InstEnv, emptyInstEnv, extendInstEnv,
extendInstEnvList, lookupInstEnv, instEnvElts,
- classInstances,
+ classInstances, instanceBindFun,
instanceCantMatch, roughMatchTcs
) where
import Name
import TcType
import TyCon
-import TcGadt
import Unify
import Outputable
import BasicTypes
import UniqFM
import Id
+import FastString
import Data.Maybe ( isJust, isNothing )
\end{code}
-- Prints the Instance as an instance declaration
pprInstance ispec
= hang (pprInstanceHdr ispec)
- 2 (ptext SLIT("--") <+> pprNameLoc (getName ispec))
+ 2 (ptext (sLit "--") <+> pprNameLoc (getName 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
+ = ptext (sLit "instance") <+> ppr flag
<+> sep [pprThetaArrow theta, pprClassPred clas tys]
where
(_, theta, clas, tys) = instanceHead ispec
%************************************************************************
%* *
-\subsection{Looking up an instance}
+ Looking up an instance
%* *
%************************************************************************
)
-- 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
+ case tcUnifyTys instanceBindFun tpl_tys tys of
Just _ -> find ms (item:us) rest
Nothing -> find ms us rest
---------------
-bind_fn :: TyVar -> BindFlag
-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 :: InstMatch -> [InstMatch] -> [InstMatch]
-- Add a new solution, knocking out strictly less specific ones
_ -> True
\end{code}
+
+%************************************************************************
+%* *
+ Binding decisions
+%* *
+%************************************************************************
+
+\begin{code}
+instanceBindFun :: TyVar -> BindFlag
+instanceBindFun tv | isTcTyVar tv && isExistentialTyVar tv = Skolem
+ | otherwise = BindMe
+ -- Note [Binding when looking up instances]
+\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 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'