Add standaloneDeriving

src/GHC/SourceGen/Decl.hs

@@ -30,6 +30,10 @@ module GHC.SourceGen.Decl
 #if MIN_VERSION_ghc(8,6,0)
     , derivingVia
 #endif
+    , standaloneDeriving
+    , standaloneDerivingStock
+    , standaloneDerivingNewtype
+    , standaloneDerivingAnyclass
       -- * Class declarations
     , class'
     , ClassDecl
@@ -56,8 +60,10 @@ import GHC.Hs.Types
     ( ConDeclField(..)
     , FieldOcc(..)
     , HsConDetails(..)
+    , HsImplicitBndrs (..)
     , HsSrcBang(..)
     , HsType(..)
+    , HsWildCardBndrs (..)
 #if MIN_VERSION_ghc(8,8,0)
     , HsArg(..)
 #endif
@@ -411,6 +417,31 @@ derivingVia :: HsType' -> [HsType'] -> HsDerivingClause'
 derivingVia t = derivingWay (Just $ ViaStrategy $ sigType t)
 #endif
 
+standaloneDeriving :: HsType' -> HsDecl'
+standaloneDeriving = standaloneDerivingWay Nothing
+
+standaloneDerivingStock :: HsType' -> HsDecl'
+standaloneDerivingStock = standaloneDerivingWay (Just StockStrategy)
+
+standaloneDerivingNewtype :: HsType' -> HsDecl'
+standaloneDerivingNewtype = standaloneDerivingWay (Just NewtypeStrategy)
+
+standaloneDerivingAnyclass :: HsType' -> HsDecl'
+standaloneDerivingAnyclass = standaloneDerivingWay (Just AnyclassStrategy)
+
+standaloneDerivingWay :: Maybe DerivStrategy' -> HsType' -> HsDecl'
+standaloneDerivingWay way ty = noExt DerivD derivDecl
+  where derivDecl =
+          noExt DerivDecl (hsWC hsIB) (fmap builtLoc way) Nothing
+        hsIB =
+          withPlaceHolder $ noExtOrPlaceHolder HsIB (builtLoc ty)
+        hsWC =
+#if MIN_VERSION_ghc(8,6,0)
+          noExt HsWC
+#else
+          id
+#endif
+
 -- | Declares multiple pattern signatures of the same type.
 --
 -- > pattern F, G :: T

tests/pprint_test.hs

@@ -329,6 +329,18 @@ declsTest dflags = testGroup "Decls"
                (prefixCon "Const" [field $ var "a"])
                [deriving' [var "Show"]]
         ]
+    , test "standaloneDeriving"
+        [ "deriving instance Show Int"
+            :~ standaloneDeriving (var "Show" @@ var "Int")
+        , "deriving instance Show a => Show (Maybe a)"
+            :~ standaloneDeriving ([var "Show" @@ var "a"] ==> var "Show" @@ (var "Maybe" @@ var "a"))
+        , "deriving stock instance Show Int"
+            :~ standaloneDerivingStock (var "Show" @@ var "Int")
+        , "deriving newtype instance Show a => Show (Identity a)"
+            :~ standaloneDerivingNewtype ([var "Show" @@ var "a"] ==> var "Show" @@ (var "Identity" @@ var "a"))
+        , "deriving anyclass instance Show Person"
+            :~ standaloneDerivingAnyclass (var "Show" @@ var "Person")
+        ]
     ]
   where
     test = testDecls dflags