 +61,15 @@ test-suite example
   ghc-options: -Wall
   build-depends:
       base
-    , lucid
     , servant-lucid
     , servant-server >=0.14     && <0.21
     , wai            >=3.0.3.0  && <3.3
     , warp           >=3.0.13.1 && <3.4
+  if flag(lucid2)
+    build-depends: lucid2
+  else
+    build-depends: lucid
   default-language: Haskell2010
+
+  if flag(lucid2)
+    cpp-options: -DLUCID2
diff --git a/src/Lucid/Servant.hs b/src/Lucid/Servant.hs
index 176933d..0a63652 100644
--- a/src/Lucid/Servant.hs
+++ b/src/Lucid/Servant.hs
@@ -1,5 +1,6 @@
 {-# LANGUAGE ConstraintKinds   #-}
 {-# LANGUAGE OverloadedStrings #-}
+{-# LANGUAGE CPP #-}
 
 -- | Some helper functions for creating values in
 -- [lucid](https://hackage.haskell.org/package/lucid) DSLs that work
@@ -18,8 +19,6 @@ import           Data.Proxy
 import           Data.Semigroup
                  ((<>))
 import qualified Data.Text           as T
-import           Lucid
-                 (Attribute)
 import           Lucid.Html5
                  (href_)
 import           Servant.API
@@ -27,6 +26,18 @@ import           Servant.API
 import           Servant.Links
                  (HasLink, IsElem, Link, MkLink, safeLink')
 
+#if defined(LUCID2)
+import qualified Lucid
+#else
+import qualified Lucid
+#endif
+
+#if defined(LUCID2)
+type Attribute = Lucid.Attributes
+#else
+type Attribute = Lucid.Attribute
+#endif
+
 -- | 'safeLink' variant which creates lucid's 'Attribute' given base url.
 --
 -- >>> type API = "path" :> Get '[JSON] Int

The problem is that the only way I know how to use this is to add the +lucid2 flag to my local cabal.project. Which, of course, every downstream user would also have to do. Ideally there would be something I could do in my package.cabal file that would force the +lucid2 flag to get turned on automatically.

[chreekat]

(sorry, wrong patch got pasted in originally)

[srid]

Looks like there is a fork (by @danidiaz) that provides this:

https://github.com/danidiaz/dani-servant-lucid2