diff options
Diffstat (limited to 'compiler/GHC/Tc/Gen/Default.hs')
| -rw-r--r-- | compiler/GHC/Tc/Gen/Default.hs | 21 |
1 files changed, 15 insertions, 6 deletions
diff --git a/compiler/GHC/Tc/Gen/Default.hs b/compiler/GHC/Tc/Gen/Default.hs index d37f26df40..c8106858b9 100644 --- a/compiler/GHC/Tc/Gen/Default.hs +++ b/compiler/GHC/Tc/Gen/Default.hs @@ -12,6 +12,8 @@ import GHC.Prelude import GHC.Hs import GHC.Core.Class +import GHC.Core.Type ( typeKind ) +import GHC.Types.Var( tyVarKind ) import GHC.Tc.Utils.Monad import GHC.Tc.Utils.Env import GHC.Tc.Gen.HsType @@ -82,13 +84,20 @@ tc_default_ty deflt_clss hs_ty ; return ty } check_instance :: Type -> Class -> TcM Bool - -- Check that ty is an instance of cls - -- We only care about whether it worked or not; return a boolean +-- Check that ty is an instance of cls +-- We only care about whether it worked or not; return a boolean +-- This checks that cls :: k -> Constraint +-- with just one argument and no polymorphism; if we need to add +-- polymorphism we can make it more complicated. For now we are +-- concerned with classes like +-- Num :: Type -> Constraint +-- Foldable :: (Type->Type) -> Constraint check_instance ty cls - = do { (_, success) <- discardErrs $ - askNoErrs $ - simplifyDefault [mkClassPred cls [ty]] - ; return success } + | [cls_tv] <- classTyVars cls + , tyVarKind cls_tv `tcEqType` typeKind ty + = simplifyDefault [mkClassPred cls [ty]] + | otherwise + = return False defaultDeclCtxt :: SDoc defaultDeclCtxt = text "When checking the types in a default declaration" |