+-- If there are no superclasses, matters are simpler, because we don't need the case
+-- see Note [Newtype deriving superclasses] in TcDeriv.lhs
+
+tc_inst_decl2 dfun_id (NewTypeDerived coi)
+ = do { let rigid_info = InstSkol
+ origin = SigOrigin rigid_info
+ inst_ty = idType dfun_id
+ inst_tvs = fst (tcSplitForAllTys inst_ty)
+ ; (inst_tvs', theta, inst_head_ty) <- tcSkolSigType rigid_info inst_ty
+ -- inst_head_ty is a PredType
+
+ ; let (cls, cls_inst_tys) = tcSplitDFunHead inst_head_ty
+ (class_tyvars, sc_theta, _, _) = classBigSig cls
+ cls_tycon = classTyCon cls
+ sc_theta' = substTheta (zipOpenTvSubst class_tyvars cls_inst_tys) sc_theta
+ Just (initial_cls_inst_tys, last_ty) = snocView cls_inst_tys
+
+ (rep_ty, wrapper)
+ = case coi of
+ IdCo -> (last_ty, idHsWrapper)
+ ACo co -> (snd (coercionKind co'), WpCast (mk_full_coercion co'))
+ where
+ co' = substTyWith inst_tvs (mkTyVarTys inst_tvs') co
+ -- NB: the free variable of coi are bound by the
+ -- universally quantified variables of the dfun_id
+ -- This is weird, and maybe we should make NewTypeDerived
+ -- carry a type-variable list too; but it works fine
+
+ -----------------------
+ -- mk_full_coercion
+ -- The inst_head looks like (C s1 .. sm (T a1 .. ak))
+ -- But we want the coercion (C s1 .. sm (sym (CoT a1 .. ak)))
+ -- with kind (C s1 .. sm (T a1 .. ak) ~ C s1 .. sm <rep_ty>)
+ -- where rep_ty is the (eta-reduced) type rep of T
+ -- So we just replace T with CoT, and insert a 'sym'
+ -- NB: we know that k will be >= arity of CoT, because the latter fully eta-reduced
+
+ mk_full_coercion co = mkTyConApp cls_tycon
+ (initial_cls_inst_tys ++ [mkSymCoercion co])
+ -- Full coercion : (Foo Int (Tree [a]) ~ Foo Int (N a)
+
+ rep_pred = mkClassPred cls (initial_cls_inst_tys ++ [rep_ty])
+ -- In our example, rep_pred is (Foo Int (Tree [a]))
+
+ ; sc_loc <- getInstLoc InstScOrigin
+ ; sc_dicts <- newDictBndrs sc_loc sc_theta'
+ ; inst_loc <- getInstLoc origin
+ ; dfun_dicts <- newDictBndrs inst_loc theta
+ ; rep_dict <- newDictBndr inst_loc rep_pred
+ ; this_dict <- newDictBndr inst_loc (mkClassPred cls cls_inst_tys)
+
+ -- Figure out bindings for the superclass context from dfun_dicts
+ -- Don't include this_dict in the 'givens', else
+ -- sc_dicts get bound by just selecting from this_dict!!
+ ; sc_binds <- addErrCtxt superClassCtxt $
+ tcSimplifySuperClasses inst_loc this_dict dfun_dicts
+ (rep_dict:sc_dicts)
+
+ -- It's possible that the superclass stuff might unified something
+ -- in the envt with one of the clas_tyvars
+ ; checkSigTyVars inst_tvs'
+
+ ; let coerced_rep_dict = wrapId wrapper (instToId rep_dict)
+
+ ; body <- make_body cls_tycon cls_inst_tys sc_dicts coerced_rep_dict
+ ; let dict_bind = mkVarBind (instToId this_dict) (noLoc body)
+
+ ; return (unitBag $ noLoc $
+ AbsBinds inst_tvs' (map instToVar dfun_dicts)
+ [(inst_tvs', dfun_id, instToId this_dict, [])]
+ (dict_bind `consBag` sc_binds)) }
+ where
+ -----------------------
+ -- (make_body C tys scs coreced_rep_dict)
+ -- returns
+ -- (case coerced_rep_dict of { C _ ops -> C scs ops })
+ -- But if there are no superclasses, it returns just coerced_rep_dict
+ -- See Note [Newtype deriving superclasses] in TcDeriv.lhs
+
+ make_body cls_tycon cls_inst_tys sc_dicts coerced_rep_dict
+ | null sc_dicts -- Case (a)
+ = return coerced_rep_dict
+ | otherwise -- Case (b)
+ = do { op_ids <- newSysLocalIds (fsLit "op") op_tys
+ ; dummy_sc_dict_ids <- newSysLocalIds (fsLit "sc") (map idType sc_dict_ids)
+ ; let the_pat = ConPatOut { pat_con = noLoc cls_data_con, pat_tvs = [],
+ pat_dicts = dummy_sc_dict_ids,
+ pat_binds = emptyLHsBinds,
+ pat_args = PrefixCon (map nlVarPat op_ids),
+ pat_ty = pat_ty}
+ the_match = mkSimpleMatch [noLoc the_pat] the_rhs
+ the_rhs = mkHsConApp cls_data_con cls_inst_tys $
+ map HsVar (sc_dict_ids ++ op_ids)
+
+ -- Warning: this HsCase scrutinises a value with a PredTy, which is
+ -- never otherwise seen in Haskell source code. It'd be
+ -- nicer to generate Core directly!
+ ; return (HsCase (noLoc coerced_rep_dict) $
+ MatchGroup [the_match] (mkFunTy pat_ty pat_ty)) }
+ where
+ sc_dict_ids = map instToId sc_dicts
+ pat_ty = mkTyConApp cls_tycon cls_inst_tys
+ cls_data_con = head (tyConDataCons cls_tycon)
+ cls_arg_tys = dataConInstArgTys cls_data_con cls_inst_tys
+ op_tys = dropList sc_dict_ids cls_arg_tys
+
+------------------------
+-- Ordinary instances
+
+tc_inst_decl2 dfun_id (VanillaInst monobinds uprags standalone_deriv)
+ = do { let rigid_info = InstSkol
+ inst_ty = idType dfun_id
+ loc = getSrcSpan dfun_id
+
+ -- Instantiate the instance decl with skolem constants
+ ; (inst_tyvars', dfun_theta', inst_head') <- tcSkolSigType rigid_info inst_ty
+ -- These inst_tyvars' scope over the 'where' part
+ -- Those tyvars are inside the dfun_id's type, which is a bit
+ -- bizarre, but OK so long as you realise it!
+ ; let
+ (clas, inst_tys') = tcSplitDFunHead inst_head'
+ (class_tyvars, sc_theta, sc_sels, op_items) = classBigSig clas
+
+ -- Instantiate the super-class context with inst_tys
+ sc_theta' = substTheta (zipOpenTvSubst class_tyvars inst_tys') sc_theta
+ origin = SigOrigin rigid_info
+
+ -- Create dictionary Ids from the specified instance contexts.
+ ; inst_loc <- getInstLoc origin
+ ; dfun_dicts <- newDictBndrs inst_loc dfun_theta' -- Includes equalities
+ ; this_dict <- newDictBndr inst_loc (mkClassPred clas inst_tys')
+ -- Default-method Ids may be mentioned in synthesised RHSs,
+ -- but they'll already be in the environment.
+
+
+ -- Cook up a binding for "this = df d1 .. dn",
+ -- to use in each method binding
+ -- Need to clone the dict in case it is floated out, and
+ -- then clashes with its friends
+ ; cloned_this <- cloneDict this_dict
+ ; let cloned_this_bind = mkVarBind (instToId cloned_this) $
+ L loc $ wrapId app_wrapper dfun_id
+ app_wrapper = mkWpApps dfun_lam_vars <.> mkWpTyApps (mkTyVarTys inst_tyvars')
+ dfun_lam_vars = map instToVar dfun_dicts -- Includes equalities
+ nested_this_pair
+ | null inst_tyvars' && null dfun_theta' = (this_dict, emptyBag)
+ | otherwise = (cloned_this, unitBag cloned_this_bind)
+
+ -- Deal with 'SPECIALISE instance' pragmas
+ -- See Note [SPECIALISE instance pragmas]
+ ; let spec_inst_sigs = filter isSpecInstLSig uprags
+ -- The filter removes the pragmas for methods
+ ; spec_inst_prags <- mapM (wrapLocM (tcSpecInst dfun_id)) spec_inst_sigs
+
+ -- Typecheck the methods
+ ; let prag_fn = mkPragFun uprags
+ tc_meth = tcInstanceMethod loc standalone_deriv
+ clas inst_tyvars'
+ dfun_dicts inst_tys'
+ nested_this_pair
+ prag_fn spec_inst_prags monobinds
+
+ ; (meth_ids, meth_binds) <- tcExtendTyVarEnv inst_tyvars' $
+ mapAndUnzipM tc_meth op_items
+
+ -- Figure out bindings for the superclass context
+ ; sc_loc <- getInstLoc InstScOrigin
+ ; sc_dicts <- newDictOccs sc_loc sc_theta' -- These are wanted
+ ; let tc_sc = tcSuperClass inst_loc inst_tyvars' dfun_dicts nested_this_pair
+ ; (sc_ids, sc_binds) <- mapAndUnzipM tc_sc (sc_sels `zip` sc_dicts)
+
+ -- It's possible that the superclass stuff might unified
+ -- something in the envt with one of the inst_tyvars'
+ ; checkSigTyVars inst_tyvars'
+
+ -- Create the result bindings
+ ; let dict_constr = classDataCon clas
+ this_dict_id = instToId this_dict
+ dict_bind = mkVarBind this_dict_id dict_rhs
+ dict_rhs = foldl mk_app inst_constr (sc_ids ++ meth_ids)
+ inst_constr = L loc $ wrapId (mkWpTyApps inst_tys')
+ (dataConWrapId dict_constr)
+ -- We don't produce a binding for the dict_constr; instead we
+ -- rely on the simplifier to unfold this saturated application
+ -- We do this rather than generate an HsCon directly, because
+ -- it means that the special cases (e.g. dictionary with only one
+ -- member) are dealt with by the common MkId.mkDataConWrapId code rather
+ -- than needing to be repeated here.
+
+ mk_app :: LHsExpr Id -> Id -> LHsExpr Id
+ mk_app fun arg_id = L loc (HsApp fun (L loc (wrapId arg_wrapper arg_id)))
+ arg_wrapper = mkWpApps dfun_lam_vars <.> mkWpTyApps (mkTyVarTys inst_tyvars')
+
+ dfun_id_w_fun | isNewTyCon (classTyCon clas)
+ = dfun_id -- Just let the dfun inline; see Note [Single-method classes]
+ | otherwise
+ = dfun_id -- Do not inline; instead give it a magic DFunFunfolding
+ -- See Note [ClassOp/DFun selection]
+ `setIdUnfolding` mkDFunUnfolding dict_constr (sc_ids ++ meth_ids)
+ `setInlinePragma` dfunInlinePragma
+
+ main_bind = noLoc $ AbsBinds
+ inst_tyvars'
+ dfun_lam_vars
+ [(inst_tyvars', dfun_id_w_fun, this_dict_id, spec_inst_prags)]
+ (unitBag dict_bind)
+
+ ; showLIE (text "instance")
+ ; return (unitBag main_bind `unionBags`
+ listToBag meth_binds `unionBags`
+ listToBag sc_binds) }
+
+
+------------------------------
+tcSuperClass :: InstLoc -> [TyVar] -> [Inst]
+ -> (Inst, LHsBinds Id)
+ -> (Id, Inst) -> TcM (Id, LHsBind Id)
+-- Build a top level decl like
+-- sc_op = /\a \d. let this = ... in
+-- let sc = ... in
+-- sc
+-- The "this" part is just-in-case (discarded if not used)
+-- See Note [Recursive superclasses]
+tcSuperClass inst_loc tyvars dicts (this_dict, this_bind)
+ (sc_sel, sc_dict)
+ = addErrCtxt superClassCtxt $
+ do { sc_binds <- tcSimplifySuperClasses inst_loc
+ this_dict dicts [sc_dict]
+ -- Don't include this_dict in the 'givens', else
+ -- sc_dicts get bound by just selecting from this_dict!!
+
+ ; uniq <- newUnique
+ ; let sc_op_ty = mkSigmaTy tyvars (map dictPred dicts)
+ (mkPredTy (dictPred sc_dict))
+ sc_op_name = mkDerivedInternalName mkClassOpAuxOcc uniq
+ (getName sc_sel)
+ sc_op_id = mkLocalId sc_op_name sc_op_ty
+ sc_id = instToVar sc_dict
+ sc_op_bind = AbsBinds tyvars
+ (map instToVar dicts)
+ [(tyvars, sc_op_id, sc_id, [])]
+ (this_bind `unionBags` sc_binds)
+
+ ; return (sc_op_id, noLoc sc_op_bind) }
+\end{code}
+
+Note [Recursive superclasses]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+See Trac #1470 for why we would *like* to add "this_dict" to the
+available instances here. But we can't do so because then the superclases
+get satisfied by selection from this_dict, and that leads to an immediate
+loop. What we need is to add this_dict to Avails without adding its
+superclasses, and we currently have no way to do that.
+
+Note [SPECIALISE instance pragmas]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Consider
+
+ instance (Ix a, Ix b) => Ix (a,b) where
+ {-# SPECIALISE instance Ix (Int,Int) #-}
+ range (x,y) = ...
+
+We do *not* want to make a specialised version of the dictionary
+function. Rather, we want specialised versions of each method.
+Thus we should generate something like this:
+
+ $dfIx :: (Ix a, Ix x) => Ix (a,b)
+ {- DFUN [$crange, ...] -}
+ $dfIx da db = Ix ($crange da db) (...other methods...)
+
+ $dfIxPair :: (Ix a, Ix x) => Ix (a,b)
+ {- DFUN [$crangePair, ...] -}
+ $dfIxPair = Ix ($crangePair da db) (...other methods...)
+
+ $crange :: (Ix a, Ix b) -> ((a,b),(a,b)) -> [(a,b)]
+ {-# SPECIALISE $crange :: ((Int,Int),(Int,Int)) -> [(Int,Int)] #-}
+ $crange da db = <blah>
+
+ {-# RULE range ($dfIx da db) = $crange da db #-}
+
+Note that
+
+ * The RULE is unaffected by the specialisation. We don't want to
+ specialise $dfIx, because then it would need a specialised RULE
+ which is a pain. The single RULE works fine at all specialisations.
+ See Note [How instance declarations are translated] above