X-Git-Url: http://git.megacz.com/?p=ghc-hetmet.git;a=blobdiff_plain;f=compiler%2Ftypecheck%2FTcInstDcls.lhs;h=2e74b6ae887ce877e1af0bb9b6ac770bad64f38b;hp=daf611af48fccf5b0ff0e5b3869741573f16517f;hb=7fc01c4671980ea3c66d549c0ece4d82fd3f5ade;hpb=85255a966b21172ce5a26c8a9cb0cdaf7315be95 diff --git a/compiler/typecheck/TcInstDcls.lhs b/compiler/typecheck/TcInstDcls.lhs index daf611a..2e74b6a 100644 --- a/compiler/typecheck/TcInstDcls.lhs +++ b/compiler/typecheck/TcInstDcls.lhs @@ -19,34 +19,41 @@ import Inst import InstEnv import FamInst import FamInstEnv +import MkCore ( nO_METHOD_BINDING_ERROR_ID ) import TcDeriv import TcEnv +import RnSource ( addTcgDUs ) +import TcSimplify( simplifySuperClass ) import TcHsType import TcUnify -import TcSimplify import Type import Coercion import TyCon -import TypeRep import DataCon import Class import Var +import VarSet ( emptyVarSet ) +import CoreUtils ( mkPiTypes ) +import CoreUnfold ( mkDFunUnfolding ) +import CoreSyn ( Expr(Var) ) +import Id import MkId import Name import NameSet import DynFlags import SrcLoc -import ListSetOps import Util import Outputable import Bag import BasicTypes import HscTypes import FastString - +import Maybes ( orElse ) import Data.Maybe import Control.Monad import Data.List + +#include "HsVersions.h" \end{code} Typechecking instance declarations is done in two passes. The first @@ -59,67 +66,238 @@ pass, when the class-instance envs and GVE contain all the info from all the instance and value decls. Indeed that's the reason we need two passes over the instance decls. -Here is the overall algorithm. -Assume that we have an instance declaration - - instance c => k (t tvs) where b - -\begin{enumerate} -\item -$LIE_c$ is the LIE for the context of class $c$ -\item -$betas_bar$ is the free variables in the class method type, excluding the - class variable -\item -$LIE_cop$ is the LIE constraining a particular class method -\item -$tau_cop$ is the tau type of a class method -\item -$LIE_i$ is the LIE for the context of instance $i$ -\item -$X$ is the instance constructor tycon -\item -$gammas_bar$ is the set of type variables of the instance -\item -$LIE_iop$ is the LIE for a particular class method instance -\item -$tau_iop$ is the tau type for this instance of a class method -\item -$alpha$ is the class variable -\item -$LIE_cop' = LIE_cop [X gammas_bar \/ alpha, fresh betas_bar]$ -\item -$tau_cop' = tau_cop [X gammas_bar \/ alpha, fresh betas_bar]$ -\end{enumerate} - -ToDo: Update the list above with names actually in the code. - -\begin{enumerate} -\item -First, make the LIEs for the class and instance contexts, which means -instantiate $thetaC [X inst_tyvars \/ alpha ]$, yielding LIElistC' and LIEC', -and make LIElistI and LIEI. -\item -Then process each method in turn. -\item -order the instance methods according to the ordering of the class methods -\item -express LIEC' in terms of LIEI, yielding $dbinds_super$ or an error -\item -Create final dictionary function from bindings generated already -\begin{pseudocode} -df = lambda inst_tyvars - lambda LIEI - let Bop1 - Bop2 - ... - Bopn - and dbinds_super - in -\end{pseudocode} -Here, Bop1 \ldots Bopn bind the methods op1 \ldots opn, -and $dbinds_super$ bind the superclass dictionaries sd1 \ldots sdm. -\end{enumerate} + +Note [How instance declarations are translated] +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +Here is how we translation instance declarations into Core + +Running example: + class C a where + op1, op2 :: Ix b => a -> b -> b + op2 = + + instance C a => C [a] + {-# INLINE [2] op1 #-} + op1 = +===> + -- Method selectors + op1,op2 :: forall a. C a => forall b. Ix b => a -> b -> b + op1 = ... + op2 = ... + + -- Default methods get the 'self' dictionary as argument + -- so they can call other methods at the same type + -- Default methods get the same type as their method selector + $dmop2 :: forall a. C a => forall b. Ix b => a -> b -> b + $dmop2 = /\a. \(d:C a). /\b. \(d2: Ix b). + -- NB: type variables 'a' and 'b' are *both* in scope in + -- Note [Tricky type variable scoping] + + -- A top-level definition for each instance method + -- Here op1_i, op2_i are the "instance method Ids" + -- The INLINE pragma comes from the user pragma + {-# INLINE [2] op1_i #-} -- From the instance decl bindings + op1_i, op2_i :: forall a. C a => forall b. Ix b => [a] -> b -> b + op1_i = /\a. \(d:C a). + let this :: C [a] + this = df_i a d + -- Note [Subtle interaction of recursion and overlap] + + local_op1 :: forall b. Ix b => [a] -> b -> b + local_op1 = + -- Source code; run the type checker on this + -- NB: Type variable 'a' (but not 'b') is in scope in + -- Note [Tricky type variable scoping] + + in local_op1 a d + + op2_i = /\a \d:C a. $dmop2 [a] (df_i a d) + + -- The dictionary function itself + {-# NOINLINE CONLIKE df_i #-} -- Never inline dictionary functions + df_i :: forall a. C a -> C [a] + df_i = /\a. \d:C a. MkC (op1_i a d) (op2_i a d) + -- But see Note [Default methods in instances] + -- We can't apply the type checker to the default-method call + + -- Use a RULE to short-circuit applications of the class ops + {-# RULE "op1@C[a]" forall a, d:C a. + op1 [a] (df_i d) = op1_i a d #-} + +Note [Instances and loop breakers] +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +* Note that df_i may be mutually recursive with both op1_i and op2_i. + It's crucial that df_i is not chosen as the loop breaker, even + though op1_i has a (user-specified) INLINE pragma. + +* Instead the idea is to inline df_i into op1_i, which may then select + methods from the MkC record, and thereby break the recursion with + df_i, leaving a *self*-recurisve op1_i. (If op1_i doesn't call op at + the same type, it won't mention df_i, so there won't be recursion in + the first place.) + +* If op1_i is marked INLINE by the user there's a danger that we won't + inline df_i in it, and that in turn means that (since it'll be a + loop-breaker because df_i isn't), op1_i will ironically never be + inlined. But this is OK: the recursion breaking happens by way of + a RULE (the magic ClassOp rule above), and RULES work inside InlineRule + unfoldings. See Note [RULEs enabled in SimplGently] in SimplUtils + +Note [ClassOp/DFun selection] +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +One thing we see a lot is stuff like + op2 (df d1 d2) +where 'op2' is a ClassOp and 'df' is DFun. Now, we could inline *both* +'op2' and 'df' to get + case (MkD ($cop1 d1 d2) ($cop2 d1 d2) ... of + MkD _ op2 _ _ _ -> op2 +And that will reduce to ($cop2 d1 d2) which is what we wanted. + +But it's tricky to make this work in practice, because it requires us to +inline both 'op2' and 'df'. But neither is keen to inline without having +seen the other's result; and it's very easy to get code bloat (from the +big intermediate) if you inline a bit too much. + +Instead we use a cunning trick. + * We arrange that 'df' and 'op2' NEVER inline. + + * We arrange that 'df' is ALWAYS defined in the sylised form + df d1 d2 = MkD ($cop1 d1 d2) ($cop2 d1 d2) ... + + * We give 'df' a magical unfolding (DFunUnfolding [$cop1, $cop2, ..]) + that lists its methods. + + * We make CoreUnfold.exprIsConApp_maybe spot a DFunUnfolding and return + a suitable constructor application -- inlining df "on the fly" as it + were. + + * We give the ClassOp 'op2' a BuiltinRule that extracts the right piece + iff its argument satisfies exprIsConApp_maybe. This is done in + MkId mkDictSelId + + * We make 'df' CONLIKE, so that shared uses stil match; eg + let d = df d1 d2 + in ...(op2 d)...(op1 d)... + +Note [Single-method classes] +~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +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 = + +We translate the class decl into a newtype, which just gives +a top-level axiom: + + axiom Co:C a :: C a ~ (a->a) + + 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] + {-# NOINLINE df DFun[ $cop_list ] #-} + df = /\a. \d. MkC ($cop_list a d) + + $cop_list :: forall a. C a => [a] -> [a] + $cop_list = + +The "constructor" MkC expands to a cast, as does the class-op selector. +The RULE works just like for multi-field dictionaries: + + * (df a d) returns (Just (MkC,..,[$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 from 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 $cop_list but it's important that +$cop_list only inlines when it's applied to *two* arguments (the +dictionary and the list argument + +The danger is that we'll get something like + op_list :: C a => [a] -> [a] + op_list = /\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 around trying to fix it. +Look at the test for Trac #3772. + + (Note: re-reading the above, I can't see how using the + uniform story solves the problem.) + +Note [Subtle interaction of recursion and overlap] +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +Consider this + class C a where { op1,op2 :: a -> a } + instance C a => C [a] where + op1 x = op2 x ++ op2 x + op2 x = ... + instance C [Int] where + ... + +When type-checking the C [a] instance, we need a C [a] dictionary (for +the call of op2). If we look up in the instance environment, we find +an overlap. And in *general* the right thing is to complain (see Note +[Overlapping instances] in InstEnv). But in *this* case it's wrong to +complain, because we just want to delegate to the op2 of this same +instance. + +Why is this justified? Because we generate a (C [a]) constraint in +a context in which 'a' cannot be instantiated to anything that matches +other overlapping instances, or else we would not be excecuting this +version of op1 in the first place. + +It might even be a bit disguised: + + nullFail :: C [a] => [a] -> [a] + nullFail x = op2 x ++ op2 x + + instance C a => C [a] where + op1 x = nullFail x + +Precisely this is used in package 'regex-base', module Context.hs. +See the overlapping instances for RegexContext, and the fact that they +call 'nullFail' just like the example above. The DoCon package also +does the same thing; it shows up in module Fraction.hs + +Conclusion: when typechecking the methods in a C [a] instance, we want +to have C [a] available. That is why we have the strange local +definition for 'this' in the definition of op1_i in the example above. +We can typecheck the defintion of local_op1, and when doing tcSimplifyCheck +we supply 'this' as a given dictionary. Only needed, though, if there +are some type variables involved; otherwise there can be no overlap and +none of this arises. + +Note [Tricky type variable scoping] +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +In our example + class C a where + op1, op2 :: Ix b => a -> b -> b + op2 = + + instance C a => C [a] + {-# INLINE [2] op1 #-} + op1 = + +note that 'a' and 'b' are *both* in scope in , but only 'a' is +in scope in . In particular, we must make sure that 'b' is in +scope when typechecking . This is achieved by subFunTys, +which brings appropriate tyvars into scope. This happens for both + and for , but that doesn't matter: the *renamer* will have +complained if 'b' is mentioned in . + %************************************************************************ @@ -147,33 +325,33 @@ tcInstDecls1 tycl_decls inst_decls deriv_decls -- round) -- (1) Do class and family instance declarations - ; let { idxty_decls = filter (isFamInstDecl . unLoc) tycl_decls } - ; local_info_tycons <- mapM tcLocalInstDecl1 inst_decls - ; idx_tycons <- mapM tcIdxTyInstDeclTL idxty_decls - - ; let { (local_infos, - at_tycons) = unzip local_info_tycons - ; local_info = concat local_infos - ; at_idx_tycon = concat at_tycons ++ catMaybes idx_tycons + ; idx_tycons <- mapAndRecoverM (tcFamInstDecl TopLevel) $ + filter (isFamInstDecl . unLoc) tycl_decls + ; local_info_tycons <- mapAndRecoverM tcLocalInstDecl1 inst_decls + + ; let { (local_info, + at_tycons_s) = unzip local_info_tycons + ; at_idx_tycons = concat at_tycons_s ++ idx_tycons ; clas_decls = filter (isClassDecl.unLoc) tycl_decls - ; implicit_things = concatMap implicitTyThings at_idx_tycon + ; implicit_things = concatMap implicitTyThings at_idx_tycons + ; aux_binds = mkRecSelBinds at_idx_tycons } -- (2) Add the tycons of indexed types and their implicit -- tythings to the global environment - ; tcExtendGlobalEnv (at_idx_tycon ++ implicit_things) $ do { + ; tcExtendGlobalEnv (at_idx_tycons ++ implicit_things) $ do { -- (3) Instances from generic class declarations ; generic_inst_info <- getGenericInstances clas_decls -- Next, construct the instance environment so far, consisting -- of - -- a) local instance decls - -- b) generic instances - -- c) local family instance decls - ; addInsts local_info $ do { - ; addInsts generic_inst_info $ do { - ; addFamInsts at_idx_tycon $ do { + -- (a) local instance decls + -- (b) generic instances + -- (c) local family instance decls + ; addInsts local_info $ + addInsts generic_inst_info $ + addFamInsts at_idx_tycons $ do { -- (4) Compute instances from "deriving" clauses; -- This stuff computes a context for the derived instance @@ -183,37 +361,13 @@ tcInstDecls1 tycl_decls inst_decls deriv_decls failIfErrsM -- If the addInsts stuff gave any errors, don't -- try the deriving stuff, becuase that may give -- more errors still - ; (deriv_inst_info, deriv_binds) <- tcDeriving tycl_decls inst_decls - deriv_decls - ; addInsts deriv_inst_info $ do { - - ; gbl_env <- getGblEnv - ; return (gbl_env, + ; (deriv_inst_info, deriv_binds, deriv_dus) + <- tcDeriving tycl_decls inst_decls deriv_decls + ; gbl_env <- addInsts deriv_inst_info getGblEnv + ; return ( addTcgDUs gbl_env deriv_dus, generic_inst_info ++ deriv_inst_info ++ local_info, - deriv_binds) - }}}}}} - where - -- Make sure that toplevel type instance are not for associated types. - -- !!!TODO: Need to perform this check for the TyThing of type functions, - -- too. - tcIdxTyInstDeclTL ldecl@(L loc decl) = - do { tything <- tcFamInstDecl ldecl - ; setSrcSpan loc $ - when (isAssocFamily tything) $ - addErr $ assocInClassErr (tcdName decl) - ; return tything - } - isAssocFamily (Just (ATyCon tycon)) = - case tyConFamInst_maybe tycon of - Nothing -> panic "isAssocFamily: no family?!?" - Just (fam, _) -> isTyConAssoc fam - isAssocFamily (Just _ ) = panic "isAssocFamily: no tycon?!?" - isAssocFamily Nothing = False - -assocInClassErr :: Name -> SDoc -assocInClassErr name = - ptext (sLit "Associated type") <+> quotes (ppr name) <+> - ptext (sLit "must be inside a class instance") + aux_binds `plusHsValBinds` deriv_binds) + }}} addInsts :: [InstInfo Name] -> TcM a -> TcM a addInsts infos thing_inside @@ -230,15 +384,13 @@ addFamInsts tycons thing_inside \begin{code} tcLocalInstDecl1 :: LInstDecl Name - -> TcM ([InstInfo Name], [TyThing]) -- [] if there was an error + -> TcM (InstInfo Name, [TyThing]) -- A source-file instance declaration -- Type-check all the stuff before the "where" -- -- We check for respectable instance type, and context tcLocalInstDecl1 (L loc (InstDecl poly_ty binds uprags ats)) - = -- Prime error recovery, set source location - recoverM (return ([], [])) $ - setSrcSpan loc $ + = setSrcSpan loc $ addErrCtxt (instDeclCtxt1 poly_ty) $ do { is_boot <- tcIsHsBoot @@ -247,14 +399,16 @@ tcLocalInstDecl1 (L loc (InstDecl poly_ty binds uprags ats)) ; (tyvars, theta, tau) <- tcHsInstHead poly_ty - -- Next, process any associated types. - ; idx_tycons <- mapM tcFamInstDecl ats - -- Now, check the validity of the instance. - ; (clas, inst_tys) <- checkValidInstHead tau - ; checkValidInstance tyvars theta clas inst_tys - ; checkValidAndMissingATs clas (tyvars, inst_tys) - (zip ats idx_tycons) + ; (clas, inst_tys) <- checkValidInstance poly_ty tyvars theta tau + + -- Next, process any associated types. + ; idx_tycons <- recoverM (return []) $ + do { idx_tycons <- checkNoErrs $ + mapAndRecoverM (tcFamInstDecl NotTopLevel) ats + ; checkValidAndMissingATs clas (tyvars, inst_tys) + (zip ats idx_tycons) + ; return idx_tycons } -- Finally, construct the Core representation of the instance. -- (This no longer includes the associated types.) @@ -266,9 +420,9 @@ tcLocalInstDecl1 (L loc (InstDecl poly_ty binds uprags ats)) dfun = mkDictFunId dfun_name tyvars theta' clas inst_tys ispec = mkLocalInstance dfun overlap_flag - ; return ([InstInfo { iSpec = ispec, - iBinds = VanillaInst binds uprags }], - catMaybes idx_tycons) + ; return (InstInfo { iSpec = ispec, + iBinds = VanillaInst binds uprags False }, + idx_tycons) } where -- We pass in the source form and the type checked form of the ATs. We @@ -277,7 +431,7 @@ tcLocalInstDecl1 (L loc (InstDecl poly_ty binds uprags ats)) checkValidAndMissingATs :: Class -> ([TyVar], [TcType]) -- instance types -> [(LTyClDecl Name, -- source form of AT - Maybe TyThing)] -- Core form of AT + TyThing)] -- Core form of AT -> TcM () checkValidAndMissingATs clas inst_tys ats = do { -- Issue a warning for each class AT that is not defined in this @@ -295,13 +449,11 @@ tcLocalInstDecl1 (L loc (InstDecl poly_ty binds uprags ats)) ; mapM_ (checkIndexes clas inst_tys) ats } - checkIndexes _ _ (_, Nothing) = - return () -- skip, we already had an error here - checkIndexes clas inst_tys (hsAT, Just (ATyCon tycon)) = + checkIndexes clas inst_tys (hsAT, ATyCon tycon) -- !!!TODO: check that this does the Right Thing for indexed synonyms, too! - checkIndexes' clas inst_tys hsAT - (tyConTyVars tycon, - snd . fromJust . tyConFamInst_maybe $ tycon) + = checkIndexes' clas inst_tys hsAT + (tyConTyVars tycon, + snd . fromJust . tyConFamInst_maybe $ tycon) checkIndexes _ _ _ = panic "checkIndexes" checkIndexes' clas (instTvs, instTys) hsAT (atTvs, atTys) @@ -311,11 +463,7 @@ tcLocalInstDecl1 (L loc (InstDecl poly_ty binds uprags ats)) addErrCtxt (atInstCtxt atName) $ case find ((atName ==) . tyConName) (classATs clas) of Nothing -> addErrTc $ badATErr clas atName -- not in this class - Just atDecl -> - case assocTyConArgPoss_maybe atDecl of - Nothing -> panic "checkIndexes': AT has no args poss?!?" - Just poss -> - + Just atycon -> -- The following is tricky! We need to deal with three -- complications: (1) The AT possibly only uses a subset of -- the class parameters as indexes and those it uses may be in @@ -323,6 +471,13 @@ tcLocalInstDecl1 (L loc (InstDecl poly_ty binds uprags ats)) -- which must be type variables; and (3) variables in AT and -- instance head will be different `Name's even if their -- source lexemes are identical. + -- + -- e.g. class C a b c where + -- data D b a :: * -> * -- NB (1) b a, omits c + -- instance C [x] Bool Char where + -- data D Bool [x] v = MkD x [v] -- NB (2) v + -- -- NB (3) the x in 'instance C...' have differnt + -- -- Names to x's in 'data D...' -- -- Re (1), `poss' contains a permutation vector to extract the -- class parameters in the right order. @@ -336,7 +491,19 @@ tcLocalInstDecl1 (L loc (InstDecl poly_ty binds uprags ats)) -- instance types with the instance type variable sharing its -- source lexeme. -- - let relevantInstTys = map (instTys !!) poss + let poss :: [Int] + -- For *associated* type families, gives the position + -- of that 'TyVar' in the class argument list (0-indexed) + -- e.g. class C a b c where { type F c a :: *->* } + -- Then we get Just [2,0] + poss = catMaybes [ tv `elemIndex` classTyVars clas + | tv <- tyConTyVars atycon] + -- We will get Nothings for the "extra" type + -- variables in an associated data type + -- e.g. class C a where { data D a :: *->* } + -- here D gets arity 2 and has two tyvars + + relevantInstTys = map (instTys !!) poss instArgs = map Just relevantInstTys ++ repeat Nothing -- extra arguments renaming = substSameTyVar atTvs instTvs @@ -367,13 +534,13 @@ tcLocalInstDecl1 (L loc (InstDecl poly_ty binds uprags ats)) %************************************************************************ %* * -\subsection{Type-checking instance declarations, pass 2} + Type-checking instance declarations, pass 2 %* * %************************************************************************ \begin{code} tcInstDecls2 :: [LTyClDecl Name] -> [InstInfo Name] - -> TcM (LHsBinds Id, TcLclEnv) + -> TcM (LHsBinds Id) -- (a) From each class declaration, -- generate any default-method bindings -- (b) From each instance decl @@ -381,471 +548,601 @@ tcInstDecls2 :: [LTyClDecl Name] -> [InstInfo Name] tcInstDecls2 tycl_decls inst_decls = do { -- (a) Default methods from class decls - (dm_binds_s, dm_ids_s) <- mapAndUnzipM tcClassDecl2 $ - filter (isClassDecl.unLoc) tycl_decls - ; tcExtendIdEnv (concat dm_ids_s) $ do - + let class_decls = filter (isClassDecl . unLoc) tycl_decls + ; dm_binds_s <- mapM tcClassDecl2 class_decls + ; let dm_binds = unionManyBags dm_binds_s + -- (b) instance declarations - ; inst_binds_s <- mapM tcInstDecl2 inst_decls + ; let dm_ids = collectHsBindsBinders dm_binds + -- Add the default method Ids (again) + -- See Note [Default methods and instances] + ; inst_binds_s <- tcExtendIdEnv dm_ids $ + mapM tcInstDecl2 inst_decls -- Done - ; let binds = unionManyBags dm_binds_s `unionBags` - unionManyBags inst_binds_s - ; tcl_env <- getLclEnv -- Default method Ids in here - ; return (binds, tcl_env) } + ; return (dm_binds `unionBags` unionManyBags inst_binds_s) } + +tcInstDecl2 :: InstInfo Name -> TcM (LHsBinds Id) +tcInstDecl2 (InstInfo { iSpec = ispec, iBinds = ibinds }) + = recoverM (return emptyLHsBinds) $ + setSrcSpan loc $ + addErrCtxt (instDeclCtxt2 (idType dfun_id)) $ + tc_inst_decl2 dfun_id ibinds + where + dfun_id = instanceDFunId ispec + loc = getSrcSpan dfun_id \end{code} -======= New documentation starts here (Sept 92) ============== +See Note [Default methods and instances] +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +The default method Ids are already in the type environment (see Note +[Default method Ids and Template Haskell] in TcTyClsDcls), BUT they +don't have their InlinePragmas yet. Usually that would not matter, +because the simplifier propagates information from binding site to +use. But, unusually, when compiling instance decls we *copy* the +INLINE pragma from the default method to the method for that +particular operation (see Note [INLINE and default methods] below). -The main purpose of @tcInstDecl2@ is to return a @HsBinds@ which defines -the dictionary function for this instance declaration. For example +So right here in tcInstDecl2 we must re-extend the type envt with +the default method Ids replete with their INLINE pragmas. Urk. - instance Foo a => Foo [a] where - op1 x = ... - op2 y = ... +\begin{code} +tc_inst_decl2 :: Id -> InstBindings Name -> TcM (LHsBinds Id) +-- Returns a binding for the dfun +tc_inst_decl2 dfun_id inst_binds + = 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 -might generate something like + -- Instantiate the super-class context with inst_tys + sc_theta' = substTheta (zipOpenTvSubst class_tyvars inst_tys') sc_theta - dfun.Foo.List dFoo_a = let op1 x = ... - op2 y = ... - in - Dict [op1, op2] + -- Create dictionary Ids from the specified instance contexts. + ; dfun_ev_vars <- newEvVars dfun_theta' + ; self_dict <- newSelfDict clas inst_tys' + -- Default-method Ids may be mentioned in synthesised RHSs, + -- but they'll already be in the environment. -HOWEVER, if the instance decl has no context, then it returns a -bigger @HsBinds@ with declarations for each method. For example + -- Cook up a binding for "self = df d1 .. dn", + -- to use in each method binding + -- Why? See Note [Subtle interaction of recursion and overlap] + ; let self_ev_bind = EvBind self_dict $ + EvDFunApp dfun_id (mkTyVarTys inst_tyvars') dfun_ev_vars - instance Foo [a] where - op1 x = ... - op2 y = ... + -- Deal with 'SPECIALISE instance' pragmas + -- See Note [SPECIALISE instance pragmas] + ; spec_info <- tcSpecInstPrags dfun_id inst_binds -might produce + -- Typecheck the methods + ; (meth_ids, meth_binds) + <- tcExtendTyVarEnv inst_tyvars' $ + tcInstanceMethods dfun_id clas inst_tyvars' dfun_ev_vars + inst_tys' self_ev_bind spec_info + op_items inst_binds + + -- Figure out bindings for the superclass context + ; let tc_sc = tcSuperClass inst_tyvars' dfun_ev_vars self_ev_bind + (sc_eqs, sc_dicts) = splitAt (classSCNEqs clas) sc_theta' + ; (sc_dict_ids, sc_binds) <- ASSERT( equalLength sc_sels sc_dicts ) + ASSERT( all isEqPred sc_eqs ) + mapAndUnzipM tc_sc (sc_sels `zip` sc_dicts) + + -- NOT FINISHED! + ; (_eq_sc_binds, sc_eq_vars) <- checkConstraints InstSkol emptyVarSet + inst_tyvars' dfun_ev_vars $ + emitWanteds ScOrigin sc_eqs + + -- Create the result bindings + ; let dict_constr = classDataCon clas + dict_bind = mkVarBind self_dict dict_rhs + dict_rhs = foldl mk_app inst_constr dict_and_meth_ids + dict_and_meth_ids = sc_dict_ids ++ meth_ids + inst_constr = L loc $ wrapId (mkWpEvVarApps sc_eq_vars + <.> 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 = mkWpEvVarApps dfun_ev_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 inst_ty (map Var dict_and_meth_ids) + -- Not right for equality superclasses + `setInlinePragma` dfunInlinePragma + + (spec_inst_prags, _) = spec_info + main_bind = AbsBinds { abs_tvs = inst_tyvars' + , abs_ev_vars = dfun_ev_vars + , abs_exports = [(inst_tyvars', dfun_id_w_fun, self_dict, + SpecPrags spec_inst_prags)] + , abs_ev_binds = emptyTcEvBinds + , abs_binds = unitBag dict_bind } + + ; return (unitBag (L loc main_bind) `unionBags` + listToBag meth_binds `unionBags` + listToBag sc_binds) + } + +------------------------------ +tcSpecInstPrags :: DFunId -> InstBindings Name + -> TcM ([Located TcSpecPrag], PragFun) +tcSpecInstPrags _ (NewTypeDerived {}) + = return ([], \_ -> []) +tcSpecInstPrags dfun_id (VanillaInst binds uprags _) + = do { spec_inst_prags <- mapM (wrapLocM (tcSpecInst dfun_id)) $ + filter isSpecInstLSig uprags + -- The filter removes the pragmas for methods + ; return (spec_inst_prags, mkPragFun uprags binds) } + +------------------------------ +tcSuperClass :: [TyVar] -> [EvVar] + -> EvBind + -> (Id, PredType) -> 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 tyvars dicts + self_ev_bind@(EvBind self_dict _) + (sc_sel, sc_pred) + = do { (ev_binds, wanted, sc_dict) + <- newImplication InstSkol emptyVarSet tyvars dicts $ + emitWanted ScOrigin sc_pred + + ; simplifySuperClass self_dict wanted + -- We include self_dict in the 'givens'; the simplifier + -- is clever enough to stop sc_pred geting bound by just + -- selecting from self_dict!! + + ; uniq <- newUnique + ; let sc_op_ty = mkForAllTys tyvars $ mkPiTypes dicts (varType sc_dict) + sc_op_name = mkDerivedInternalName mkClassOpAuxOcc uniq + (getName sc_sel) + sc_op_id = mkLocalId sc_op_name sc_op_ty + sc_op_bind = VarBind { var_id = sc_op_id, var_inline = False + , var_rhs = L noSrcSpan $ wrapId sc_wrapper sc_dict } + sc_wrapper = mkWpTyLams tyvars + <.> mkWpLams dicts + <.> mkWpLet (EvBinds (unitBag self_ev_bind)) + <.> mkWpLet ev_binds + + ; return (sc_op_id, noLoc sc_op_bind) } +\end{code} - dfun.Foo.List a = Dict [Foo.op1.List a, Foo.op2.List a] - const.Foo.op1.List a x = ... - const.Foo.op2.List a y = ... +Note [Recursive superclasses] +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +See Trac #1470 for why we would *like* to add "self_dict" to the +available instances here. But we can't do so because then the superclases +get satisfied by selection from self_dict, and that leads to an immediate +loop. What we need is to add self_dict to Avails without adding its +superclasses, and we currently have no way to do that. -This group may be mutually recursive, because (for example) there may -be no method supplied for op2 in which case we'll get +Note [SPECIALISE instance pragmas] +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +Consider - const.Foo.op2.List a = default.Foo.op2 (dfun.Foo.List a) + instance (Ix a, Ix b) => Ix (a,b) where + {-# SPECIALISE instance Ix (Int,Int) #-} + range (x,y) = ... -that is, the default method applied to the dictionary at this type. -What we actually produce in either case is: +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: - AbsBinds [a] [dfun_theta_dicts] - [(dfun.Foo.List, d)] ++ (maybe) [(const.Foo.op1.List, op1), ...] - { d = (sd1,sd2, ..., op1, op2, ...) - op1 = ... - op2 = ... - } + $dfIx :: (Ix a, Ix x) => Ix (a,b) + {- DFUN [$crange, ...] -} + $dfIx da db = Ix ($crange da db) (...other methods...) -The "maybe" says that we only ask AbsBinds to make global constant methods -if the dfun_theta is empty. + $dfIxPair :: (Ix a, Ix x) => Ix (a,b) + {- DFUN [$crangePair, ...] -} + $dfIxPair = Ix ($crangePair da db) (...other methods...) -For an instance declaration, say, + $crange :: (Ix a, Ix b) -> ((a,b),(a,b)) -> [(a,b)] + {-# SPECIALISE $crange :: ((Int,Int),(Int,Int)) -> [(Int,Int)] #-} + $crange da db = - instance (C1 a, C2 b) => C (T a b) where - ... + {-# RULE range ($dfIx da db) = $crange da db #-} -where the {\em immediate} superclasses of C are D1, D2, we build a dictionary -function whose type is +Note that - (C1 a, C2 b, D1 (T a b), D2 (T a b)) => C (T a b) + * 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 -Notice that we pass it the superclass dictionaries at the instance type; this -is the ``Mark Jones optimisation''. The stuff before the "=>" here -is the @dfun_theta@ below. + * 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} -tcInstDecl2 :: InstInfo Name -> TcM (LHsBinds Id) --- Returns a binding for the dfun +tcSpecInst :: Id -> Sig Name -> TcM TcSpecPrag +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 <- tcSubType (SpecPragOrigin name) (SigSkol SpecInstCtxt) + (idType dfun_id) spec_ty + ; return (SpecPrag co_fn defaultInlinePragma) } + where + spec_ctxt prag = hang (ptext (sLit "In the SPECIALISE pragma")) 2 (ppr prag) ------------------------- --- Derived newtype instances; surprisingly tricky! --- --- class Show a => Foo a b where ... --- newtype N a = MkN (Tree [a]) deriving( Foo Int ) +tcSpecInst _ _ = panic "tcSpecInst" +\end{code} + +%************************************************************************ +%* * + Type-checking an instance method +%* * +%************************************************************************ + +tcInstanceMethod +- Make the method bindings, as a [(NonRec, HsBinds)], one per method +- Remembering to use fresh Name (the instance method Name) as the binder +- Bring the instance method Ids into scope, for the benefit of tcInstSig +- Use sig_fn mapping instance method Name -> instance tyvars +- Ditto prag_fn +- Use tcValBinds to do the checking + +\begin{code} +tcInstanceMethods :: DFunId -> Class -> [TcTyVar] + -> [EvVar] + -> [TcType] + -> EvBind -- "This" and its binding + -> ([Located TcSpecPrag], PragFun) + -> [(Id, DefMeth)] + -> InstBindings Name + -> TcM ([Id], [LHsBind Id]) + -- The returned inst_meth_ids all have types starting + -- forall tvs. theta => ... +tcInstanceMethods dfun_id clas tyvars dfun_ev_vars inst_tys + self_dict_ev (spec_inst_prags, prag_fn) + op_items (VanillaInst binds _ standalone_deriv) + = mapAndUnzipM tc_item op_items + where + ---------------------- + tc_item :: (Id, DefMeth) -> TcM (Id, LHsBind Id) + tc_item (sel_id, dm_info) + = case findMethodBind (idName sel_id) binds of + Just user_bind -> tc_body sel_id standalone_deriv user_bind + Nothing -> tc_default sel_id dm_info + + ---------------------- + tc_body :: Id -> Bool -> LHsBind Name -> TcM (TcId, LHsBind Id) + tc_body sel_id generated_code rn_bind + = add_meth_ctxt generated_code rn_bind $ + do { (meth_id, local_meth_id) <- mkMethIds clas tyvars dfun_ev_vars + inst_tys sel_id + ; (meth_id1, spec_prags) <- tcPrags NonRecursive False True + meth_id (prag_fn (idName sel_id)) + + ; bind <- tcInstanceMethodBody InstSkol + tyvars dfun_ev_vars + mb_dict_ev + meth_id1 local_meth_id + meth_sig_fn + (SpecPrags (spec_inst_prags ++ spec_prags)) + rn_bind + ; return (meth_id1, bind) } + + ---------------------- + tc_default :: Id -> DefMeth -> TcM (TcId, LHsBind Id) + tc_default sel_id GenDefMeth -- Derivable type classes stuff + = do { meth_bind <- mkGenericDefMethBind clas inst_tys sel_id + ; tc_body sel_id False {- Not generated code? -} meth_bind } + + tc_default sel_id NoDefMeth -- No default method at all + = do { warnMissingMethod sel_id + ; (meth_id, _) <- mkMethIds clas tyvars dfun_ev_vars + inst_tys sel_id + ; return (meth_id, mkVarBind meth_id $ + mkLHsWrap lam_wrapper error_rhs) } + where + error_rhs = L loc $ HsApp error_fun error_msg + error_fun = L loc $ wrapId (WpTyApp meth_tau) nO_METHOD_BINDING_ERROR_ID + error_msg = L loc (HsLit (HsStringPrim (mkFastString error_string))) + meth_tau = funResultTy (applyTys (idType sel_id) inst_tys) + error_string = showSDoc (hcat [ppr loc, text "|", ppr sel_id ]) + lam_wrapper = mkWpTyLams tyvars <.> mkWpLams dfun_ev_vars + + tc_default sel_id (DefMeth dm_name) -- A polymorphic default method + = do { -- Build the typechecked version directly, + -- without calling typecheck_method; + -- see Note [Default methods in instances] + -- Generate /\as.\ds. let this = df as ds + -- in $dm inst_tys this + -- The 'let' is necessary only because HsSyn doesn't allow + -- you to apply a function to a dictionary *expression*. + + ; (meth_id, local_meth_id) <- mkMethIds clas tyvars dfun_ev_vars + inst_tys sel_id + ; dm_id <- tcLookupId dm_name + ; let dm_inline_prag = idInlinePragma dm_id + EvBind self_dict _ = self_dict_ev + rhs = HsWrap (mkWpEvVarApps [self_dict] <.> mkWpTyApps inst_tys) $ + HsVar dm_id + + meth_bind = L loc $ VarBind { var_id = local_meth_id + , var_rhs = L loc rhs + , var_inline = False } + meth_id1 = meth_id `setInlinePragma` dm_inline_prag + -- Copy the inline pragma (if any) from the default + -- method to this version. Note [INLINE and default methods] + + bind = AbsBinds { abs_tvs = tyvars, abs_ev_vars = dfun_ev_vars + , abs_exports = [( tyvars, meth_id1, local_meth_id + , SpecPrags spec_inst_prags)] + , abs_ev_binds = EvBinds (unitBag self_dict_ev) + , abs_binds = unitBag meth_bind } + -- Default methods in an instance declaration can't have their own + -- INLINE or SPECIALISE pragmas. It'd be possible to allow them, but + -- currently they are rejected with + -- "INLINE pragma lacks an accompanying binding" + + ; return (meth_id1, L loc bind) } + + ---------------------- + loc = getSrcSpan dfun_id + meth_sig_fn _ = Just ([],loc) -- The 'Just' says "yes, there's a type sig" + -- But there are no scoped type variables from local_method_id + -- Only the ones from the instance decl itself, which are already + -- in scope. Example: + -- class C a where { op :: forall b. Eq b => ... } + -- instance C [c] where { op = } + -- In , 'c' is scope but 'b' is not! + + mb_dict_ev = if null tyvars then Nothing else Just self_dict_ev + -- Only need the self_dict stuff if there are type + -- variables involved; otherwise overlap is not possible + -- See Note [Subtle interaction of recursion and overlap] + -- in TcInstDcls + + -- For instance decls that come from standalone deriving clauses + -- we want to print out the full source code if there's an error + -- because otherwise the user won't see the code at all + add_meth_ctxt generated_code rn_bind thing + | generated_code = addLandmarkErrCtxt (derivBindCtxt clas inst_tys rn_bind) thing + | otherwise = thing + + +tcInstanceMethods dfun_id clas tyvars dfun_ev_vars inst_tys + _ _ op_items (NewTypeDerived coi _) + +-- Running example: +-- class Show b => Foo a b where +-- op :: a -> b -> b +-- newtype N a = MkN (Tree [a]) +-- deriving instance (Show p, Foo Int p) => Foo Int (N p) +-- -- NB: standalone deriving clause means +-- -- that the contex is user-specified +-- Hence op :: forall a b. Foo a b => a -> b -> b -- --- The newtype gives an FC axiom looking like --- axiom CoN a :: N a :=: Tree [a] --- (see Note [Newtype coercions] in TyCon for this unusual form of axiom) +-- We're going to make an instance like +-- instance (Show p, Foo Int p) => Foo Int (N p) +-- op = $copT -- --- So all need is to generate a binding looking like: --- dfunFooT :: forall a. (Foo Int (Tree [a], Show (N a)) => Foo Int (N a) --- dfunFooT = /\a. \(ds:Show (N a)) (df:Foo (Tree [a])). --- case df `cast` (Foo Int (sym (CoN a))) of --- Foo _ op1 .. opn -> Foo ds op1 .. opn +-- $copT :: forall p. (Show p, Foo Int p) => Int -> N p -> N p +-- $copT p (d1:Show p) (d2:Foo Int p) +-- = op Int (Tree [p]) rep_d |> op_co +-- where +-- rep_d :: Foo Int (Tree [p]) = ...d1...d2... +-- op_co :: (Int -> Tree [p] -> Tree [p]) ~ (Int -> T p -> T p) +-- We get op_co by substituting [Int/a] and [co/b] in type for op +-- where co : [p] ~ T p -- --- If there are no superclasses, matters are simpler, because we don't need the case --- see Note [Newtype deriving superclasses] in TcDeriv.lhs - -tcInstDecl2 (InstInfo { iSpec = ispec, iBinds = NewTypeDerived }) - = do { let dfun_id = instanceDFunId ispec - rigid_info = InstSkol - origin = SigOrigin rigid_info - inst_ty = idType dfun_id - ; (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 - (nt_tycon, tc_args) = tcSplitTyConApp last_ty -- Can't fail - rep_ty = newTyConInstRhs nt_tycon tc_args - - rep_pred = mkClassPred cls (initial_cls_inst_tys ++ [rep_ty]) - -- In our example, rep_pred is (Foo Int (Tree [a])) - the_coercion = make_coercion cls_tycon initial_cls_inst_tys nt_tycon tc_args - -- Coercion of kind (Foo Int (Tree [a]) ~ Foo Int (N a) - - ; inst_loc <- getInstLoc origin - ; sc_loc <- getInstLoc InstScOrigin - ; dfun_dicts <- newDictBndrs inst_loc theta - ; sc_dicts <- newDictBndrs sc_loc sc_theta' - ; this_dict <- newDictBndr inst_loc (mkClassPred cls cls_inst_tys) - ; rep_dict <- newDictBndr inst_loc rep_pred - - -- Figure out bindings for the superclass context from dfun_dicts - -- 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_dicts (rep_dict:sc_dicts) - - ; let coerced_rep_dict = mkHsWrap the_coercion (HsVar (instToId rep_dict)) - - ; body <- make_body cls_tycon cls_inst_tys sc_dicts coerced_rep_dict - ; let dict_bind = noLoc $ VarBind (instToId this_dict) (noLoc body) - - ; return (unitBag $ noLoc $ - AbsBinds tvs (map instToVar dfun_dicts) - [(tvs, dfun_id, instToId this_dict, [])] - (dict_bind `consBag` sc_binds)) } +-- Notice that the dictionary bindings "..d1..d2.." must be generated +-- by the constraint solver, since the may be +-- user-specified. + + = do { rep_d_stuff <- checkConstraints InstSkol emptyVarSet tyvars dfun_ev_vars $ + emitWanted ScOrigin rep_pred + + ; mapAndUnzipM (tc_item rep_d_stuff) op_items } where - ----------------------- - -- make_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 ) - -- 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 - - make_coercion cls_tycon initial_cls_inst_tys nt_tycon tc_args - | Just co_con <- newTyConCo_maybe nt_tycon - , let co = mkSymCoercion (mkTyConApp co_con tc_args) - = WpCast (mkTyConApp cls_tycon (initial_cls_inst_tys ++ [co])) - | otherwise -- The newtype is transparent; no need for a cast - = idHsWrapper - - ----------------------- - -- (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 - -tcInstDecl2 (InstInfo { iSpec = ispec, iBinds = VanillaInst monobinds uprags }) - = let - dfun_id = instanceDFunId ispec - rigid_info = InstSkol - inst_ty = idType dfun_id - loc = getSrcSpan dfun_id - in - -- Prime error recovery - recoverM (return emptyLHsBinds) $ - setSrcSpan loc $ - addErrCtxt (instDeclCtxt2 (idType dfun_id)) $ do + loc = getSrcSpan dfun_id + + inst_tvs = fst (tcSplitForAllTys (idType dfun_id)) + Just (init_inst_tys, _) = snocView inst_tys + rep_ty = fst (coercionKind co) -- [p] + rep_pred = mkClassPred clas (init_inst_tys ++ [rep_ty]) + + -- co : [p] ~ T p + co = substTyWith inst_tvs (mkTyVarTys tyvars) $ + case coi of { IdCo ty -> ty ; + ACo co -> mkSymCoercion co } + + ---------------- + tc_item :: (TcEvBinds, EvVar) -> (Id, DefMeth) -> TcM (TcId, LHsBind TcId) + tc_item (rep_ev_binds, rep_d) (sel_id, _) + = do { (meth_id, local_meth_id) <- mkMethIds clas tyvars dfun_ev_vars + inst_tys sel_id + + ; let meth_rhs = wrapId (mk_op_wrapper sel_id rep_d) sel_id + meth_bind = VarBind { var_id = local_meth_id + , var_rhs = L loc meth_rhs + , var_inline = False } + + bind = AbsBinds { abs_tvs = tyvars, abs_ev_vars = dfun_ev_vars + , abs_exports = [(tyvars, meth_id, + local_meth_id, noSpecPrags)] + , abs_ev_binds = rep_ev_binds + , abs_binds = unitBag $ L loc meth_bind } + + ; return (meth_id, L loc bind) } + + ---------------- + mk_op_wrapper :: Id -> EvVar -> HsWrapper + mk_op_wrapper sel_id rep_d + = WpCast (substTyWith sel_tvs (init_inst_tys ++ [co]) local_meth_ty) + <.> WpEvApp (EvId rep_d) + <.> mkWpTyApps (init_inst_tys ++ [rep_ty]) + where + (sel_tvs, sel_rho) = tcSplitForAllTys (idType sel_id) + (_, local_meth_ty) = tcSplitPredFunTy_maybe sel_rho + `orElse` pprPanic "tcInstanceMethods" (ppr sel_id) + +---------------------- +mkMethIds :: Class -> [TcTyVar] -> [EvVar] -> [TcType] -> Id -> TcM (TcId, TcId) +mkMethIds clas tyvars dfun_ev_vars inst_tys sel_id + = do { uniq <- newUnique + ; let meth_name = mkDerivedInternalName mkClassOpAuxOcc uniq sel_name + ; local_meth_name <- newLocalName sel_name + -- Base the local_meth_name on the selector name, becuase + -- type errors from tcInstanceMethodBody come from here + + ; let meth_id = mkLocalId meth_name meth_ty + local_meth_id = mkLocalId local_meth_name local_meth_ty + ; return (meth_id, local_meth_id) } + where + local_meth_ty = instantiateMethod clas sel_id inst_tys + meth_ty = mkForAllTys tyvars $ mkPiTypes dfun_ev_vars local_meth_ty + sel_name = idName sel_id + +---------------------- +wrapId :: HsWrapper -> id -> HsExpr id +wrapId wrapper id = mkHsWrap wrapper (HsVar id) + +derivBindCtxt :: Class -> [Type ] -> LHsBind Name -> SDoc +derivBindCtxt clas tys bind + = vcat [ ptext (sLit "When typechecking a standalone-derived method for") + <+> quotes (pprClassPred clas tys) <> colon + , nest 2 $ pprSetDepth AllTheWay $ ppr bind ] + +warnMissingMethod :: Id -> TcM () +warnMissingMethod sel_id + = do { warn <- doptM Opt_WarnMissingMethods + ; warnTc (warn -- Warn only if -fwarn-missing-methods + && not (startsWithUnderscore (getOccName sel_id))) + -- Don't warn about _foo methods + (ptext (sLit "No explicit method nor default method for") + <+> quotes (ppr sel_id)) } +\end{code} - -- 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 +Note [Export helper functions] +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +We arrange to export the "helper functions" of an instance declaration, +so that they are not subject to preInlineUnconditionally, even if their +RHS is trivial. Reason: they are mentioned in the DFunUnfolding of +the dict fun as Ids, not as CoreExprs, so we can't substitute a +non-variable for them. - -- 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' +We could change this by making DFunUnfoldings have CoreExprs, but it +seems a bit simpler this way. - -- 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. +Note [Default methods in instances] +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +Consider this - -- 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 = dfun_insts ++ given_sc_insts - - (meth_ids, meth_binds) <- tcMethods origin clas inst_tyvars' - dfun_theta' inst_tys' this_dict 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 - where - unEqPred (EqPred ty1 ty2) = (ty1, ty2) - unEqPred _ = panic "TcInstDcls.mkCoVars" + class Baz v x where + foo :: x -> x + foo y = -mkMetaCoVars :: [PredType] -> TcM [TyVar] -mkMetaCoVars = mapM eqPredToCoVar - where - eqPredToCoVar (EqPred ty1 ty2) = newMetaCoVar ty1 ty2 - eqPredToCoVar _ = panic "TcInstDcls.mkMetaCoVars" - -tcMethods :: InstOrigin -> Class -> [TcTyVar] -> TcThetaType -> [TcType] - -> Inst -> [Inst] -> [(Id, DefMeth)] -> LHsBindsLR Name Name - -> [LSig Name] - -> TcM ([Id], Bag (LHsBind Id)) -tcMethods origin clas inst_tyvars' dfun_theta' inst_tys' - this_dict extra_insts op_items monobinds uprags = do - -- Check that all the method bindings come from this class - let - sel_names = [idName sel_id | (sel_id, _) <- op_items] - bad_bndrs = collectHsBindBinders monobinds `minusList` sel_names - - mapM (addErrTc . badMethodErr clas) bad_bndrs - - -- Make the method bindings - let - mk_method_id (sel_id, _) = mkMethId origin clas sel_id inst_tys' - - (meth_insts, meth_ids) <- mapAndUnzipM mk_method_id op_items - - -- And type check them - -- It's really worth making meth_insts available to the tcMethodBind - -- Consider instance Monad (ST s) where - -- {-# INLINE (>>) #-} - -- (>>) = ...(>>=)... - -- If we don't include meth_insts, we end up with bindings like this: - -- rec { dict = MkD then bind ... - -- then = inline_me (... (GHC.Base.>>= dict) ...) - -- bind = ... } - -- The trouble is that (a) 'then' and 'dict' are mutually recursive, - -- and (b) the inline_me prevents us inlining the >>= selector, which - -- would unravel the loop. Result: (>>) ends up as a loop breaker, and - -- is not inlined across modules. Rather ironic since this does not - -- happen without the INLINE pragma! - -- - -- Solution: make meth_insts available, so that 'then' refers directly - -- to the local 'bind' rather than going via the dictionary. - -- - -- BUT WATCH OUT! If the method type mentions the class variable, then - -- this optimisation is not right. Consider - -- class C a where - -- op :: Eq a => a - -- - -- instance C Int where - -- op = op - -- The occurrence of 'op' on the rhs gives rise to a constraint - -- op at Int - -- The trouble is that the 'meth_inst' for op, which is 'available', also - -- looks like 'op at Int'. But they are not the same. - let - prag_fn = mkPragFun uprags - all_insts = extra_insts ++ catMaybes meth_insts - sig_fn _ = Just [] -- No scoped type variables, but every method has - -- a type signature, in effect, so that we check - -- the method has the right type - tc_method_bind = tcMethodBind origin inst_tyvars' dfun_theta' this_dict - all_insts sig_fn prag_fn monobinds - - meth_binds_s <- zipWithM tc_method_bind op_items meth_ids - - return (meth_ids, unionManyBags meth_binds_s) -\end{code} + instance Baz Int Int + +From the class decl we get + + $dmfoo :: forall v x. Baz v x => x -> x + $dmfoo y = +Notice that the type is ambiguous. That's fine, though. The instance +decl generates - ------------------------------ - [Inline dfuns] Inlining dfuns unconditionally - ------------------------------ - -The code above unconditionally inlines dict funs. Here's why. -Consider this program: - - test :: Int -> Int -> Bool - test x y = (x,y) == (y,x) || test y x - -- Recursive to avoid making it inline. - -This needs the (Eq (Int,Int)) instance. If we inline that dfun -the code we end up with is good: - - Test.$wtest = - \r -> case ==# [ww ww1] of wild { - PrelBase.False -> Test.$wtest ww1 ww; - PrelBase.True -> - case ==# [ww1 ww] of wild1 { - PrelBase.False -> Test.$wtest ww1 ww; - PrelBase.True -> PrelBase.True []; - }; - }; - Test.test = \r [w w1] - case w of w2 { - PrelBase.I# ww -> - case w1 of w3 { PrelBase.I# ww1 -> Test.$wtest ww ww1; }; - }; - -If we don't inline the dfun, the code is not nearly as good: - - (==) = case PrelTup.$fEq(,) PrelBase.$fEqInt PrelBase.$fEqInt of tpl { - PrelBase.:DEq tpl1 tpl2 -> tpl2; - }; - - Test.$wtest = - \r [ww ww1] - let { y = PrelBase.I#! [ww1]; } in - let { x = PrelBase.I#! [ww]; } in - let { sat_slx = PrelTup.(,)! [y x]; } in - let { sat_sly = PrelTup.(,)! [x y]; - } in - case == sat_sly sat_slx of wild { - PrelBase.False -> Test.$wtest ww1 ww; - PrelBase.True -> PrelBase.True []; - }; - - Test.test = - \r [w w1] - case w of w2 { - PrelBase.I# ww -> - case w1 of w3 { PrelBase.I# ww1 -> Test.$wtest ww ww1; }; - }; - -Why doesn't GHC inline $fEq? Because it looks big: - - PrelTup.zdfEqZ1T{-rcX-} - = \ @ a{-reT-} :: * @ b{-reS-} :: * - zddEq{-rf6-} _Ks :: {PrelBase.Eq{-23-} a{-reT-}} - zddEq1{-rf7-} _Ks :: {PrelBase.Eq{-23-} b{-reS-}} -> - let { - zeze{-rf0-} _Kl :: (b{-reS-} -> b{-reS-} -> PrelBase.Bool{-3c-}) - zeze{-rf0-} = PrelBase.zeze{-01L-}@ b{-reS-} zddEq1{-rf7-} } in - let { - zeze1{-rf3-} _Kl :: (a{-reT-} -> a{-reT-} -> PrelBase.Bool{-3c-}) - zeze1{-rf3-} = PrelBase.zeze{-01L-} @ a{-reT-} zddEq{-rf6-} } in - let { - zeze2{-reN-} :: ((a{-reT-}, b{-reS-}) -> (a{-reT-}, b{-reS-})-> PrelBase.Bool{-3c-}) - zeze2{-reN-} = \ ds{-rf5-} _Ks :: (a{-reT-}, b{-reS-}) - ds1{-rf4-} _Ks :: (a{-reT-}, b{-reS-}) -> - case ds{-rf5-} - of wild{-reW-} _Kd { (a1{-rf2-} _Ks, a2{-reZ-} _Ks) -> - case ds1{-rf4-} - of wild1{-reX-} _Kd { (b1{-rf1-} _Ks, b2{-reY-} _Ks) -> - PrelBase.zaza{-r4e-} - (zeze1{-rf3-} a1{-rf2-} b1{-rf1-}) - (zeze{-rf0-} a2{-reZ-} b2{-reY-}) - } - } } in - let { - a1{-reR-} :: ((a{-reT-}, b{-reS-})-> (a{-reT-}, b{-reS-})-> PrelBase.Bool{-3c-}) - a1{-reR-} = \ a2{-reV-} _Ks :: (a{-reT-}, b{-reS-}) - b1{-reU-} _Ks :: (a{-reT-}, b{-reS-}) -> - PrelBase.not{-r6I-} (zeze2{-reN-} a2{-reV-} b1{-reU-}) - } in - PrelBase.zdwZCDEq{-r8J-} @ (a{-reT-}, b{-reS-}) a1{-reR-} zeze2{-reN-}) - -and it's not as bad as it seems, because it's further dramatically -simplified: only zeze2 is extracted and its body is simplified. + $dBazIntInt = MkBaz fooIntInt + fooIntInt = $dmfoo Int Int $dBazIntInt + +BUT this does mean we must generate the dictionary translation of +fooIntInt directly, rather than generating source-code and +type-checking it. That was the bug in Trac #1061. In any case it's +less work to generate the translated version! + +Note [INLINE and default methods] +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +Default methods need special case. They are supposed to behave rather like +macros. For exmample + + class Foo a where + op1, op2 :: Bool -> a -> a + + {-# INLINE op1 #-} + op1 b x = op2 (not b) x + + instance Foo Int where + -- op1 via default method + op2 b x = + +The instance declaration should behave + + just as if 'op1' had been defined with the + code, and INLINE pragma, from its original + definition. + +That is, just as if you'd written + + instance Foo Int where + op2 b x = + + {-# INLINE op1 #-} + op1 b x = op2 (not b) x + +So for the above example we generate: + + + {-# INLINE $dmop1 #-} + -- $dmop1 has an InlineCompulsory unfolding + $dmop1 d b x = op2 d (not b) x + + $fFooInt = MkD $cop1 $cop2 + + {-# INLINE $cop1 #-} + $cop1 = $dmop1 $fFooInt + + $cop2 = + +Note carefullly: + +* We *copy* any INLINE pragma from the default method $dmop1 to the + instance $cop1. Otherwise we'll just inline the former in the + latter and stop, which isn't what the user expected + +* Regardless of its pragma, we give the default method an + unfolding with an InlineCompulsory source. That means + that it'll be inlined at every use site, notably in + each instance declaration, such as $cop1. This inlining + must happen even though + a) $dmop1 is not saturated in $cop1 + b) $cop1 itself has an INLINE pragma + + It's vital that $dmop1 *is* inlined in this way, to allow the mutual + recursion between $fooInt and $cop1 to be broken + +* To communicate the need for an InlineCompulsory to the desugarer + (which makes the Unfoldings), we use the IsDefaultMethod constructor + in TcSpecPrags. %************************************************************************ @@ -865,14 +1162,11 @@ instDeclCtxt2 :: Type -> SDoc instDeclCtxt2 dfun_ty = inst_decl_ctxt (ppr (mkClassPred cls tys)) where - (_,_,cls,tys) = tcSplitDFunTy dfun_ty + (_,cls,tys) = tcSplitDFunTy dfun_ty inst_decl_ctxt :: SDoc -> SDoc inst_decl_ctxt doc = ptext (sLit "In the instance declaration for") <+> quotes doc -superClassCtxt :: SDoc -superClassCtxt = ptext (sLit "When checking the super-classes of an instance declaration") - atInstCtxt :: Name -> SDoc atInstCtxt name = ptext (sLit "In the associated type instance for") <+> quotes (ppr name) @@ -887,7 +1181,7 @@ mustBeVarArgErr ty = wrongATArgErr :: Type -> Type -> SDoc wrongATArgErr ty instTy = sep [ ptext (sLit "Type indexes must match class instance head") - , ptext (sLit "Found") <+> ppr ty <+> ptext (sLit "but expected") <+> - ppr instTy + , ptext (sLit "Found") <+> quotes (ppr ty) + <+> ptext (sLit "but expected") <+> quotes (ppr instTy) ] \end{code}