+-- Returns a binding for the dfun
+
+--
+-- Derived newtype instances
+--
+-- We need to make a copy of the dictionary we are deriving from
+-- because we may need to change some of the superclass dictionaries
+-- see Note [Newtype deriving superclasses] in TcDeriv.lhs
+--
+-- In the case of a newtype, things are rather easy
+-- class Show a => Foo a b where ...
+-- newtype T a = MkT (Tree [a]) deriving( Foo Int )
+-- The newtype gives an FC axiom looking like
+-- axiom CoT a :: Tree [a] = T a
+--
+-- So all need is to generate a binding looking like
+-- dfunFooT :: forall a. (Foo Int (Tree [a], Show (T a)) => Foo Int (T a)
+-- dfunFooT = /\a. \(ds:Show (T a)) (df:Foo (Tree [a])).
+-- case df `cast` (Foo Int (CoT a)) of
+-- Foo _ op1 .. opn -> Foo ds op1 .. opn
+
+tcInstDecl2 (InstInfo { iSpec = ispec,
+ iBinds = NewTypeDerived tycon rep_tys })
+ = do { let dfun_id = instanceDFunId ispec
+ rigid_info = InstSkol dfun_id
+ origin = SigOrigin rigid_info
+ inst_ty = idType dfun_id
+ maybe_co_con = newTyConCo tycon
+ ; (tvs, theta, inst_head) <- tcSkolSigType rigid_info inst_ty
+ ; dicts <- newDicts origin theta
+ ; uniqs <- newUniqueSupply
+ ; let (rep_dict_id:sc_dict_ids) = map instToId dicts
+ -- (Here, wee are relying on the order of dictionary
+ -- arguments built by NewTypeDerived in TcDeriv.)
+
+ wrap_fn = CoTyLams tvs <.> CoLams dict_ids
+
+ coerced_rep_dict = mkHsCoerce co_fn (HsVar rep_dict_id)
+
+ body | null sc_dicts = coerced_rep_dict
+ | otherwise = HsCase coerced_rep_dict $
+ MatchGroup [the_match] inst_head
+ the_match = mkSimpleMatch [the_pat] the_rhs
+ op_ids = zipWith (mkSysLocal FSLIT("op"))
+ (uniqsFromSupply uniqs) op_tys
+ the_pat = ConPatOut { pat_con = cls_data_con, pat_tvs = [],
+ pat_dicts = map (WildPat . idType) sc_dict_ids,
+ pat_binds = emptyDictBinds,
+ pat_args = PrefixCon (map VarPat op_ids),
+ pat_ty = <type of pattern> }
+ the_rhs = mkHsApps (dataConWrapId cls_data_con) types sc_dict_ids (map HsVar op_ids)
+
+ ; return (unitBag (VarBind dfun_id (mkHsCoerce wrap_fn body))) }
+ where
+ co_fn :: ExprCoFn
+ co_fn | Just co_con <- newTyConCo tycon
+ = ExprCoFn (mkAppCoercion (mkAppsCoercion tycon rep_tys)
+ (mkTyConApp co_con tvs))
+ | otherwise
+ = idCoerecion
+
+tcMethods origin clas inst_tyvars' dfun_theta' inst_tys'
+ avail_insts op_items (NewTypeDerived rep_tys)
+ = getInstLoc origin `thenM` \ inst_loc ->
+ mapAndUnzip3M (do_one inst_loc) op_items `thenM` \ (meth_ids, meth_binds, rhs_insts) ->
+
+ tcSimplifyCheck
+ (ptext SLIT("newtype derived instance"))
+ inst_tyvars' avail_insts rhs_insts `thenM` \ lie_binds ->
+
+ -- I don't think we have to do the checkSigTyVars thing
+
+ returnM (meth_ids, lie_binds `unionBags` listToBag meth_binds)
+
+ where
+ do_one inst_loc (sel_id, _)
+ = -- The binding is like "op @ NewTy = op @ RepTy"
+ -- Make the *binder*, like in mkMethodBind
+ tcInstClassOp inst_loc sel_id inst_tys' `thenM` \ meth_inst ->
+
+ -- Make the *occurrence on the rhs*
+ tcInstClassOp inst_loc sel_id rep_tys' `thenM` \ rhs_inst ->
+ let
+ meth_id = instToId meth_inst
+ in
+ return (meth_id, noLoc (VarBind meth_id (nlHsVar (instToId rhs_inst))), rhs_inst)
+
+ -- Instantiate rep_tys with the relevant type variables
+ -- This looks a bit odd, because inst_tyvars' are the skolemised version
+ -- of the type variables in the instance declaration; but rep_tys doesn't
+ -- have the skolemised version, so we substitute them in here
+ rep_tys' = substTys subst rep_tys
+ subst = zipOpenTvSubst inst_tyvars' (mkTyVarTys inst_tyvars')
+
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