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/11-extensions/datakinds.blt

@@ -0,0 +1,74 @@
+#group extensions/datakinds-promoted-types
+
+### The spaces before commas are undesirable
+#test 1
+test :: Proxy '[ 'True ]
+test = Proxy @'[ 'True ]
+#test 2
+test :: Proxy '[True]
+test = Proxy @'[True]
+#test 3
+test :: Proxy '[ 'True , False ]
+test = Proxy @'[ 'True , False ]
+#test 4
+test :: Proxy '[True , False]
+test = Proxy @'[True , False]
+#test 5
+test :: Proxy '[True , 'False]
+test = Proxy @'[True , 'False]
+#test 6
+test :: Proxy '[ 'True , 'False ]
+test = Proxy @'[ 'True , 'False ]
+#test 7
+test :: Proxy '[ 'Just 'True , False ]
+test = Proxy @'[ 'Just 'True , False ]
+#test 8
+test :: Proxy '[Just True , False]
+test = Proxy @'[Just True , False]
+#test 9
+test :: Proxy '[('True)]
+test = Proxy @'[('True)]
+#test 10
+test :: Proxy ('Just 'True)
+test = Proxy @('Just 'True)
+#test 11
+test :: Proxy ('True)
+test = Proxy @('True)
+
+#test with-comment-1
+test
+  :: Proxy '[-- comment
+              'True ]
+test = Proxy @'[-- comment
+                 'True ]
+
+#test with-comment-2
+test
+  :: Proxy '[-- comment
+             True]
+test = Proxy @'[-- comment
+                True]
+
+#test with-comment-3
+test
+  :: Proxy '[{- comment -} 'True ]
+test = Proxy @'[{- comment -} 'True ]
+
+#test with-comment-4
+test
+  :: Proxy '[{- comment -}
+                           'True ]
+test = Proxy @'[{- comment -}
+                              'True ]
+
+#test with-comment-5
+test
+  :: Proxy '[{- comment -} True]
+test = Proxy @'[{- comment -} True]
+
+#test with-comment-6
+test
+  :: Proxy '[{- comment -}
+                           True]
+test = Proxy @'[{- comment -}
+                              True]

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

@@ -158,7 +158,7 @@ layoutType ltype@(L _ typ) = docHandleComms ltype $ case typ of
   HsTyVar epAnn promoted name -> docHandleComms epAnn $ do
     t <- lrdrNameToTextAnnTypeEqualityIsSpecial name
     case promoted of
-      IsPromoted -> docSeq [docSeparator, docTick, docHandleComms name $ docLit t]
+      IsPromoted -> docSeq [docTick, docHandleComms name $ docLit t]
       NotPromoted -> docHandleComms name $ docLit t
   HsForAllTy{} -> do
     parts <- splitArrowType ltype
@@ -429,14 +429,25 @@ layoutType ltype@(L _ typ) = docHandleComms ltype $ case typ of
   HsRecTy{} -> -- TODO
     briDocByExactInlineOnly "HsRecTy{}" ltype
   HsExplicitListTy epAnn _ typs -> docHandleComms epAnn $ 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 <- typs `forM` (shareDoc . docHandleListElemComms layoutType)
     let 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 "'[]" -- TODO92 comments AnnOpenS
@@ -444,7 +455,9 @@ layoutType ltype@(L _ typ) = docHandleComms ltype $ case typ of
         FirstLastSingleton e -> docAlt
           [ docSeq
             [ docLit $ Text.pack "'["
+            , sepIfHeadPromoted
             , docForceSingleline e -- TODO92 comments AnnOpenS
+            , sepIfHeadPromoted
             , docLit $ Text.pack "]"
             ]
           , docSetBaseY $ docLines
@@ -459,7 +472,7 @@ layoutType ltype@(L _ typ) = docHandleComms 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
@@ -541,3 +554,10 @@ withoutSpecificity :: LHsTyVarBndr GHC.Types.Var.Specificity GhcPs -> LHsTyVarBn
 withoutSpecificity = fmap $ \case
   UserTyVar a _ c -> UserTyVar a () c
   KindedTyVar a _ c d -> KindedTyVar a () c d
+
+-- | Determine if the type starts with a tick mark (single quote) when rendered.
+startsWithTick :: HsType GhcPs -> Bool
+startsWithTick = \case
+  HsTyVar _ IsPromoted _ -> True
+  HsAppTy _ (L _ t) _ -> startsWithTick t
+  _ -> False