import Util
import FastString
+import Data.List ( nubBy )
import Data.Maybe ( isJust )
\end{code}
-> TyVarSet -> [Type] -- Proposed new instance type
-> [Instance]
badFunDeps cls_insts clas ins_tv_set ins_tys
- = [ ispec | fd <- fds, -- fds is often empty
+ = nubBy eq_inst $
+ [ ispec | fd <- fds, -- fds is often empty, so do this first!
let trimmed_tcs = trimRoughMatchTcs clas_tvs fd rough_tcs,
ispec@(Instance { is_tcs = inst_tcs, is_tvs = tvs,
is_tys = tys }) <- cls_insts,
where
(clas_tvs, fds) = classTvsFds clas
rough_tcs = roughMatchTcs ins_tys
+ eq_inst i1 i2 = instanceDFunId i1 == instanceDFunId i2
+ -- An single instance may appear twice in the un-nubbed conflict list
+ -- because it may conflict with more than one fundep. E.g.
+ -- class C a b c | a -> b, a -> c
+ -- instance C Int Bool Bool
+ -- instance C Int Char Char
+ -- The second instance conflicts with the first by *both* fundeps
trimRoughMatchTcs :: [TyVar] -> FunDep TyVar -> [Maybe Name] -> [Maybe Name]
-- Computing rough_tcs for a particular fundep
--- class C a b c | a -> b where ...
+-- class C a b c | a -> b where ...
-- For each instance .... => C ta tb tc
--- we want to match only on the types ta, tc; so our
+-- we want to match only on the type ta; so our
-- rough-match thing must similarly be filtered.
--- Hence, we Nothing-ise the tb type right here
-trimRoughMatchTcs clas_tvs (_,rtvs) mb_tcs
+-- Hence, we Nothing-ise the tb and tc types right here
+trimRoughMatchTcs clas_tvs (ltvs, _) mb_tcs
= zipWith select clas_tvs mb_tcs
where
- select clas_tv mb_tc | clas_tv `elem` rtvs = Nothing
- | otherwise = mb_tc
+ select clas_tv mb_tc | clas_tv `elem` ltvs = mb_tc
+ | otherwise = Nothing
\end{code}