Fix Trac #1954: newtype deriving caused 'defined but not used' error

[ghc-hetmet.git] / compiler / typecheck / TcEnv.lhs

diff --git a/compiler/typecheck/TcEnv.lhs b/compiler/typecheck/TcEnv.lhs
index fae782a..0a369a2 100644 (file)
--- a/compiler/typecheck/TcEnv.lhs
+++ b/compiler/typecheck/TcEnv.lhs
@@ -613,13 +613,20 @@ data InstBindings a
                                -- specialised instances
        Bool                    -- True <=> This code came from a standalone deriving clause
 
-  | NewTypeDerived              -- Used for deriving instances of newtypes, where the
-       CoercionI               -- witness dictionary is identical to the argument 
-                               -- dictionary.  Hence no bindings, no pragmas.
-               -- The coercion maps from newtype to the representation type
-               -- (mentioning type variables bound by the forall'd iSpec variables)
-               -- E.g.   newtype instance N [a] = N1 (Tree a)
-               --        co : N [a] ~ Tree a
+  | NewTypeDerived      -- Used for deriving instances of newtypes, where the
+                       -- witness dictionary is identical to the argument 
+                       -- dictionary.  Hence no bindings, no pragmas.
+
+       CoercionI       -- The coercion maps from newtype to the representation type
+                       -- (mentioning type variables bound by the forall'd iSpec variables)
+                       -- E.g.   newtype instance N [a] = N1 (Tree a)
+                       --        co : N [a] ~ Tree a
+
+       TyCon           -- The TyCon is the newtype N.  If it's indexed, then it's the 
+                       -- representation TyCon, so that tyConDataCons returns [N1], 
+                       -- the "data constructor".
+                       -- See Note [Newtype deriving and unused constructors]
+                        -- in TcDeriv
 
 pprInstInfo :: InstInfo a -> SDoc
 pprInstInfo info = vcat [ptext (sLit "InstInfo:") <+> ppr (idType (iDFunId info))]
@@ -628,7 +635,7 @@ pprInstInfoDetails :: OutputableBndr a => InstInfo a -> SDoc
 pprInstInfoDetails info = pprInstInfo info $$ nest 2 (details (iBinds info))
   where
     details (VanillaInst b _ _) = pprLHsBinds b
-    details (NewTypeDerived _)  = text "Derived from the representation type"
+    details (NewTypeDerived {}) = text "Derived from the representation type"
 
 simpleInstInfoClsTy :: InstInfo a -> (Class, Type)
 simpleInstInfoClsTy info = case instanceHead (iSpec info) of