Support promoted type applications.

Closes #370.

Up to GHC 8.10,

    foo @ 'Bar

was a valid type application. In GHC 9 it's not, which means brittany
needs to allow

    foo @'Bar

which it now does.

The reason the space was needed was to allow a promoted type variable at
the head of a type-level list. That is,

    '['Foo]

is invalid syntax, because it initially parses as the character `'['`.
So the promoted type variable was always given a separator at the
beginning, and we'd get

    '[ 'Foo]

which was valid. Now we handle this case by specifically examining the
head of a type-level list; if it's promoted we introduce spaces, so

    '[ 'Foo ]
    '[Foo]

I've added tests for this and some related cases. In doing so I noticed
that unnecessary spaces get added in front of commas in these lists; I
believe that's a separate bug, and I've written a comment explaining why
it happens, but I haven't tried to fix it.

I'm not sure when the first alternates in the `FirstLastSingleton`
and `FirstLast` branches would ever be hit, so I'm not entirely sure if
the separators are necessary there. But since `docSeparator` disappears
at the end of a line and merges with adjacent separators, they should be
harmless.

data/Test316.hs

@@ -1 +1,50 @@
-test :: Proxy '[ 'True]
+-- The spaces before commas are undesirable
+test :: Proxy '[ 'True ]
+test :: Proxy '[True]
+test :: Proxy '[ 'True , False ]
+test :: Proxy '[True , False]
+test :: Proxy '[True , 'False]
+test :: Proxy '[ 'True , 'False ]
+test :: Proxy '[ 'Just 'True , False ]
+test :: Proxy '[Just True , False]
+test :: Proxy '[('True)]
+test :: Proxy ('Just 'True)
+test :: Proxy ('True)
+
+test = Proxy @'[ 'True ]
+test = Proxy @'[True]
+test = Proxy @'[ 'True , False ]
+test = Proxy @'[True , False]
+test = Proxy @'[True , 'False]
+test = Proxy @'[ 'True , 'False ]
+test = Proxy @'[ 'Just 'True , False ]
+test = Proxy @'[Just True , False]
+test = Proxy @'[('True)]
+test = Proxy @('Just 'True)
+test = Proxy @('True)
+
+test
+  :: Proxy '[-- comment
+              'True ]
+test
+  :: Proxy '[-- comment
+             True]
+test
+  :: Proxy '[{- comment -}
+              'True ]
+test
+  :: Proxy '[{- comment -}
+             True]
+
+test =
+  Proxy @'[-- comment
+            'True ]
+test =
+  Proxy @'[-- comment
+           True]
+test =
+  Proxy @'[{- comment -}
+            'True ]
+test =
+  Proxy @'[{- comment -}
+           True]

source/library/Language/Haskell/Brittany/Internal/Layouters/Type.hs

@@ -24,7 +24,7 @@ layoutType ltype@(L _ typ) = docWrapNode ltype $ case typ of
   HsTyVar _ promoted name -> do
     t <- lrdrNameToTextAnnTypeEqualityIsSpecial name
     case promoted of
-      IsPromoted -> docSeq [docSeparator, docTick, docWrapNode name $ docLit t]
+      IsPromoted -> docSeq [docTick, docWrapNode name $ docLit t]
       NotPromoted -> docWrapNode name $ docLit t
   HsForAllTy _ hsf (L _ (HsQualTy _ (L _ cntxts) typ2)) -> do
     let bndrs = getBinders hsf
@@ -522,14 +522,25 @@ layoutType ltype@(L _ typ) = docWrapNode ltype $ case typ of
   HsRecTy{} -> -- TODO
     briDocByExactInlineOnly "HsRecTy{}" ltype
   HsExplicitListTy _ _ typs -> do
+    -- `'['Foo]` isn't valid because it parses as a character. So if the list
+    -- starts with a promoted type var, we swap to `'[ 'Foo ]`.
+    let
+      sepIfHeadPromoted = case typs of
+        (L _ t) : _ | startsWithTick t -> docSeparator
+        _ -> docEmpty
+
+    -- When rendering on multiple lines, this causes commas to line up with the
+    -- opening bracket. Unfortunately it also adds unnecessary space when
+    -- rendering on a single line.
+    let specialCommaSep = appSep $ docLit $ Text.pack " ,"
+
     typDocs <- docSharedWrapper layoutType `mapM` typs
     hasComments <- hasAnyCommentsBelow ltype
-    let specialCommaSep = appSep $ docLit $ Text.pack " ,"
     docAlt
       [ docSeq
-      $ [docLit $ Text.pack "'["]
+      $ [docLit $ Text.pack "'[", sepIfHeadPromoted]
       ++ List.intersperse specialCommaSep (docForceSingleline <$> typDocs)
-      ++ [docLit $ Text.pack "]"]
+      ++ [sepIfHeadPromoted, docLit $ Text.pack "]"]
       , case splitFirstLast typDocs of
         FirstLastEmpty -> docSeq
           [ docLit $ Text.pack "'["
@@ -538,7 +549,9 @@ layoutType ltype@(L _ typ) = docWrapNode ltype $ case typ of
         FirstLastSingleton e -> docAlt
           [ docSeq
             [ docLit $ Text.pack "'["
+            , sepIfHeadPromoted
             , docNodeAnnKW ltype (Just AnnOpenS) $ docForceSingleline e
+            , sepIfHeadPromoted
             , docLit $ Text.pack "]"
             ]
           , docSetBaseY $ docLines
@@ -553,7 +566,7 @@ layoutType ltype@(L _ typ) = docWrapNode ltype $ case typ of
         FirstLast e1 ems eN -> runFilteredAlternative $ do
           addAlternativeCond (not hasComments)
             $ docSeq
-            $ [docLit $ Text.pack "'["]
+            $ [docLit $ Text.pack "'[", sepIfHeadPromoted]
             ++ List.intersperse
                  specialCommaSep
                  (docForceSingleline
@@ -633,3 +646,10 @@ withoutSpecificity = fmap $ \case
   UserTyVar a _ c -> UserTyVar a () c
   KindedTyVar a _ c d -> KindedTyVar a () c d
   XTyVarBndr a -> XTyVarBndr a
+
+-- | Determine if the type starts with a tick mark (single quote) when rendered.
+startsWithTick :: HsType pass -> Bool
+startsWithTick = \case
+  HsTyVar _ IsPromoted _ -> True
+  HsAppTy _ (L _ t) _ -> startsWithTick t
+  _ -> False