%
% (c) The University of Glasgow 2006
% (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
%
\section[InstEnv]{Utilities for typechecking instance declarations}
The bits common to TcInstDcls and TcDeriv.
\begin{code}
module InstEnv (
DFunId, OverlapFlag(..),
Instance(..), pprInstance, pprInstanceHdr, pprInstances,
instanceHead, mkLocalInstance, mkImportedInstance,
instanceDFunId, setInstanceDFunId, instanceRoughTcs,
InstEnv, emptyInstEnv, extendInstEnv,
extendInstEnvList, lookupInstEnv, instEnvElts,
classInstances, instanceBindFun,
instanceCantMatch, roughMatchTcs
) where
#include "HsVersions.h"
import Class
import Var
import VarSet
import Name
import TcType
import TyCon
import Unify
import Outputable
import BasicTypes
import UniqFM
import Id
import FastString
import Data.Maybe ( isJust, isNothing )
\end{code}
%************************************************************************
%* *
\subsection{The key types}
%* *
%************************************************************************
\begin{code}
data Instance
= Instance { is_cls :: Name
, is_tcs :: [Maybe Name]
, is_tvs :: TyVarSet
, is_tys :: [Type]
, is_dfun :: DFunId
, is_flag :: OverlapFlag
}
\end{code}
Note [Rough-match field]
~~~~~~~~~~~~~~~~~~~~~~~~~~~
The is_cls, is_tcs 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!
Note [Proper-match fields]
~~~~~~~~~~~~~~~~~~~~~~~~~
The is_tvs, is_tys fields are simply cached 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.)
Note [Haddock assumptions]
~~~~~~~~~~~~~~~~~~~~~~~~~~
For normal user-written instances, Haddock relies on
* the SrcSpan of
* the Name of
* the is_dfun of
* an Instance
being equal to
* the SrcSpan of
* the instance head type of
* the InstDecl used to construct the Instance.
\begin{code}
instanceDFunId :: Instance -> DFunId
instanceDFunId = is_dfun
setInstanceDFunId :: Instance -> DFunId -> Instance
setInstanceDFunId ispec dfun
= ASSERT( idType dfun `eqType` idType (is_dfun ispec) )
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}
\begin{code}
instance NamedThing Instance where
getName ispec = getName (is_dfun ispec)
instance Outputable Instance where
ppr = pprInstance
pprInstance :: Instance -> SDoc
pprInstance ispec
= hang (pprInstanceHdr ispec)
2 (ptext (sLit "--") <+> pprNameLoc (getName ispec))
pprInstanceHdr :: Instance -> SDoc
pprInstanceHdr ispec@(Instance { is_flag = flag })
= ptext (sLit "instance") <+> ppr flag
<+> sep [pprThetaArrowTy theta, ppr res_ty]
where
dfun = is_dfun ispec
(_, theta, res_ty) = tcSplitSigmaTy (idType dfun)
pprInstances :: [Instance] -> SDoc
pprInstances ispecs = vcat (map pprInstance ispecs)
instanceHead :: Instance -> ([TyVar], ThetaType, Class, [Type])
instanceHead ispec = (tvs, theta, cls, tys)
where
(tvs, theta, tau) = tcSplitSigmaTy (idType dfun)
(cls, tys) = tcSplitDFunHead tau
dfun = is_dfun ispec
mkLocalInstance :: DFunId
-> OverlapFlag
-> Instance
mkLocalInstance dfun oflag
= Instance { is_flag = oflag, is_dfun = dfun,
is_tvs = mkVarSet tvs, is_tys = tys,
is_cls = className cls, is_tcs = roughMatchTcs tys }
where
(tvs, _, cls, tys) = tcSplitDFunTy (idType dfun)
mkImportedInstance :: Name -> [Maybe Name]
-> DFunId -> OverlapFlag -> Instance
mkImportedInstance cls mb_tcs dfun oflag
= Instance { is_flag = oflag, is_dfun = dfun,
is_tvs = mkVarSet tvs, is_tys = tys,
is_cls = cls, is_tcs = mb_tcs }
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
instanceCantMatch :: [Maybe Name] -> [Maybe Name] -> Bool
instanceCantMatch (Just t : ts) (Just a : as) = t/=a || instanceCantMatch ts as
instanceCantMatch _ _ = False
\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:
[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
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
newtype ClsInstEnv
= ClsIE [Instance]
instance Outputable ClsInstEnv where
ppr (ClsIE is) = pprInstances is
emptyInstEnv :: InstEnv
emptyInstEnv = emptyUFM
instEnvElts :: InstEnv -> [Instance]
instEnvElts ie = [elt | ClsIE elts <- eltsUFM ie, elt <- elts]
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 -> []
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 })
= addToUFM_C add inst_env cls_nm (ClsIE [ins_item])
where
add (ClsIE cur_insts) _ = ClsIE (ins_item : cur_insts)
\end{code}
%************************************************************************
%* *
Looking up an instance
%* *
%************************************************************************
@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}
type InstTypes = [Either TyVar Type]
type InstMatch = (Instance, InstTypes)
\end{code}
Note [InstTypes: instantiating types]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
A successful match is an Instance, together with the types at which
the dfun_id in the Instance should be instantiated
The instantiating types are (Mabye Type)s because the dfun
might have some tyvars that *only* appear in arguments
dfun :: forall a b. C a b, Ord b => D [a]
When we match this against D [ty], we return the instantiating types
[Right ty, Left b]
where the Nothing indicates that 'b' can be freely instantiated.
(The caller instantiates it to a flexi type variable, which will presumably
presumably later become fixed via functional dependencies.)
\begin{code}
lookupInstEnv :: (InstEnv, InstEnv)
-> Class -> [Type]
-> ([InstMatch],
[Instance],
Bool)
lookupInstEnv (pkg_ie, home_ie) cls tys
= (safe_matches, all_unifs, safe_fail)
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 = foldr insert_overlapping [] all_matches
(safe_matches, safe_fail) = if length pruned_matches == 1
then check_safe (head pruned_matches) all_matches
else (pruned_matches, False)
check_safe match@(inst,_) others
= case isSafeOverlap (is_flag inst) of
False -> ([match], False)
True -> (go [] others, True)
where
go bad [] = match:bad
go bad (i@(x,_):unchecked) =
if inSameMod x
then go bad unchecked
else go (i:bad) unchecked
inSameMod b =
let na = getName $ getName inst
la = isInternalName na
nb = getName $ getName b
lb = isInternalName nb
in (la && lb) || (nameModule na == nameModule nb)
lookup env = case lookupUFM env cls of
Nothing -> ([],[])
Just (ClsIE insts) -> 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)
| instanceCantMatch rough_tcs mb_tcs
= find ms us rest
| Just subst <- tcMatchTys tpl_tvs tpl_tys tys
= let
(dfun_tvs, _) = tcSplitForAllTys (idType dfun)
in
ASSERT( all (`elemVarSet` tpl_tvs) dfun_tvs )
find ((item, map (lookup_tv subst) dfun_tvs) : ms) us rest
| Incoherent _ <- oflag
= find ms us rest
| otherwise
= ASSERT2( tyVarsOfTypes tys `disjointVarSet` tpl_tvs,
(ppr cls <+> ppr tys <+> ppr all_tvs) $$
(ppr dfun <+> ppr tpl_tvs <+> ppr tpl_tys)
)
case tcUnifyTys instanceBindFun tpl_tys tys of
Just _ -> find ms (item:us) rest
Nothing -> find ms us rest
lookup_tv :: TvSubst -> TyVar -> Either TyVar Type
lookup_tv subst tv = case lookupTyVar subst tv of
Just ty -> Right ty
Nothing -> Left tv
insert_overlapping :: InstMatch -> [InstMatch] -> [InstMatch]
insert_overlapping new_item [] = [new_item]
insert_overlapping new_item (item:items)
| new_beats_old && old_beats_new = item : insert_overlapping new_item items
| new_beats_old = insert_overlapping new_item items
| old_beats_new = item : items
| otherwise = item : insert_overlapping new_item items
where
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))
where
overlap_ok = case (is_flag instA, is_flag instB) of
(NoOverlap _, NoOverlap _) -> False
_ -> True
\end{code}
%************************************************************************
%* *
Binding decisions
%* *
%************************************************************************
\begin{code}
instanceBindFun :: TyVar -> BindFlag
instanceBindFun tv | isTcTyVar tv && isOverlappableTyVar tv = Skolem
| otherwise = BindMe
\end{code}
Note [Binding when looking up instances]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
When looking up in the instance environment, or family-instance environment,
we are careful about multiple matches, as described above in
Note [Overlapping instances]
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 isOverlappableTyVar 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'