%
-% (c) The GRASP/AQUA Project, Glasgow University, 1993-1998
+% (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
%
-\section{Class Instance environments}
+\section[InstEnv]{Utilities for typechecking instance declarations}
+
+The bits common to TcInstDcls and TcDeriv.
\begin{code}
module InstEnv (
- InstEnv, emptyInstEnv, addToInstEnv, lookupInstEnv
+ DFunId, OverlapFlag(..),
+ Instance(..), pprInstance, pprInstanceHdr, pprInstances,
+ instanceHead, mkLocalInstance, mkImportedInstance,
+ instanceDFunId, setInstanceDFunId, instanceRoughTcs,
+
+ InstEnv, emptyInstEnv, extendInstEnv,
+ extendInstEnvList, lookupInstEnv, instEnvElts,
+ classInstances,
+ instanceCantMatch, roughMatchTcs
) where
#include "HsVersions.h"
-import Var ( TyVar, Id )
+import Class ( Class )
+import Var ( Id, TyVar, isTcTyVar )
import VarSet
-import VarEnv ( TyVarSubstEnv )
-import Type ( Type, tyVarsOfTypes )
-import Unify ( unifyTyListsX, matchTys )
+import Name ( Name, NamedThing(..), getSrcLoc, nameIsLocalOrFrom, nameModule )
+import OccName ( OccName )
+import NameSet ( unionNameSets, unitNameSet, nameSetToList )
+import Type ( TvSubst )
+import TcType ( Type, PredType, tcEqType,
+ tcSplitDFunTy, tyVarsOfTypes, isExistentialTyVar,
+ pprThetaArrow, pprClassPred,
+ tyClsNamesOfType, tcSplitTyConApp_maybe
+ )
+import TyCon ( tyConName )
+import Unify ( tcMatchTys, tcUnifyTys, BindFlag(..) )
import Outputable
-import Maybes
+import UniqFM ( UniqFM, lookupUFM, emptyUFM, addToUFM_C, eltsUFM )
+import Id ( idType, idName )
+import SrcLoc ( pprDefnLoc )
+import Maybe ( isJust, isNothing )
\end{code}
%************************************************************************
%* *
-\section{InstEnv}
+\subsection{The key types}
%* *
%************************************************************************
\begin{code}
-type InstEnv = [(TyVarSet, [Type], Id)]
+type DFunId = Id
+data Instance
+ = Instance { is_cls :: Name -- Class name
+
+ -- Used for "rough matching"; see note below
+ , is_tcs :: [Maybe Name] -- Top of type args
+
+ -- Used for "proper matching"; see note
+ , is_tvs :: TyVarSet -- Template tyvars for full match
+ , is_tys :: [Type] -- Full arg types
+
+ , is_dfun :: DFunId
+ , is_flag :: OverlapFlag
+
+ , is_orph :: Maybe OccName }
+
+-- The "rough-match" fields
+-- ~~~~~~~~~~~~~~~~~~~~~~~~~
+-- The is_cls, is_args fields allow a "rough match" to be done
+-- without poking inside the DFunId. Poking the DFunId forces
+-- us to suck in all the type constructors etc it involves,
+-- which is a total waste of time if it has no chance of matching
+-- So the Name, [Maybe Name] fields allow us to say "definitely
+-- does not match", based only on the Name.
+--
+-- In is_tcs,
+-- Nothing means that this type arg is a type variable
+--
+-- (Just n) means that this type arg is a
+-- TyConApp with a type constructor of n.
+-- This is always a real tycon, never a synonym!
+-- (Two different synonyms might match, but two
+-- different real tycons can't.)
+-- NB: newtypes are not transparent, though!
+--
+-- The "proper-match" fields
+-- ~~~~~~~~~~~~~~~~~~~~~~~~~
+-- The is_tvs, is_tys fields are simply cahced values, pulled
+-- out (lazily) from the dfun id. They are cached here simply so
+-- that we don't need to decompose the DFunId each time we want
+-- to match it. The hope is that the fast-match fields mean
+-- that we often never poke th proper-match fields
+--
+-- However, note that:
+-- * is_tvs must be a superset of the free vars of is_tys
+--
+-- * The is_dfun must itself be quantified over exactly is_tvs
+-- (This is so that we can use the matching substitution to
+-- instantiate the dfun's context.)
+--
+-- The "orphan" field
+-- ~~~~~~~~~~~~~~~~~~
+-- An instance is an orphan if its head (after the =>) mentions
+-- nothing defined in this module.
+--
+-- Just n The head mentions n, which is defined in this module
+-- This is used for versioning; the instance decl is
+-- considered part of the defn of n when computing versions
+--
+-- Nothing The head mentions nothing defined in this modle
+--
+-- If a module contains any orphans, then its interface file is read
+-- regardless, so that its instances are not missed.
+--
+-- Functional dependencies worsen the situation a bit. Consider
+-- class C a b | a -> b
+-- In some other module we might have
+-- module M where
+-- data T = ...
+-- instance C Int T where ...
+-- This isn't considered an orphan, so we will only read M's interface
+-- if something from M is used (e.g. T). So there's a risk we'll
+-- miss the improvement from the instance. Workaround: import M.
+
+instanceDFunId :: Instance -> DFunId
+instanceDFunId = is_dfun
+
+setInstanceDFunId :: Instance -> DFunId -> Instance
+setInstanceDFunId ispec dfun
+ = ASSERT( idType dfun `tcEqType` idType (is_dfun ispec) )
+ -- We need to create the cached fields afresh from
+ -- the new dfun id. In particular, the is_tvs in
+ -- the Instance must match those in the dfun!
+ -- We assume that the only thing that changes is
+ -- the quantified type variables, so the other fields
+ -- are ok; hence the assert
+ ispec { is_dfun = dfun, is_tvs = mkVarSet tvs, is_tys = tys }
+ where
+ (tvs, _, _, tys) = tcSplitDFunTy (idType dfun)
+
+instanceRoughTcs :: Instance -> [Maybe Name]
+instanceRoughTcs = is_tcs
\end{code}
-In some InstEnvs overlap is prohibited; that is, no pair of templates unify.
+\begin{code}
+instance NamedThing Instance where
+ getName ispec = getName (is_dfun ispec)
+
+instance Outputable Instance where
+ ppr = pprInstance
+
+pprInstance :: Instance -> SDoc
+-- Prints the Instance as an instance declaration
+pprInstance ispec@(Instance { is_flag = flag })
+ = hang (pprInstanceHdr ispec)
+ 2 (ptext SLIT("--") <+> (pprDefnLoc (getSrcLoc ispec)))
+
+-- * pprInstanceHdr is used in VStudio to populate the ClassView tree
+pprInstanceHdr :: Instance -> SDoc
+-- Prints the Instance as an instance declaration
+pprInstanceHdr ispec@(Instance { is_flag = flag })
+ = ptext SLIT("instance") <+> ppr flag
+ <+> sep [pprThetaArrow theta, pprClassPred clas tys]
+ where
+ (_, theta, clas, tys) = instanceHead ispec
+ -- Print without the for-all, which the programmer doesn't write
+
+pprInstances :: [Instance] -> SDoc
+pprInstances ispecs = vcat (map pprInstance ispecs)
+
+instanceHead :: Instance -> ([TyVar], [PredType], Class, [Type])
+instanceHead ispec = tcSplitDFunTy (idType (is_dfun ispec))
+
+mkLocalInstance :: DFunId -> OverlapFlag -> Instance
+-- Used for local instances, where we can safely pull on the DFunId
+mkLocalInstance dfun oflag
+ = Instance { is_flag = oflag, is_dfun = dfun,
+ is_tvs = mkVarSet tvs, is_tys = tys,
+ is_cls = cls_name, is_tcs = roughMatchTcs tys,
+ is_orph = orph }
+ where
+ (tvs, _, cls, tys) = tcSplitDFunTy (idType dfun)
+ mod = nameModule (idName dfun)
+ cls_name = getName cls
+ tycl_names = foldr (unionNameSets . tyClsNamesOfType)
+ (unitNameSet cls_name) tys
+ orph = case filter (nameIsLocalOrFrom mod) (nameSetToList tycl_names) of
+ [] -> Nothing
+ (n:ns) -> Just (getOccName n)
+
+mkImportedInstance :: Name -> [Maybe Name] -> Maybe OccName
+ -> DFunId -> OverlapFlag -> Instance
+-- Used for imported instances, where we get the rough-match stuff
+-- from the interface file
+mkImportedInstance cls mb_tcs orph dfun oflag
+ = Instance { is_flag = oflag, is_dfun = dfun,
+ is_tvs = mkVarSet tvs, is_tys = tys,
+ is_cls = cls, is_tcs = mb_tcs, is_orph = orph }
+ where
+ (tvs, _, _, tys) = tcSplitDFunTy (idType dfun)
+
+roughMatchTcs :: [Type] -> [Maybe Name]
+roughMatchTcs tys = map rough tys
+ where
+ rough ty = case tcSplitTyConApp_maybe ty of
+ Just (tc,_) -> Just (tyConName tc)
+ Nothing -> Nothing
-In others, overlap is permitted, but only in such a way that one can make
+instanceCantMatch :: [Maybe Name] -> [Maybe Name] -> Bool
+-- (instanceCantMatch tcs1 tcs2) returns True if tcs1 cannot
+-- possibly be instantiated to actual, nor vice versa;
+-- False is non-committal
+instanceCantMatch (Just t : ts) (Just a : as) = t/=a || instanceCantMatch ts as
+instanceCantMatch ts as = False -- Safe
+
+---------------------------------------------------
+data OverlapFlag
+ = NoOverlap -- This instance must not overlap another
+
+ | OverlapOk -- Silently ignore this instance if you find a
+ -- more specific one that matches the constraint
+ -- you are trying to resolve
+ --
+ -- Example: constraint (Foo [Int])
+ -- instances (Foo [Int])
+ -- (Foo [a]) OverlapOk
+ -- Since the second instance has the OverlapOk flag,
+ -- the first instance will be chosen (otherwise
+ -- its ambiguous which to choose)
+
+ | Incoherent -- Like OverlapOk, but also ignore this instance
+ -- if it doesn't match the constraint you are
+ -- trying to resolve, but could match if the type variables
+ -- in the constraint were instantiated
+ --
+ -- Example: constraint (Foo [b])
+ -- instances (Foo [Int]) Incoherent
+ -- (Foo [a])
+ -- Without the Incoherent flag, we'd complain that
+ -- instantiating 'b' would change which instance
+ -- was chosen
+
+instance Outputable OverlapFlag where
+ ppr NoOverlap = empty
+ ppr OverlapOk = ptext SLIT("[overlap ok]")
+ ppr Incoherent = ptext SLIT("[incoherent]")
+\end{code}
+
+
+Note [Overlapping instances]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+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:
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
+
+BUT NOTE [Nov 2001]: we must actually *unify* not reverse-match in
+this test. Suppose the instance envt had
+ ..., forall a b. C a a b, ..., forall a b c. C a b c, ...
+(still most specific first)
+Now suppose we are looking for (C x y Int), where x and y are unconstrained.
+ C x y Int doesn't match the template {a,b} C a a b
+but neither does
+ C a a b match the template {x,y} C x y Int
+But still x and y might subsequently be unified so they *do* match.
+
+Simple story: unify, don't match.
+
+
+%************************************************************************
+%* *
+ InstEnv, ClsInstEnv
+%* *
+%************************************************************************
+
+A @ClsInstEnv@ 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
+
\begin{code}
+---------------------------------------------------
+type InstEnv = UniqFM ClsInstEnv -- Maps Class to instances for that class
+
+data ClsInstEnv
+ = ClsIE [Instance] -- The instances for a particular class, in any order
+ Bool -- True <=> there is an instance of form C a b c
+ -- If *not* then the common case of looking up
+ -- (C a b c) can fail immediately
+
+-- INVARIANTS:
+-- * The is_tvs are distinct in each Instance
+-- of a ClsInstEnv (so we can safely unify them)
+
+-- 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.
+
emptyInstEnv :: InstEnv
-emptyInstEnv = []
+emptyInstEnv = emptyUFM
-isEmptyInstEnv env = null env
-\end{code}
+instEnvElts :: InstEnv -> [Instance]
+instEnvElts ie = [elt | ClsIE elts _ <- eltsUFM ie, elt <- elts]
-@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.
+classInstances :: (InstEnv,InstEnv) -> Class -> [Instance]
+classInstances (pkg_ie, home_ie) cls
+ = get home_ie ++ get pkg_ie
+ where
+ get env = case lookupUFM env cls of
+ Just (ClsIE insts _) -> insts
+ Nothing -> []
-\begin{code}
-lookupInstEnv :: SDoc -- For error report
- -> InstEnv -- The envt
- -> [Type] -- Key
- -> Maybe (TyVarSubstEnv, Id)
-
-lookupInstEnv doc env key
- = find env
+extendInstEnvList :: InstEnv -> [Instance] -> InstEnv
+extendInstEnvList inst_env ispecs = foldl extendInstEnv inst_env ispecs
+
+extendInstEnv :: InstEnv -> Instance -> InstEnv
+extendInstEnv inst_env ins_item@(Instance { is_cls = cls_nm, is_tcs = mb_tcs })
+ = addToUFM_C add inst_env cls_nm (ClsIE [ins_item] ins_tyvar)
where
- find [] = Nothing
- find ((tpl_tyvars, tpl, val) : rest)
- = case matchTys tpl_tyvars tpl key of
- Nothing -> find rest
- Just (subst, leftovers) -> ASSERT( null leftovers )
- Just (subst, val)
-\end{code}
+ add (ClsIE cur_insts cur_tyvar) _ = ClsIE (ins_item : cur_insts)
+ (ins_tyvar || cur_tyvar)
+ ins_tyvar = not (any isJust mb_tcs)
+\end{code}
-@addToInstEnv@ extends a @InstEnv@, checking for overlaps.
-A boolean flag controls overlap reporting.
+%************************************************************************
+%* *
+\subsection{Looking up an instance}
+%* *
+%************************************************************************
-True => overlap is permitted, but only if one template matches the other;
- not if they unify but neither is
+@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}
-addToInstEnv :: Bool -- True <=> overlap permitted
- -> InstEnv -- Envt
- -> [TyVar] -> [Type] -> Id -- New item
- -> MaybeErr InstEnv -- Success...
- ([Type], Id) -- Failure: Offending overlap
-
-addToInstEnv overlap_ok env ins_tvs ins_tys value
- = insert env
+lookupInstEnv :: (InstEnv -- External package inst-env
+ ,InstEnv) -- Home-package inst-env
+ -> Class -> [Type] -- What we are looking for
+ -> ([(TvSubst, Instance)], -- Successful matches
+ [Instance]) -- These don't match but do unify
+ -- The second component of the tuple 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 report that Foo [b] is a match (mapping b->a)
+ -- but Foo [Int] is a unifier. This gives the caller a better chance of
+ -- giving a suitable error messagen
+
+lookupInstEnv (pkg_ie, home_ie) cls tys
+ = (pruned_matches, all_unifs)
+ where
+ rough_tcs = roughMatchTcs tys
+ all_tvs = all isNothing rough_tcs
+ (home_matches, home_unifs) = lookup home_ie
+ (pkg_matches, pkg_unifs) = lookup pkg_ie
+ all_matches = home_matches ++ pkg_matches
+ all_unifs = home_unifs ++ pkg_unifs
+ pruned_matches
+ | null all_unifs = foldr insert_overlapping [] all_matches
+ | otherwise = all_matches -- Non-empty unifs is always an error situation,
+ -- so don't attempt to pune the matches
+
+ --------------
+ lookup env = case lookupUFM env cls of
+ Nothing -> ([],[]) -- No instances for this class
+ Just (ClsIE insts has_tv_insts)
+ | all_tvs && not has_tv_insts
+ -> ([],[]) -- Short cut for common case
+ -- The thing we are looking up is of form (C a b c), and
+ -- the ClsIE has no instances of that form, so don't bother to search
+
+ | otherwise
+ -> find [] [] insts
+
+ --------------
+ find ms us [] = (ms, us)
+ find ms us (item@(Instance { is_tcs = mb_tcs, is_tvs = tpl_tvs,
+ is_tys = tpl_tys, is_flag = oflag,
+ is_dfun = dfun }) : rest)
+ -- Fast check for no match, uses the "rough match" fields
+ | instanceCantMatch rough_tcs mb_tcs
+ = find ms us rest
+
+ | Just subst <- tcMatchTys tpl_tvs tpl_tys tys
+ = find ((subst,item):ms) us rest
+
+ -- Does not match, so next check whether the things unify
+ -- See Note [overlapping instances] above
+ | Incoherent <- oflag
+ = find ms us rest
+
+ | otherwise
+ = ASSERT2( not (tyVarsOfTypes tys `intersectsVarSet` tpl_tvs),
+ (ppr cls <+> ppr tys <+> ppr all_tvs) $$
+ (ppr dfun <+> ppr tpl_tvs <+> ppr tpl_tys)
+ )
+ -- Unification will break badly if the variables overlap
+ -- They shouldn't because we allocate separate uniques for them
+ case tcUnifyTys bind_fn tpl_tys tys of
+ Just _ -> find ms (item:us) rest
+ Nothing -> find ms us rest
+
+---------------
+bind_fn tv | isTcTyVar tv && isExistentialTyVar tv = Skolem
+ | otherwise = BindMe
+ -- The key_tys can contain skolem constants, and we can guarantee that those
+ -- are never going to be instantiated to anything, so we should not involve
+ -- them in the unification test. Example:
+ -- class Foo a where { op :: a -> Int }
+ -- instance Foo a => Foo [a] -- NB overlap
+ -- instance Foo [Int] -- NB overlap
+ -- data T = forall a. Foo a => MkT a
+ -- f :: T -> Int
+ -- f (MkT x) = op [x,x]
+ -- The op [x,x] means we need (Foo [a]). Without the filterVarSet we'd
+ -- complain, saying that the choice of instance depended on the instantiation
+ -- of 'a'; but of course it isn't *going* to be instantiated.
+ --
+ -- We do this only for pattern-bound skolems. For example we reject
+ -- g :: forall a => [a] -> Int
+ -- g x = op x
+ -- on the grounds that the correct instance depends on the instantiation of 'a'
+
+---------------
+insert_overlapping :: (TvSubst, Instance) -> [(TvSubst, Instance)]
+ -> [(TvSubst, Instance)]
+-- Add a new solution, knocking out strictly less specific ones
+insert_overlapping new_item [] = [new_item]
+insert_overlapping new_item (item:items)
+ | new_beats_old && old_beats_new = item : insert_overlapping new_item items
+ -- Duplicate => keep both for error report
+ | new_beats_old = insert_overlapping new_item items
+ -- Keep new one
+ | old_beats_new = item : items
+ -- Keep old one
+ | otherwise = item : insert_overlapping new_item items
+ -- Keep both
where
- ins_tv_set = mkVarSet ins_tvs
- ins_item = (ins_tv_set, ins_tys, value)
-
- insert [] = returnMaB [ins_item]
- insert 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 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
+ new_beats_old = new_item `beats` item
+ old_beats_new = item `beats` new_item
+
+ (_, instA) `beats` (_, instB)
+ = overlap_ok &&
+ isJust (tcMatchTys (is_tvs instB) (is_tys instB) (is_tys instA))
+ -- A beats B if A is more specific than B, and B admits overlap
+ -- I.e. if B can be instantiated to match A
+ where
+ overlap_ok = case is_flag instB of
+ NoOverlap -> False
+ other -> True
\end{code}
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