TcId, TcIdSet, tcInstId,
tcLookupDataCon,
- TcEnv, ValueEnv, TcTyThing(..),
+ TcEnv, ValueEnv, TyThing(..), TyThingDetails(..), tyThingKind,
- initEnv, getEnvTyCons, getEnvClasses, getEnvAllTyCons,
+ initEnv, getEnvTyCons, getEnvClasses,
tcExtendUVarEnv, tcLookupUVar,
- tcExtendTyVarEnv, tcExtendTyVarEnvForMeths, tcExtendTypeEnv, tcGetInScopeTyVars,
+ tcExtendKindEnv, tcExtendTyVarEnv,
+ tcExtendTyVarEnvForMeths, tcExtendTypeEnv, tcGetInScopeTyVars,
tcLookupTy,
tcLookupTyConByKey,
valueEnvIds,
newLocalId, newSpecPragmaId,
+ newDefaultMethodName, newDFunName,
tcGetGlobalTyVars, tcExtendGlobalTyVars,
+ InstEnv, emptyInstEnv, addToInstEnv,
+ lookupInstEnv, InstLookupResult(..),
+ tcGetInstEnv, tcSetInstEnv, classInstEnv,
+
badCon, badPrimOp
) where
#include "HsVersions.h"
-import HsTypes ( HsTyVarBndr, getTyVarName )
import Id ( mkUserLocal, isDataConWrapId_maybe )
import MkId ( mkSpecPragmaId )
import Var ( TyVar, Id, setVarName,
)
import TcType ( TcType, TcTyVar, TcTyVarSet, TcThetaType,
tcInstTyVars, zonkTcTyVars,
- TcKind, kindToTcKind
+ TcKind,
)
-import VarEnv
import VarSet
-import Type ( Kind, superKind,
- tyVarsOfType, tyVarsOfTypes, mkTyVarTy,
+import Type ( Kind, Type, superKind,
+ tyVarsOfType, tyVarsOfTypes,
splitForAllTys, splitRhoTy, splitFunTys,
- splitAlgTyConApp_maybe, getTyVar
+ splitAlgTyConApp_maybe, getTyVar, getDFunTyKey
)
import Subst ( substTy )
import UsageSPUtils ( unannotTy )
import DataCon ( DataCon )
import TyCon ( TyCon, tyConKind, tyConArity, isSynTyCon )
-import Class ( Class, classTyCon )
+import Class ( Class, ClassOpItem, ClassContext, classTyCon )
import TcMonad
-import BasicTypes ( Arity )
import IdInfo ( vanillaIdInfo )
-import Name ( Name, OccName, nameOccName, getSrcLoc,
+import Name ( Name, OccName, Provenance(..), ExportFlag(..), NamedThing(..),
+ nameOccName, nameModule, getSrcLoc, mkGlobalName,
maybeWiredInTyConName, maybeWiredInIdName, isLocallyDefined,
- NamedThing(..),
- NameEnv, emptyNameEnv, addToNameEnv,
- extendNameEnv, lookupNameEnv, nameEnvElts
+ NameEnv, emptyNameEnv, lookupNameEnv, nameEnvElts,
+ extendNameEnv, extendNameEnvList
)
+import OccName ( mkDFunOcc, mkDefaultMethodOcc, occNameString )
+import Module ( Module )
+import Unify ( unifyTyListsX, matchTys )
import Unique ( pprUnique10, Unique, Uniquable(..) )
-import FiniteMap ( lookupFM, addToFM )
import UniqFM
import Unique ( Uniquable(..) )
import Util ( zipEqual, zipWith3Equal, mapAccumL )
-import Bag ( bagToList )
-import Maybes ( maybeToBool, catMaybes )
+import VarEnv ( TyVarSubstEnv )
import SrcLoc ( SrcLoc )
import FastString ( FastString )
+import Maybes
import Outputable
\end{code}
~~~~~~~~~~~~~~~~~~~~~
\begin{code}
-data TcEnv = TcEnv
- UsageEnv
- TypeEnv
- ValueEnv
- (TcTyVarSet, -- The in-scope TyVars
- TcRef TcTyVarSet) -- Free type variables of the value env
- -- ...why mutable? see notes with tcGetGlobalTyVars
- -- Includes the in-scope tyvars
+data TcEnv
+ = TcEnv {
+ tcGST :: GlobalSymbolTable, -- The symbol table at the moment we began this compilation
+
+ tcGEnv :: NameEnv TyThing -- The global type environment we've accumulated while
+ -- compiling this module:
+ -- types and classes (both imported and local)
+ -- imported Ids
+ -- (Ids defined in this module are in the local envt)
+ -- When type checking is over we'll augment the
+ -- global symbol table with everything in tcGEnv
+
+ tcInst :: InstEnv, -- All instances (both imported and in this module)
+
+ tcLEnv :: NameEnv TcTyThing, -- The local type environment: Ids and TyVars
+ -- defined in this module
+
+ tcTyVars :: FreeTyVars -- Type variables free in tcLST
+ }
-type UsageEnv = NameEnv UVar
-type TypeEnv = NameEnv (TcKind, TcTyThing)
-type ValueEnv = NameEnv Id
+
+type InScopeTyVars = (TcTyVarSet, -- The in-scope TyVars
+ TcRef TcTyVarSet) -- Free type variables of the value env
+ -- ...why mutable? see notes with tcGetGlobalTyVars
valueEnvIds :: ValueEnv -> [Id]
valueEnvIds ve = nameEnvElts ve
-data TcTyThing = ATyVar TcTyVar -- Mutable only so that the kind can be mutable
- -- if the kind is mutable, the tyvar must be so that
- -- zonking works
- | ADataTyCon TyCon
- | ASynTyCon TyCon Arity
- | AClass Class Arity
-
+data TcTyThing = ATyVar TyVar
+ | ATcId TcId
+ | AThing TcKind -- Used temporarily, during kind checking
+-- For example, when checking (forall a. T a Int):
+-- 1. We first bind (a -> AThink kv), where kv is a kind variable.
+-- 2. Then we kind-check the (T a Int) part.
+-- 3. Then we zonk the kind variable.
+-- 4. Now we know the kind for 'a', and we add (a -> ATyVar a::K) to the environment
+
+tyThingKind :: TyThing -> TcKind
+tyThingKind (ATyVar tv) = tyVarKind tv
+tyThingKind (ATyCon tc) = tyConKind tc
+tyThingKind (AClass cl) = tyConKind (classTyCon cl) -- For some odd reason,
+ -- a class doesn't include its kind
+tyThingKind (AThing k) = k
+
+data TyThingDetails = SynTyDetails Type
+ | DataTyDetails ClassContext [DataCon] [Class]
+ | ClassDetails ClassContext [Id] [ClassOpItem] DataCon
initEnv :: TcRef TcTyVarSet -> TcEnv
-initEnv mut = TcEnv emptyNameEnv emptyNameEnv emptyNameEnv (emptyVarSet, mut)
-
-getEnvClasses (TcEnv _ te _ _) = [cl | (_, AClass cl _) <- nameEnvElts te]
-
-getEnvTyCons (TcEnv _ te _ _) = catMaybes (map get_tc (nameEnvElts te))
- where
- get_tc (_, ADataTyCon tc) = Just tc
- get_tc (_, ASynTyCon tc _) = Just tc
- get_tc other = Nothing
-
-getEnvAllTyCons te_list = catMaybes (map get_tc te_list)
- -- The 'all' means 'including the tycons from class decls'
- where
- get_tc (_, ADataTyCon tc) = Just tc
- get_tc (_, ASynTyCon tc _) = Just tc
- get_tc (_, AClass cl _) = Just (classTyCon cl)
- get_tc other = Nothing
+initEnv mut = TcEnv emptyNameEnv emptyNameEnv emptyNameEnv emptyInstEnv (emptyVarSet, mut)
+
+getEnvClasses (TcEnv _ te _ _ _) = [cl | AClass cl <- nameEnvElts te]
+getEnvTyCons (TcEnv _ te _ _ _) = [tc | ATyCon tc <- nameEnvElts te]
\end{code}
-The UsageEnv
-~~~~~~~~~~~~
+%************************************************************************
+%* *
+\subsection{The usage environment}
+%* *
+%************************************************************************
-Extending the usage environment.
+Extending the usage environment
\begin{code}
tcExtendUVarEnv :: Name -> UVar -> TcM s r -> TcM s r
tcExtendUVarEnv uv_name uv scope
- = tcGetEnv `thenNF_Tc` \ (TcEnv ue te ve gtvs) ->
- tcSetEnv (TcEnv (addToNameEnv ue uv_name uv) te ve gtvs) scope
+ = tcGetEnv `thenNF_Tc` \ (TcEnv ue te ve ie gtvs) ->
+ tcSetEnv (TcEnv (extendNameEnv ue uv_name uv) te ve ie gtvs) scope
\end{code}
Looking up in the environments.
\begin{code}
tcLookupUVar :: Name -> NF_TcM s UVar
tcLookupUVar uv_name
- = tcGetEnv `thenNF_Tc` \ (TcEnv ue te ve gtvs) ->
+ = tcGetEnv `thenNF_Tc` \ (TcEnv ue te ve _ gtvs) ->
case lookupNameEnv ue uv_name of
Just uv -> returnNF_Tc uv
Nothing -> failWithTc (uvNameOutOfScope uv_name)
\end{code}
-The TypeEnv
-~~~~~~~~~~~~
-
-Extending the type environment.
+%************************************************************************
+%* *
+\subsection{The type environment}
+%* *
+%************************************************************************
\begin{code}
+tcExtendKindEnv :: [(Name,TcKind)] -> TcM s r -> TcM s r
+tcExtendKindEnv pairs scope
+ = tcGetEnv `thenNF_Tc` \ (TcEnv ue te ve ie gtvs) ->
+ let
+ te' = extendNameEnvList te [(n, AThing k) | (n,k) <- pairs]
+ -- No need to extend global tyvars for kind checking
+ in
+ tcSetEnv (TcEnv ue te' ve ie gtvs) scope
+
tcExtendTyVarEnv :: [TyVar] -> TcM s r -> TcM s r
tcExtendTyVarEnv tyvars scope
- = tcGetEnv `thenNF_Tc` \ (TcEnv ue te ve (in_scope_tvs, gtvs)) ->
+ = tcGetEnv `thenNF_Tc` \ (TcEnv ue te ve ie (in_scope_tvs, gtvs)) ->
let
- extend_list = [ (getName tv, (kindToTcKind (tyVarKind tv), ATyVar tv))
- | tv <- tyvars
- ]
- te' = extendNameEnv te extend_list
+ te' = extendNameEnvList te [ (getName tv, ATyVar tv) | tv <- tyvars]
new_tv_set = mkVarSet tyvars
in_scope_tvs' = in_scope_tvs `unionVarSet` new_tv_set
in
-- class and instance decls, when we mustn't generalise the class tyvars
-- when typechecking the methods.
tc_extend_gtvs gtvs new_tv_set `thenNF_Tc` \ gtvs' ->
- tcSetEnv (TcEnv ue te' ve (in_scope_tvs', gtvs')) scope
+ tcSetEnv (TcEnv ue te' ve ie (in_scope_tvs', gtvs')) scope
-- This variant, tcExtendTyVarEnvForMeths, takes *two* bunches of tyvars:
-- the signature tyvars contain the original names
tcExtendTyVarEnvForMeths :: [TyVar] -> [TcTyVar] -> TcM s r -> TcM s r
tcExtendTyVarEnvForMeths sig_tyvars inst_tyvars thing_inside
- = tcGetEnv `thenNF_Tc` \ (TcEnv ue te ve gtvs) ->
+ = tcGetEnv `thenNF_Tc` \ (TcEnv ue te ve ie gtvs) ->
let
- te' = extendNameEnv te stuff
+ te' = extendNameEnvList te stuff
in
- tcSetEnv (TcEnv ue te' ve gtvs) thing_inside
+ tcSetEnv (TcEnv ue te' ve ie gtvs) thing_inside
where
- stuff = [ (getName sig_tv, (kindToTcKind (tyVarKind inst_tv), ATyVar inst_tv))
+ stuff = [ (getName sig_tv, ATyVar inst_tv)
| (sig_tv, inst_tv) <- zipEqual "tcMeth" sig_tyvars inst_tyvars
]
tcExtendGlobalTyVars extra_global_tvs scope
- = tcGetEnv `thenNF_Tc` \ (TcEnv ue te ve (in_scope,gtvs)) ->
+ = tcGetEnv `thenNF_Tc` \ (TcEnv ue te ve ie (in_scope,gtvs)) ->
tc_extend_gtvs gtvs extra_global_tvs `thenNF_Tc` \ gtvs' ->
- tcSetEnv (TcEnv ue te ve (in_scope,gtvs')) scope
+ tcSetEnv (TcEnv ue te ve ie (in_scope,gtvs')) scope
tc_extend_gtvs gtvs extra_global_tvs
= tcReadMutVar gtvs `thenNF_Tc` \ global_tvs ->
\begin{code}
tcGetGlobalTyVars :: NF_TcM s TcTyVarSet
tcGetGlobalTyVars
- = tcGetEnv `thenNF_Tc` \ (TcEnv ue te ve (_,gtvs)) ->
+ = tcGetEnv `thenNF_Tc` \ (TcEnv ue te ve ie (_,gtvs)) ->
tcReadMutVar gtvs `thenNF_Tc` \ global_tvs ->
zonkTcTyVars (varSetElems global_tvs) `thenNF_Tc` \ global_tys' ->
let
tcGetInScopeTyVars :: NF_TcM s [TcTyVar]
tcGetInScopeTyVars
- = tcGetEnv `thenNF_Tc` \ (TcEnv ue te ve (in_scope_tvs, gtvs)) ->
+ = tcGetEnv `thenNF_Tc` \ (TcEnv ue te ve ie (in_scope_tvs, gtvs)) ->
returnNF_Tc (varSetElems in_scope_tvs)
\end{code}
Type constructors and classes
\begin{code}
-tcExtendTypeEnv :: [(Name, (TcKind, TcTyThing))] -> TcM s r -> TcM s r
+tcExtendTypeEnv :: [(Name, TyThing)] -> TcM s r -> TcM s r
tcExtendTypeEnv bindings scope
- = ASSERT( null [tv | (_, (_,ATyVar tv)) <- bindings] )
+ = ASSERT( null [tv | (_, ATyVar tv) <- bindings] )
-- Not for tyvars; use tcExtendTyVarEnv
- tcGetEnv `thenNF_Tc` \ (TcEnv ue te ve gtvs) ->
+ tcGetEnv `thenNF_Tc` \ (TcEnv ue te ve ie gtvs) ->
let
- te' = extendNameEnv te bindings
+ te' = extendNameEnvList te bindings
in
- tcSetEnv (TcEnv ue te' ve gtvs) scope
+ tcSetEnv (TcEnv ue te' ve ie gtvs) scope
\end{code}
Looking up in the environments.
\begin{code}
-tcLookupTy :: Name -> NF_TcM s (TcKind, TcTyThing)
+tcLookupTy :: Name -> NF_TcM s TyThing
tcLookupTy name
- = tcGetEnv `thenNF_Tc` \ (TcEnv ue te ve gtvs) ->
+ = tcGetEnv `thenNF_Tc` \ (TcEnv ue te ve ie gtvs) ->
case lookupNameEnv te name of {
Just thing -> returnNF_Tc thing ;
Nothing ->
case maybeWiredInTyConName name of
- Just tc | isSynTyCon tc -> returnNF_Tc (kindToTcKind (tyConKind tc), ASynTyCon tc (tyConArity tc))
- | otherwise -> returnNF_Tc (kindToTcKind (tyConKind tc), ADataTyCon tc)
+ Just tc -> returnNF_Tc (ATyCon tc)
Nothing -> -- This can happen if an interface-file
-- unfolding is screwed up
tcLookupClassByKey :: Unique -> NF_TcM s Class
tcLookupClassByKey key
- = tcGetEnv `thenNF_Tc` \ (TcEnv ue te ve gtvs) ->
+ = tcGetEnv `thenNF_Tc` \ (TcEnv ue te ve ie gtvs) ->
case lookupUFM_Directly te key of
- Just (_, AClass cl _) -> returnNF_Tc cl
- other -> pprPanic "tcLookupClassByKey:" (pprUnique10 key)
+ Just (AClass cl) -> returnNF_Tc cl
+ other -> pprPanic "tcLookupClassByKey:" (pprUnique10 key)
tcLookupClassByKey_maybe :: Unique -> NF_TcM s (Maybe Class)
tcLookupClassByKey_maybe key
- = tcGetEnv `thenNF_Tc` \ (TcEnv ue te ve gtvs) ->
+ = tcGetEnv `thenNF_Tc` \ (TcEnv ue te ve ie gtvs) ->
case lookupUFM_Directly te key of
- Just (_, AClass cl _) -> returnNF_Tc (Just cl)
- other -> returnNF_Tc Nothing
+ Just (AClass cl) -> returnNF_Tc (Just cl)
+ other -> returnNF_Tc Nothing
tcLookupTyConByKey :: Unique -> NF_TcM s TyCon
tcLookupTyConByKey key
- = tcGetEnv `thenNF_Tc` \ (TcEnv ue te ve gtvs) ->
+ = tcGetEnv `thenNF_Tc` \ (TcEnv ue te ve ie gtvs) ->
case lookupUFM_Directly te key of
- Just (_, ADataTyCon tc) -> returnNF_Tc tc
- Just (_, ASynTyCon tc _) -> returnNF_Tc tc
- other -> pprPanic "tcLookupTyConByKey:" (pprUnique10 key)
+ Just (ATyCon tc) -> returnNF_Tc tc
+ other -> pprPanic "tcLookupTyConByKey:" (pprUnique10 key)
\end{code}
\begin{code}
tcExtendGlobalValEnv :: [Id] -> TcM s a -> TcM s a
tcExtendGlobalValEnv ids scope
- = tcGetEnv `thenNF_Tc` \ (TcEnv ue te ve gtvs) ->
+ = tcGetEnv `thenNF_Tc` \ (TcEnv ue te ve ie gtvs) ->
let
ve' = addListToUFM_Directly ve [(getUnique id, id) | id <- ids]
in
- tcSetEnv (TcEnv ue te ve' gtvs) scope
+ tcSetEnv (TcEnv ue te ve' ie gtvs) scope
tcExtendLocalValEnv :: [(Name,TcId)] -> TcM s a -> TcM s a
tcExtendLocalValEnv names_w_ids scope
- = tcGetEnv `thenNF_Tc` \ (TcEnv ue te ve (in_scope_tvs,gtvs)) ->
+ = tcGetEnv `thenNF_Tc` \ (TcEnv ue te ve ie (in_scope_tvs,gtvs)) ->
tcReadMutVar gtvs `thenNF_Tc` \ global_tvs ->
let
- ve' = extendNameEnv ve names_w_ids
+ ve' = extendNameEnvList ve names_w_ids
extra_global_tyvars = tyVarsOfTypes (map (idType . snd) names_w_ids)
in
tc_extend_gtvs gtvs extra_global_tyvars `thenNF_Tc` \ gtvs' ->
- tcSetEnv (TcEnv ue te ve' (in_scope_tvs,gtvs')) scope
+ tcSetEnv (TcEnv ue te ve' ie (in_scope_tvs,gtvs')) scope
\end{code}
tcLookupValue name
= case maybeWiredInIdName name of
Just id -> returnNF_Tc id
- Nothing -> tcGetEnv `thenNF_Tc` \ (TcEnv ue te ve gtvs) ->
+ Nothing -> tcGetEnv `thenNF_Tc` \ (TcEnv ue te ve ie gtvs) ->
returnNF_Tc (lookupWithDefaultUFM ve def name)
where
- def = pprPanic "tcLookupValue:" (ppr name)
+ wired_in = case maybeWiredInIdName name of
+ Just id -> True
+ Nothing -> False
+ def = pprPanic "tcLookupValue:" (ppr name <+> ppr wired_in)
tcLookupValueMaybe :: Name -> NF_TcM s (Maybe Id)
tcLookupValueMaybe name
= case maybeWiredInIdName name of
Just id -> returnNF_Tc (Just id)
- Nothing -> tcGetEnv `thenNF_Tc` \ (TcEnv ue te ve gtvs) ->
+ Nothing -> tcGetEnv `thenNF_Tc` \ (TcEnv ue te ve ie gtvs) ->
returnNF_Tc (lookupNameEnv ve name)
tcLookupValueByKey :: Unique -> NF_TcM s Id -- Panics if not found
tcLookupValueByKey key
- = tcGetEnv `thenNF_Tc` \ (TcEnv ue te ve gtvs) ->
+ = tcGetEnv `thenNF_Tc` \ (TcEnv ue te ve ie gtvs) ->
returnNF_Tc (explicitLookupValueByKey ve key)
tcLookupValueByKeyMaybe :: Unique -> NF_TcM s (Maybe Id)
tcLookupValueByKeyMaybe key
- = tcGetEnv `thenNF_Tc` \ (TcEnv ue te ve gtvs) ->
+ = tcGetEnv `thenNF_Tc` \ (TcEnv ue te ve ie gtvs) ->
returnNF_Tc (lookupUFM_Directly ve key)
tcGetValueEnv :: NF_TcM s ValueEnv
tcGetValueEnv
- = tcGetEnv `thenNF_Tc` \ (TcEnv ue te ve gtvs) ->
+ = tcGetEnv `thenNF_Tc` \ (TcEnv ue te ve ie gtvs) ->
returnNF_Tc ve
+
tcSetValueEnv :: ValueEnv -> TcM s a -> TcM s a
tcSetValueEnv ve scope
- = tcGetEnv `thenNF_Tc` \ (TcEnv ue te _ gtvs) ->
- tcSetEnv (TcEnv ue te ve gtvs) scope
+ = tcGetEnv `thenNF_Tc` \ (TcEnv ue te _ ie gtvs) ->
+ tcSetEnv (TcEnv ue te ve ie gtvs) scope
-- Non-monadic version, environment given explicitly
explicitLookupValueByKey :: ValueEnv -> Unique -> Id
= id `lazySetIdInfo` new_info
-- The Id must be returned without a data dependency on maybe_id
where
- new_info = -- pprTrace "tcAdd" (ppr id) $
- case explicitLookupValue unf_env (getName id) of
+ new_info = case explicitLookupValue unf_env (getName id) of
Nothing -> vanillaIdInfo
Just imported_id -> idInfo imported_id
-- ToDo: could check that types are the same
\end{code}
-
-%************************************************************************
-%* *
-\subsection{Constructing new Ids}
-%* *
-%************************************************************************
+Constructing new Ids
\begin{code}
newLocalId :: OccName -> TcType -> SrcLoc -> NF_TcM s TcId
%************************************************************************
%* *
+\subsection{The instance environment}
+%* *
+%************************************************************************
+
+\begin{code}
+tcGetInstEnv :: NF_TcM s InstEnv
+tcGetInstEnv = tcGetEnv `thenNF_Tc` \ (TcEnv ue te ve ie (_,gtvs)) ->
+ returnNF_Tc ie
+
+tcSetInstEnv :: InstEnv -> TcM s a -> TcM s a
+tcSetInstEnv ie scope
+ = tcGetEnv `thenNF_Tc` \ (TcEnv ue te ve _ gtvs) ->
+ tcSetEnv (TcEnv ue te ve ie gtvs) scope
+\end{code}
+
+
+\begin{code}
+type InstEnv = UniqFM ClsInstEnv -- Maps Class to instances for that class
+type ClsInstEnv = [(TyVarSet, [Type], Id)] -- The instances for a particular class
+
+classInstEnv :: InstEnv -> Class -> ClsInstEnv
+classInstEnv env cls = lookupWithDefaultUFM env [] cls
+\end{code}
+
+A @ClsInstEnv@ lives inside a class, and identifies all the instances
+of that class. The @Id@ inside a ClsInstEnv mapping is the dfun for
+that instance.
+
+If class C maps to a list containing the item ([a,b], [t1,t2,t3], dfun), then
+
+ forall a b, C t1 t2 t3 can be constructed by dfun
+
+or, to put it another way, we have
+
+ instance (...) => C t1 t2 t3, witnessed by dfun
+
+There is an important consistency constraint in the elements of a ClsInstEnv:
+
+ * [a,b] must be a superset of the free vars of [t1,t2,t3]
+
+ * The dfun must itself be quantified over [a,b]
+
+Thus, the @ClassInstEnv@ for @Eq@ might contain the following entry:
+ [a] ===> dfun_Eq_List :: forall a. Eq a => Eq [a]
+The "a" in the pattern must be one of the forall'd variables in
+the dfun type.
+
+
+
+Notes on overlapping instances
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+In some ClsInstEnvs, overlap is prohibited; that is, no pair of templates unify.
+
+In others, overlap is permitted, but only in such a way that one can make
+a unique choice when looking up. That is, overlap is only permitted if
+one template matches the other, or vice versa. So this is ok:
+
+ [a] [Int]
+
+but this is not
+
+ (Int,a) (b,Int)
+
+If overlap is permitted, the list is kept most specific first, so that
+the first lookup is the right choice.
+
+
+For now we just use association lists.
+
+\subsection{Avoiding a problem with overlapping}
+
+Consider this little program:
+
+\begin{pseudocode}
+ class C a where c :: a
+ class C a => D a where d :: a
+
+ instance C Int where c = 17
+ instance D Int where d = 13
+
+ instance C a => C [a] where c = [c]
+ instance ({- C [a], -} D a) => D [a] where d = c
+
+ instance C [Int] where c = [37]
+
+ main = print (d :: [Int])
+\end{pseudocode}
+
+What do you think `main' prints (assuming we have overlapping instances, and
+all that turned on)? Well, the instance for `D' at type `[a]' is defined to
+be `c' at the same type, and we've got an instance of `C' at `[Int]', so the
+answer is `[37]', right? (the generic `C [a]' instance shouldn't apply because
+the `C [Int]' instance is more specific).
+
+Ghc-4.04 gives `[37]', while ghc-4.06 gives `[17]', so 4.06 is wrong. That
+was easy ;-) Let's just consult hugs for good measure. Wait - if I use old
+hugs (pre-September99), I get `[17]', and stranger yet, if I use hugs98, it
+doesn't even compile! What's going on!?
+
+What hugs complains about is the `D [a]' instance decl.
+
+\begin{pseudocode}
+ ERROR "mj.hs" (line 10): Cannot build superclass instance
+ *** Instance : D [a]
+ *** Context supplied : D a
+ *** Required superclass : C [a]
+\end{pseudocode}
+
+You might wonder what hugs is complaining about. It's saying that you
+need to add `C [a]' to the context of the `D [a]' instance (as appears
+in comments). But there's that `C [a]' instance decl one line above
+that says that I can reduce the need for a `C [a]' instance to the
+need for a `C a' instance, and in this case, I already have the
+necessary `C a' instance (since we have `D a' explicitly in the
+context, and `C' is a superclass of `D').
+
+Unfortunately, the above reasoning indicates a premature commitment to the
+generic `C [a]' instance. I.e., it prematurely rules out the more specific
+instance `C [Int]'. This is the mistake that ghc-4.06 makes. The fix is to
+add the context that hugs suggests (uncomment the `C [a]'), effectively
+deferring the decision about which instance to use.
+
+Now, interestingly enough, 4.04 has this same bug, but it's covered up
+in this case by a little known `optimization' that was disabled in
+4.06. Ghc-4.04 silently inserts any missing superclass context into
+an instance declaration. In this case, it silently inserts the `C
+[a]', and everything happens to work out.
+
+(See `basicTypes/MkId:mkDictFunId' for the code in question. Search for
+`Mark Jones', although Mark claims no credit for the `optimization' in
+question, and would rather it stopped being called the `Mark Jones
+optimization' ;-)
+
+So, what's the fix? I think hugs has it right. Here's why. Let's try
+something else out with ghc-4.04. Let's add the following line:
+
+ d' :: D a => [a]
+ d' = c
+
+Everyone raise their hand who thinks that `d :: [Int]' should give a
+different answer from `d' :: [Int]'. Well, in ghc-4.04, it does. The
+`optimization' only applies to instance decls, not to regular
+bindings, giving inconsistent behavior.
+
+Old hugs had this same bug. Here's how we fixed it: like GHC, the
+list of instances for a given class is ordered, so that more specific
+instances come before more generic ones. For example, the instance
+list for C might contain:
+ ..., C Int, ..., C a, ...
+When we go to look for a `C Int' instance we'll get that one first.
+But what if we go looking for a `C b' (`b' is unconstrained)? We'll
+pass the `C Int' instance, and keep going. But if `b' is
+unconstrained, then we don't know yet if the more specific instance
+will eventually apply. GHC keeps going, and matches on the generic `C
+a'. The fix is to, at each step, check to see if there's a reverse
+match, and if so, abort the search. This prevents hugs from
+prematurely chosing a generic instance when a more specific one
+exists.
+
+--Jeff
+
+\begin{code}
+emptyInstEnv :: InstEnv
+emptyInstEnv = emptyUFM
+\end{code}
+
+@lookupInstEnv@ looks up in a @InstEnv@, using a one-way match. Since
+the env is kept ordered, the first match must be the only one. The
+thing we are looking up can have an arbitrary "flexi" part.
+
+\begin{code}
+lookupInstEnv :: InstEnv -- The envt
+ -> Class -> [Type] -- Key
+ -> InstLookupResult
+
+data InstLookupResult
+ = FoundInst -- There is a (template,substitution) pair
+ -- that makes the template match the key,
+ -- and no template is an instance of the key
+ TyVarSubstEnv Id
+
+ | NoMatch Bool -- Boolean is true iff there is at least one
+ -- template that matches the key.
+ -- (but there are other template(s) that are
+ -- instances of the key, so we don't report
+ -- FoundInst)
+ -- The NoMatch True case happens when we look up
+ -- Foo [a]
+ -- in an InstEnv that has entries for
+ -- Foo [Int]
+ -- Foo [b]
+ -- Then which we choose would depend on the way in which 'a'
+ -- is instantiated. So we say there is no match, but identify
+ -- it as ambiguous case in the hope of giving a better error msg.
+ -- See the notes above from Jeff Lewis
+
+lookupInstEnv env key_cls key_tys
+ = find (classInstEnv env key_cls)
+ where
+ key_vars = tyVarsOfTypes key_tys
+
+ find [] = NoMatch False
+ find ((tpl_tyvars, tpl, val) : rest)
+ = case matchTys tpl_tyvars tpl key_tys of
+ Nothing ->
+ case matchTys key_vars key_tys tpl of
+ Nothing -> find rest
+ Just (_, _) -> NoMatch (any_match rest)
+ Just (subst, leftovers) -> ASSERT( null leftovers )
+ FoundInst subst val
+
+ any_match rest = or [ maybeToBool (matchTys tvs tpl key_tys)
+ | (tvs,tpl,_) <- rest
+ ]
+\end{code}
+
+@addToClsInstEnv@ extends a @ClsInstEnv@, checking for overlaps.
+
+A boolean flag controls overlap reporting.
+
+True => overlap is permitted, but only if one template matches the other;
+ not if they unify but neither is
+
+\begin{code}
+addToInstEnv :: Bool -- True <=> overlap permitted
+ -> InstEnv -- Envt
+ -> Class -> [TyVar] -> [Type] -> Id -- New item
+ -> MaybeErr InstEnv -- Success...
+ ([Type], Id) -- Failure: Offending overlap
+
+addToInstEnv overlap_ok inst_env clas ins_tvs ins_tys value
+ = case insert_into (classInstEnv inst_env clas) of
+ Failed stuff -> Failed stuff
+ Succeeded new_env -> Succeeded (addToUFM inst_env clas new_env)
+
+ where
+ ins_tv_set = mkVarSet ins_tvs
+ ins_item = (ins_tv_set, ins_tys, value)
+
+ insert_into [] = returnMaB [ins_item]
+ insert_into env@(cur_item@(tpl_tvs, tpl_tys, val) : rest)
+
+ -- FAIL if:
+ -- (a) they are the same, or
+ -- (b) they unify, and any sort of overlap is prohibited,
+ -- (c) they unify but neither is more specific than t'other
+ | identical
+ || (unifiable && not overlap_ok)
+ || (unifiable && not (ins_item_more_specific || cur_item_more_specific))
+ = failMaB (tpl_tys, val)
+
+ -- New item is an instance of current item, so drop it here
+ | ins_item_more_specific = returnMaB (ins_item : env)
+
+ -- Otherwise carry on
+ | otherwise = insert_into rest `thenMaB` \ rest' ->
+ returnMaB (cur_item : rest')
+ where
+ unifiable = maybeToBool (unifyTyListsX (ins_tv_set `unionVarSet` tpl_tvs) tpl_tys ins_tys)
+ ins_item_more_specific = maybeToBool (matchTys tpl_tvs tpl_tys ins_tys)
+ cur_item_more_specific = maybeToBool (matchTys ins_tv_set ins_tys tpl_tys)
+ identical = ins_item_more_specific && cur_item_more_specific
+\end{code}
+
+Make a name for the dict fun for an instance decl
+
+\begin{code}
+newDFunName :: Module -> Class -> [Type] -> SrcLoc -> NF_TcM s Name
+newDFunName mod clas (ty:_) loc
+ = tcGetDFunUniq dfun_string `thenNF_Tc` \ inst_uniq ->
+ tcGetUnique `thenNF_Tc` \ uniq ->
+ returnNF_Tc (mkGlobalName uniq mod
+ (mkDFunOcc dfun_string inst_uniq)
+ (LocalDef loc Exported))
+ where
+ -- Any string that is somewhat unique will do
+ dfun_string = occNameString (getOccName clas) ++ occNameString (getDFunTyKey ty)
+
+newDefaultMethodName :: Name -> SrcLoc -> NF_TcM s Name
+newDefaultMethodName op_name loc
+ = tcGetUnique `thenNF_Tc` \ uniq ->
+ returnNF_Tc (mkGlobalName uniq (nameModule op_name)
+ (mkDefaultMethodOcc (getOccName op_name))
+ (LocalDef loc Exported))
+\end{code}
+
+
+%************************************************************************
+%* *
\subsection{Errors}
%* *
%************************************************************************
projects / ghc-hetmet.git / blobdiff