import TcTyClsDecls
import TcClassDcl
import TcPat( addInlinePrags )
+import TcSimplify( simplifyTop )
import TcRnMonad
import TcMType
import TcType
import TcDeriv
import TcEnv
import RnSource ( addTcgDUs )
-import TcSimplify( simplifySuperClass )
import TcHsType
import TcUnify
import Type
import DataCon
import Class
import Var
+import VarSet
import CoreUtils ( mkPiTypes )
import CoreUnfold ( mkDFunUnfolding )
-import CoreSyn ( Expr(Var) )
+import CoreSyn ( Expr(Var), DFunArg(..), CoreExpr )
import Id
import MkId
import Name
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.
+Conclusion: when typechecking the methods in a C [a] instance, we want to
+treat the 'a' as an *existential* type variable, in the sense described
+by Note [Binding when looking up instances]. That is why isOverlappableTyVar
+responds True to an InstSkol, which is the kind of skolem we use in
+tcInstDecl2.
+
Note [Tricky type variable scoping]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
; checkTc (not is_boot || (isEmptyLHsBinds binds && null uprags))
badBootDeclErr
- ; (tyvars, theta, tau) <- tcHsInstHead poly_ty
-
- -- Now, check the validity of the instance.
- ; (clas, inst_tys) <- checkValidInstance poly_ty tyvars theta tau
+ ; (tyvars, theta, clas, inst_tys) <- tcHsInstHead poly_ty
+ ; checkValidInstance poly_ty tyvars theta clas inst_tys
-- Next, process any associated types.
; idx_tycons <- recoverM (return []) $
dfun = mkDictFunId dfun_name tyvars theta' clas inst_tys
ispec = mkLocalInstance dfun overlap_flag
- ; return (InstInfo { iSpec = ispec,
- iBinds = VanillaInst binds uprags False },
+ ; return (InstInfo { iSpec = ispec, iBinds = VanillaInst binds uprags False },
idx_tycons)
}
where
-- Done
; 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}
See Note [Default methods and instances]
the default method Ids replete with their INLINE pragmas. Urk.
\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
-
- -- Instantiate the super-class context with inst_tys
- sc_theta' = substTheta (zipOpenTvSubst class_tyvars inst_tys') sc_theta
-
- -- 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.
-
- -- 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 []
- -- Empty dependencies [], since it only
- -- depends on "given" things
+
+tcInstDecl2 :: InstInfo Name -> TcM (LHsBinds Id)
+ -- Returns a binding for the dfun
+tcInstDecl2 (InstInfo { iSpec = ispec, iBinds = ibinds })
+ = 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 skol_info (idType dfun_id)
+ ; let (clas, inst_tys) = tcSplitDFunHead inst_head
+ (class_tyvars, sc_theta, _, op_items) = classBigSig clas
+ sc_theta' = substTheta (zipOpenTvSubst class_tyvars inst_tys) sc_theta
+ n_ty_args = length inst_tyvars
+ n_silent = dfunNSilent dfun_id
+ (silent_theta, orig_theta) = splitAt n_silent dfun_theta
+
+ ; silent_ev_vars <- mapM newSilentGiven silent_theta
+ ; orig_ev_vars <- newEvVars orig_theta
+ ; let dfun_ev_vars = silent_ev_vars ++ orig_ev_vars
+
+ ; (sc_binds, sc_dicts, sc_args)
+ <- mapAndUnzip3M (tcSuperClass n_ty_args dfun_ev_vars) sc_theta'
+
+ -- Check that any superclasses gotten from a silent arguemnt
+ -- can be deduced from the originally-specified dfun arguments
+ ; ct_loc <- getCtLoc ScOrigin
+ ; _ <- checkConstraints skol_info inst_tyvars orig_ev_vars $
+ emitConstraints $ listToBag $
+ [ WcEvVar (WantedEvVar sc ct_loc)
+ | sc <- sc_dicts, isSilentEvVar sc ]
-- Deal with 'SPECIALISE instance' pragmas
-- See Note [SPECIALISE instance pragmas]
- ; spec_info <- tcSpecInstPrags dfun_id inst_binds
+ ; spec_info <- tcSpecInstPrags dfun_id ibinds
-- 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
- inst_tyvars' dfun_ev_vars $
- emitWanteds ScOrigin sc_eqs
+ <- tcExtendTyVarEnv inst_tyvars $
+ -- The 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!
+ tcInstanceMethods dfun_id clas inst_tyvars dfun_ev_vars
+ inst_tys spec_info
+ op_items ibinds
-- Create the result bindings
+ ; self_dict <- newEvVar (ClassP clas inst_tys)
; 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)
+ dict_rhs = foldl mk_app inst_constr $
+ map HsVar sc_dicts ++ map (wrapId arg_wrapper) 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
-- 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')
+ mk_app :: LHsExpr Id -> HsExpr Id -> LHsExpr Id
+ mk_app fun arg = L loc (HsApp fun (L loc arg))
+
+ 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
+ `setIdUnfolding` mkDFunUnfolding dfun_ty (sc_args ++ meth_args)
`setInlinePragma` dfunInlinePragma
+ meth_args = map (DFunPolyArg . Var) meth_ids
- (spec_inst_prags, _) = spec_info
- main_bind = AbsBinds { abs_tvs = inst_tyvars'
+ 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_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)
+ ; return (unitBag (L loc main_bind) `unionBags`
+ unionManyBags sc_binds `unionBags`
+ listToBag meth_binds)
}
+ where
+ skol_info = InstSkol -- See Note [Subtle interaction of recursion and overlap]
+ dfun_ty = idType dfun_id
+ dfun_id = instanceDFunId ispec
+ loc = getSrcSpan dfun_id
+
+------------------------------
+tcSuperClass :: Int -> [EvVar] -> PredType -> TcM (LHsBinds Id, Id, DFunArg CoreExpr)
+tcSuperClass n_ty_args ev_vars pred
+ | Just (ev, i) <- find n_ty_args ev_vars
+ = return (emptyBag, ev, DFunLamArg i)
+ | otherwise
+ = ASSERT2( isEmptyVarSet (tyVarsOfPred pred), ppr pred)
+ do { sc_dict <- newWantedEvVar pred
+ ; loc <- getCtLoc ScOrigin
+ ; ev_binds <- simplifyTop (unitBag (WcEvVar (WantedEvVar sc_dict loc)))
+ ; let ev_wrap = WpLet (EvBinds ev_binds)
+ sc_bind = mkVarBind sc_dict (noLoc $ (wrapId ev_wrap sc_dict))
+ ; return (unitBag sc_bind, sc_dict, DFunConstArg (Var sc_dict)) }
+ -- It's very important to solve the superclass constraint *in isolation*
+ -- so that it isn't generated by superclass selection from something else
+ -- We then generate the (also rather degenerate) top-level binding:
+ -- sc_dict = let sc_dict = <blah> in sc_dict
+ -- where <blah> is generated by solving the implication constraint
+ where
+ find _ [] = Nothing
+ find i (ev:evs) | pred `tcEqPred` evVarPred ev = Just (ev, i)
+ | otherwise = find (i+1) evs
------------------------------
-tcSpecInstPrags :: DFunId -> InstBindings Name
+tcSpecInstPrags :: DFunId -> InstBindings Name
-> TcM ([Located TcSpecPrag], PragFun)
tcSpecInstPrags _ (NewTypeDerived {})
= return ([], \_ -> [])
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
- (sc_sel, sc_pred)
- = do { sc_dict <- newWantedEvVar sc_pred
- ; ev_binds <- simplifySuperClass tyvars dicts sc_dict self_ev_bind
-
- ; 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 ev_binds
-
- ; return (sc_op_id, noLoc sc_op_bind) }
\end{code}
-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.
-
+Note [Silent Superclass Arguments]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Consider the following (extreme) situation:
+ class C a => D a where ...
+ instance D [a] => D [a] where ...
+Although this looks wrong (assume D [a] to prove D [a]), it is only a
+more extreme case of what happens with recursive dictionaries.
+
+To implement the dfun we must generate code for the superclass C [a],
+which we can get by superclass selection from the supplied argument!
+So we’d generate:
+ dfun :: forall a. D [a] -> D [a]
+ dfun = \d::D [a] -> MkD (scsel d) ..
+
+However this means that if we later encounter a situation where
+we have a [Wanted] dw::D [a] we could solve it thus:
+ dw := dfun dw
+Although recursive, this binding would pass the TcSMonadisGoodRecEv
+check because it appears as guarded. But in reality, it will make a
+bottom superclass. The trouble is that isGoodRecEv can't "see" the
+superclass-selection inside dfun.
+
+Our solution to this problem is to change the way ‘dfuns’ are created
+for instances, so that we pass as first arguments to the dfun some
+``silent superclass arguments’’, which are the immediate superclasses
+of the dictionary we are trying to construct. In our example:
+ dfun :: forall a. (C [a], D [a] -> D [a]
+ dfun = \(dc::C [a]) (dd::D [a]) -> DOrd dc ...
+
+This gives us:
+
+ -----------------------------------------------------------
+ DFun Superclass Invariant
+ ~~~~~~~~~~~~~~~~~~~~~~~~
+ In the body of a DFun, every superclass argument to the
+ returned dictionary is
+ either * one of the arguments of the DFun,
+ or * constant, bound at top level
+ -----------------------------------------------------------
+
+This means that no superclass is hidden inside a dfun application, so
+the counting argument in isGoodRecEv (more dfun calls than superclass
+selections) works correctly.
+
+The extra arguments required to satisfy the DFun Superclass Invariant
+always come first, and are called the "silent" arguments. DFun types
+are built (only) by MkId.mkDictFunId, so that is where we decide
+what silent arguments are to be added.
+
+This net effect is that it is safe to treat a dfun application as
+wrapping a dictionary constructor around its arguments (in particular,
+a dfun never picks superclasses from the arguments under the dictionary
+constructor).
+
+In our example, if we had [Wanted] dw :: D [a] we would get via the instance:
+ dw := dfun d1 d2
+ [Wanted] (d1 :: C [a])
+ [Wanted] (d2 :: D [a])
+ [Derived] (d :: D [a])
+ [Derived] (scd :: C [a]) scd := scsel d
+ [Derived] (scd2 :: C [a]) scd2 := scsel d2
+
+And now, though we *can* solve:
+ d2 := dw
+we will get an isGoodRecEv failure when we try to solve:
+ d1 := scsel d
+ or
+ d1 := scsel d2
+
+Test case SCLoop tests this fix.
+
Note [SPECIALISE instance pragmas]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Consider
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
+ ; (tyvars, theta, clas, tys) <- tcHsInstHead hs_ty
+ ; let (_, spec_dfun_ty) = mkDictFunTy tyvars theta clas tys
+
+ ; co_fn <- tcSubType (SpecPragOrigin name) (SigSkol SpecInstCtxt)
+ (idType dfun_id) spec_dfun_ty
; return (SpecPrag dfun_id co_fn defaultInlinePragma) }
where
spec_ctxt prag = hang (ptext (sLit "In the SPECIALISE pragma")) 2 (ppr prag)
tcInstanceMethods :: DFunId -> Class -> [TcTyVar]
-> [EvVar]
-> [TcType]
- -> EvBind -- "This" and its binding
- -> ([Located TcSpecPrag], PragFun)
+ -> ([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)
+ (spec_inst_prags, prag_fn)
op_items (VanillaInst binds _ standalone_deriv)
= mapAndUnzipM tc_item op_items
where
; meth_id1 <- addInlinePrags meth_id prags
; spec_prags <- tcSpecPrags meth_id1 prags
; bind <- tcInstanceMethodBody InstSkol
- tyvars dfun_ev_vars mb_dict_ev
+ tyvars dfun_ev_vars
meth_id1 local_meth_id meth_sig_fn
(mk_meth_spec_prags meth_id1 spec_prags)
rn_bind
= 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
+ -- Generate /\as.\ds. let self = df as ds
+ -- in $dm inst_tys self
-- The 'let' is necessary only because HsSyn doesn't allow
-- you to apply a function to a dictionary *expression*.
+ ; self_dict <- newEvVar (ClassP clas inst_tys)
+ ; let self_ev_bind = EvBind self_dict $
+ EvDFunApp dfun_id (mkTyVarTys tyvars) dfun_ev_vars
+
; (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 }
+ , 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
, mk_meth_spec_prags meth_id1 [])]
- , abs_ev_binds = EvBinds (unitBag self_dict_ev)
+ , abs_ev_binds = EvBinds (unitBag self_ev_bind)
, 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
-- instance C [c] where { op = <rhs> }
-- In <rhs>, '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
+ -- 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 sel_id generated_code rn_bind thing
tcInstanceMethods dfun_id clas tyvars dfun_ev_vars inst_tys
- _ _ op_items (NewTypeDerived coi _)
+ _ op_items (NewTypeDerived coi _)
-- Running example:
-- class Show b => Foo a b where
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