From 9a4c93a59e008ddc376fde5f9eb468b762f0d0a7 Mon Sep 17 00:00:00 2001 From: "simonpj@microsoft.com" Date: Tue, 5 Jan 2010 10:16:00 +0000 Subject: [PATCH] Undo the fix for Trac #3772 and do it a new way The main idea is that I'm now treating a single-method dictionary very much like a multi-method dictionary. In particular, it respond to exprIsConApp_maybe, even though newtypes aren't *really* proper constructors. See long comments with Note [Single-method classes] for why this slight hack is justified. --- compiler/basicTypes/MkId.lhs | 12 ++--- compiler/typecheck/TcInstDcls.lhs | 98 ++++++++++++++++++++++++++----------- 2 files changed, 74 insertions(+), 36 deletions(-) diff --git a/compiler/basicTypes/MkId.lhs b/compiler/basicTypes/MkId.lhs index aaeb3bc..b5525dc 100644 --- a/compiler/basicTypes/MkId.lhs +++ b/compiler/basicTypes/MkId.lhs @@ -467,15 +467,11 @@ mkDictSelId no_unf name clas -- becuase we use that to generate a top-level binding -- for the ClassOp - info | new_tycon = base_info - -- For newtype dictionaries, just inline the class op - -- See Note [Single-method classes] in TcInstDcls - | otherwise = base_info - `setSpecInfo` mkSpecInfo [rule] + info = base_info `setSpecInfo` mkSpecInfo [rule] `setInlinePragInfo` neverInlinePragma - -- Otherwise add a magic BuiltinRule, and never inline it - -- so that the rule is always available to fire. - -- See Note [ClassOp/DFun selection] in TcInstDcls + -- Add a magic BuiltinRule, and never inline it + -- so that the rule is always available to fire. + -- See Note [ClassOp/DFun selection] in TcInstDcls n_ty_args = length tyvars diff --git a/compiler/typecheck/TcInstDcls.lhs b/compiler/typecheck/TcInstDcls.lhs index aa61f5a..1af025e 100644 --- a/compiler/typecheck/TcInstDcls.lhs +++ b/compiler/typecheck/TcInstDcls.lhs @@ -33,7 +33,7 @@ import DataCon import Class import Var import CoreUnfold ( mkDFunUnfolding ) -import CoreUtils ( mkPiTypes ) +-- import CoreUtils ( mkPiTypes ) import PrelNames ( inlineIdName ) import Id import MkId @@ -180,8 +180,8 @@ Instead we use a cunning trick. Note [Single-method classes] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~ -If the class has just one method (or, more accurately, just one elemen -of {superclasses + methods}), then we want a different strategy. +If the class has just one method (or, more accurately, just one element +of {superclasses + methods}), then we still use the *same* strategy class C a where op :: a -> a instance C a => C [a] where op = @@ -194,34 +194,39 @@ a top-level axiom: op :: forall a. C a -> (a -> a) op a d = d |> (Co:C a) + MkC :: forall a. (a->a) -> C a + MkC = /\a.\op. op |> (sym Co:C a) + df :: forall a. C a => C [a] - {-# INLINE df #-} - df = $cop_list |> (forall a. C a -> (sym (Co:C a)) + {-# NOINLINE df DFun[ $cop_list ] #-} + df = /\a. \d. MkD ($cop_list a d) $cop_list :: forall a. C a => a -> a $cop_list = -So the ClassOp is just a cast; and so is the dictionary function. -(The latter doesn't even have any lambdas.) We can inline both freely. -No need for fancy BuiltIn rules. Indeed the BuiltinRule stuff does -not work well for newtypes because it uses exprIsConApp_maybe. - -The INLINE on df is vital, else $cop_list occurs just once and is inlined, -which is a disaster if $cop_list *itself* has an INLINE pragma. - -Notice, also, that we go to the trouble of generating a complicated cast, -rather than do this: - df = /\a. \d. MkD ($cop_list a d) -where the MkD "constructor" willl expand to a suitable cast: - df = /\a. \d. ($cop_list a d) |> (...) -Reason: suppose $cop_list has an INLINE pragma. We want to avoid the -nasty possibility that we eta-expand df, to get - df = (/\a \d \x. $cop_list a d x) |> (...) -and now $cop_list may get inlined into the df, rather than at -the actual call site. Of course, eta reduction may get there first, -but it seems less fragile to generate the Right Thing in the first place. -See Trac #3772. - +The "constructor" MkD expands to a cast, as does the class-op selector. +The RULE works just like for multi-field dictionaries: + * (df a d) returns (Just (MkD,..,[$cop_list a d])) + to exprIsConApp_Maybe + + * The RULE for op picks the right result + +This is a bit of a hack, because (df a d) isn't *really* a constructor +application. But it works just fine in this case, exprIsConApp_maybe +is otherwise used only when we hit a case expression which will have +a real data constructor in it. + +The biggest reason for doing it this way, apart form uniformity, is +that we want to be very careful when we have + instance C a => C [a] where + {-# INLINE op #-} + op = ... +then we'll get an INLINE pragma on $cop_list. The danger is that +we'll get something like + foo = /\a.\d. $cop_list a d +and then we'll eta expand, and then we'll inline TOO EARLY. This happened in +Trac #3772 and I spent far too long fiddling arond trying to fix it. +Look at the test for Trac #3772. Note [Subtle interaction of recursion and overlap] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ @@ -769,6 +774,44 @@ tc_inst_decl2 dfun_id (VanillaInst monobinds uprags standalone_deriv) ; 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') + + -- Do not inline the dfun; instead give it a magic DFunFunfolding + -- See Note [ClassOp/DFun selection] + -- See also note [Single-method classes] + dfun_id_w_fun = dfun_id + `setIdUnfolding` mkDFunUnfolding dict_constr (sc_ids ++ meth_ids) + `setInlinePragma` dfunInlinePragma + + main_bind = 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 (L loc main_bind) `unionBags` + listToBag meth_binds `unionBags` + listToBag sc_binds) + } + +{- + -- Create the result bindings ; let this_dict_id = instToId this_dict arg_ids = sc_ids ++ meth_ids arg_binds = listToBag meth_binds `unionBags` @@ -819,8 +862,7 @@ tc_inst_decl2 dfun_id (VanillaInst monobinds uprags standalone_deriv) the_meth_id = ASSERT( length arg_ids == 1 ) head arg_ids nt_cast = WpCast $ mkPiTypes (inst_tyvars' ++ dfun_lam_vars) $ mkSymCoercion (mkTyConApp the_nt_co inst_tys') - } - +-} ------------------------------ tcSuperClass :: InstLoc -> [TyVar] -> [Inst] -- 1.7.10.4