-tcInstDecl2 (InstInfo { iSpec = ispec, iBinds = VanillaInst monobinds uprags })
- = let
- dfun_id = instanceDFunId ispec
- rigid_info = InstSkol
- inst_ty = idType dfun_id
- loc = srcLocSpan (getSrcLoc dfun_id)
- in
- -- Prime error recovery
- recoverM (return emptyLHsBinds) $
- setSrcSpan loc $
- addErrCtxt (instDeclCtxt2 (idType dfun_id)) $ do
-
- -- 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, _, op_items) = classBigSig clas
-
- -- Instantiate the super-class context with inst_tys
- sc_theta' = substTheta (zipOpenTvSubst class_tyvars inst_tys') sc_theta
- (eq_sc_theta',dict_sc_theta') = partition isEqPred sc_theta'
- origin = SigOrigin rigid_info
- (eq_dfun_theta',dict_dfun_theta') = partition isEqPred dfun_theta'
-
- -- Create dictionary Ids from the specified instance contexts.
- sc_loc <- getInstLoc InstScOrigin
- sc_dicts <- newDictBndrs sc_loc dict_sc_theta'
- inst_loc <- getInstLoc origin
- sc_covars <- mkMetaCoVars eq_sc_theta'
- wanted_sc_eqs <- mkEqInsts eq_sc_theta' (map mkWantedCo sc_covars)
- dfun_covars <- mkCoVars eq_dfun_theta'
- dfun_eqs <- mkEqInsts eq_dfun_theta' (map mkGivenCo $ mkTyVarTys dfun_covars)
- dfun_dicts <- newDictBndrs inst_loc dict_dfun_theta'
- 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.
-
- -- Typecheck the methods
- let -- These insts are in scope; quite a few, eh?
- dfun_insts = dfun_eqs ++ dfun_dicts
- wanted_sc_insts = wanted_sc_eqs ++ sc_dicts
- given_sc_eqs = map (updateEqInstCoercion (mkGivenCo . TyVarTy . fromWantedCo "tcInstDecl2") ) wanted_sc_eqs
- given_sc_insts = given_sc_eqs ++ sc_dicts
- avail_insts = [this_dict] ++ dfun_insts ++ given_sc_insts
-
- (meth_ids, meth_binds) <- tcMethods origin clas inst_tyvars'
- dfun_theta' inst_tys' avail_insts
- op_items monobinds uprags
-
- -- Figure out bindings for the superclass context
- -- Don't include this_dict in the 'givens', else
- -- wanted_sc_insts get bound by just selecting from this_dict!!
- sc_binds <- addErrCtxt superClassCtxt
- (tcSimplifySuperClasses inst_loc dfun_insts wanted_sc_insts)
-
- -- It's possible that the superclass stuff might unified one
- -- of the inst_tyavars' with something in the envt
- checkSigTyVars inst_tyvars'
-
- -- Deal with 'SPECIALISE instance' pragmas
- prags <- tcPrags dfun_id (filter isSpecInstLSig uprags)
-
- -- Create the result bindings
- let
- dict_constr = classDataCon clas
- scs_and_meths = map instToId sc_dicts ++ meth_ids
- this_dict_id = instToId this_dict
- inline_prag | null dfun_insts = []
- | otherwise = [L loc (InlinePrag (Inline AlwaysActive True))]
- -- Always inline the dfun; this is an experimental decision
- -- because it makes a big performance difference sometimes.
- -- Often it means we can do the method selection, and then
- -- inline the method as well. Marcin's idea; see comments below.
- --
- -- BUT: don't inline it if it's a constant dictionary;
- -- we'll get all the benefit without inlining, and we get
- -- a **lot** of code duplication if we inline it
- --
- -- See Note [Inline dfuns] below
-
- dict_rhs
- = mkHsConApp dict_constr (inst_tys' ++ mkTyVarTys sc_covars) (map HsVar scs_and_meths)
- -- 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.
-
- dict_bind = noLoc (VarBind this_dict_id dict_rhs)
- all_binds = dict_bind `consBag` (sc_binds `unionBags` meth_binds)
-
- main_bind = noLoc $ AbsBinds
- (inst_tyvars' ++ dfun_covars)
- (map instToId dfun_dicts)
- [(inst_tyvars' ++ dfun_covars, dfun_id, this_dict_id, inline_prag ++ prags)]
- all_binds
-
- showLIE (text "instance")
- return (unitBag main_bind)
-
-mkCoVars :: [PredType] -> TcM [TyVar]
-mkCoVars = newCoVars . map unEqPred
+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
+
+ * Instead, we want to specialise the *method*, $crange
+
+In practice, rather than faking up a SPECIALISE pragama for each
+method (which is painful, since we'd have to figure out its
+specialised type), we call tcSpecPrag *as if* were going to specialise
+$dfIx -- you can see that in the call to tcSpecInst. That generates a
+SpecPrag which, as it turns out, can be used unchanged for each method.
+The "it turns out" bit is delicate, but it works fine!
+
+\begin{code}
+tcSpecInst :: Id -> Sig Name -> TcM SpecPrag
+tcSpecInst dfun_id prag@(SpecInstSig hs_ty)
+ = addErrCtxt (spec_ctxt prag) $
+ do { let name = idName dfun_id
+ ; (tyvars, theta, tau) <- tcHsInstHead hs_ty
+ ; let spec_ty = mkSigmaTy tyvars theta tau
+ ; co_fn <- tcSubExp (SpecPragOrigin name) (idType dfun_id) spec_ty
+ ; return (SpecPrag co_fn defaultInlinePragma) }