Preserve empty ‘forall’s

CHANGELOG.md

@@ -18,6 +18,9 @@
   inside the export list. See [issue
   430](https://github.com/tweag/ormolu/issues/430).
 
+* Empty `forall`s are now correctly preserved. See [issue
+  429](https://github.com/tweag/ormolu/issues/429).
+
 ## Ormolu 0.0.1.0
 
 * Initial release.

data/examples/other/empty-forall-out.hs

@@ -0,0 +1,18 @@
+-- Empty foralls are handled correctly in different situations.
+
+data D = forall. D Int
+
+data G where
+  G :: forall. Int -> G
+
+f :: forall. a -> a
+f x = x
+
+type family T x where
+  forall. T x = x
+
+{-# RULES
+"r"
+  r a =
+    ()
+  #-}

data/examples/other/empty-forall.hs

@@ -0,0 +1,17 @@
+-- Empty foralls are handled correctly in different situations.
+
+data D = forall. D Int
+
+data G where
+  G :: forall. Int -> G
+
+f :: forall. a -> a
+f x = x
+
+type family T x where
+  forall. T x = x
+
+{-# RULES
+  "r"
+  forall. r a = ()
+#-}

src/Ormolu/Printer/Meat/Declaration/Data.hs

@@ -104,8 +104,9 @@ p_conDecl = \case
                 then newline
                 else breakpoint
         interArgBreak
-        p_forallBndrs p_hsTyVarBndr (hsq_explicit con_qvars)
-        unless (null $ hsq_explicit con_qvars) interArgBreak
+        when (unLoc con_forall) $ do
+          p_forallBndrs p_hsTyVarBndr (hsq_explicit con_qvars)
+          interArgBreak
         forM_ con_mb_cxt p_lhsContext
         case con_args of
           PrefixCon xs -> do
@@ -126,12 +127,14 @@ p_conDecl = \case
     mapM_ (p_hsDocString Pipe True) con_doc
     let conDeclSpn =
           [getLoc con_name]
+            <> [getLoc con_forall]
             <> fmap getLoc con_ex_tvs
             <> maybeToList (fmap getLoc con_mb_cxt)
             <> conArgsSpans con_args
     switchLayout conDeclSpn $ do
-      p_forallBndrs p_hsTyVarBndr con_ex_tvs
-      unless (null con_ex_tvs) breakpoint
+      when (unLoc con_forall) $ do
+        p_forallBndrs p_hsTyVarBndr con_ex_tvs
+        breakpoint
       forM_ con_mb_cxt p_lhsContext
       case con_args of
         PrefixCon xs -> do

src/Ormolu/Printer/Meat/Declaration/Rule.hs

@@ -8,7 +8,7 @@ module Ormolu.Printer.Meat.Declaration.Rule
 where
 
 import BasicTypes
-import Data.Maybe (fromMaybe)
+import Control.Monad (unless)
 import GHC
 import Ormolu.Printer.Combinators
 import Ormolu.Printer.Meat.Common
@@ -31,9 +31,16 @@ p_ruleDecl = \case
     space
     p_activation activation
     space
-    p_forallBndrs p_hsTyVarBndr (fromMaybe [] tyvars)
-    space
-    p_forallBndrs p_ruleBndr ruleBndrs
+    case tyvars of
+      Nothing -> return ()
+      Just xs -> do
+        p_forallBndrs p_hsTyVarBndr xs
+        space
+    -- NOTE It appears that there is no way to tell if there was an empty
+    -- forall in the input or no forall at all. We do not want to add
+    -- redundant foralls, so let's just skip the empty ones.
+    unless (null ruleBndrs) $
+      p_forallBndrs p_ruleBndr ruleBndrs
     breakpoint
     inci $ do
       located lhs p_hsExpr

src/Ormolu/Printer/Meat/Type.hs

@@ -184,7 +184,7 @@ p_hsTyVarBndr = \case
 
 -- | Render several @forall@-ed variables.
 p_forallBndrs :: Data a => (a -> R ()) -> [Located a] -> R ()
-p_forallBndrs _ [] = return ()
+p_forallBndrs _ [] = txt "forall."
 p_forallBndrs p tyvars =
   switchLayout (getLoc <$> tyvars) $ do
     txt "forall"