Another round of External Core fixes

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

diff --git a/compiler/typecheck/TcDeriv.lhs b/compiler/typecheck/TcDeriv.lhs
index 859b988..2edd836 100644 (file)
--- a/compiler/typecheck/TcDeriv.lhs
+++ b/compiler/typecheck/TcDeriv.lhs
@@ -46,6 +46,7 @@ import SrcLoc
 import Util
 import ListSetOps
 import Outputable
+import FastString
 import Bag
 \end{code}
 
@@ -265,7 +266,7 @@ tcDeriving  :: [LTyClDecl Name]  -- All type constructors
                    HsValBinds Name)    -- Extra generated top-level bindings
 
 tcDeriving tycl_decls inst_decls deriv_decls
-  = recoverM (returnM ([], emptyValBindsOut)) $
+  = recoverM (return ([], emptyValBindsOut)) $
     do {       -- Fish the "deriving"-related information out of the TcEnv
                -- And make the necessary "equations".
        ; early_specs <- makeDerivSpecs tycl_decls inst_decls deriv_decls
@@ -286,8 +287,8 @@ tcDeriving tycl_decls inst_decls deriv_decls
        ; let inst_info = insts1 ++ insts2
 
        ; dflags <- getDOpts
-       ; ioToTcRn (dumpIfSet_dyn dflags Opt_D_dump_deriv "Derived instances" 
-                  (ddump_deriving inst_info rn_binds))
+       ; liftIO (dumpIfSet_dyn dflags Opt_D_dump_deriv "Derived instances"
+                (ddump_deriving inst_info rn_binds))
 
        ; return (inst_info, rn_binds) }
   where
@@ -530,21 +531,24 @@ mk_data_eqn orig tvs cls tycon tc_args rep_tc rep_tc_args mtheta
                                dataConInstOrigArgTys data_con rep_tc_args,
                    not (isUnLiftedType arg_ty) ] -- No constraints for unlifted types?
 
+                       -- See Note [Superclasses of derived instance]
+             sc_constraints = substTheta (zipOpenTvSubst (classTyVars cls) inst_tys)
+                                         (classSCTheta cls)
+             inst_tys =  [mkTyConApp tycon tc_args]
+
              stupid_subst = zipTopTvSubst (tyConTyVars rep_tc) rep_tc_args
              stupid_constraints = substTheta stupid_subst (tyConStupidTheta rep_tc)
-             all_constraints = stupid_constraints ++ ordinary_constraints
-                        -- see Note [Data decl contexts] above
+             all_constraints = stupid_constraints ++ sc_constraints ++ ordinary_constraints
 
              spec = DS { ds_loc = loc, ds_orig = orig
                        , ds_name = dfun_name, ds_tvs = tvs 
-                       , ds_cls = cls, ds_tys = [mkTyConApp tycon tc_args]
+                       , ds_cls = cls, ds_tys = inst_tys
                        , ds_theta =  mtheta `orElse` all_constraints
                        , ds_newtype = False }
 
        ; return (if isJust mtheta then Just (Right spec)       -- Specified context
                                   else Just (Left spec)) }     -- Infer context
 
-
 mk_typeable_eqn orig tvs cls tycon tc_args rep_tc _rep_tc_args mtheta
        -- The Typeable class is special in several ways
        --        data T a b = ... deriving( Typeable )
@@ -695,6 +699,30 @@ new_dfun_name clas tycon   -- Just a simple wrapper
        -- a suitable string; hence the empty type arg list
 \end{code}
 
+Note [Superclasses of derived instance] 
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+In general, a derived instance decl needs the superclasses of the derived
+class too.  So if we have
+       data T a = ...deriving( Ord )
+then the initial context for Ord (T a) should include Eq (T a).  Often this is 
+redundant; we'll also generate an Ord constraint for each constructor argument,
+and that will probably generate enough constraints to make the Eq (T a) constraint 
+be satisfied too.  But not always; consider:
+
+ data S a = S
+ instance Eq (S a)
+ instance Ord (S a)
+
+ data T a = MkT (S a) deriving( Ord )
+ instance Num a => Eq (T a)
+
+The derived instance for (Ord (T a)) must have a (Num a) constraint!
+Similarly consider:
+       data T a = MkT deriving( Data, Typeable )
+Here there *is* no argument field, but we must nevertheless generate
+a context for the Data instances:
+       instance Typable a => Data (T a) where ...
+
 
 %************************************************************************
 %*                                                                     *
@@ -771,10 +799,10 @@ mkNewTypeEqn orig mayDeriveDataTypeable newtype_deriving tvs
                -- Want to drop 1 arg from (T s a) and (ST s a)
                -- to get       instance Monad (ST s) => Monad (T s)
 
-       -- Note [newtype representation]
-       -- Need newTyConRhs *not* newTyConRep to get the representation 
-       -- type, because the latter looks through all intermediate newtypes
-       -- For example
+       -- Note [Newtype representation]
+       -- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+       -- Need newTyConRhs (*not* a recursive representation finder) 
+       -- to get the representation type. For example
        --      newtype B = MkB Int
        --      newtype A = MkA B deriving( Num )
        -- We want the Num instance of B, *not* the Num instance of Int,