Attempting to fix #301

src/IHaskell/Eval/Util.hs

@@ -36,11 +36,15 @@ import RdrName
 import           NameSet
 import           Name
 import           PprTyThing
+import           InstEnv (ClsInst(..))
+import           Unify (tcMatchTys)
+import           VarSet (mkVarSet)
 import qualified Pretty
 
 import           Control.Monad (void)
 import           Data.Function (on)
 import           Data.String.Utils (replace)
+import           Data.List (nubBy)
 
 -- | A extension flag that can be set or unset.
 data ExtFlag
@@ -197,9 +201,26 @@ evalImport imports = do
 evalDeclarations :: GhcMonad m => String -> m [String]
 evalDeclarations decl = do
   names <- runDecls decl
+  cleanUpDuplicateInstances
   flags <- getSessionDynFlags
   return $ map (replace ":Interactive." "" . showPpr flags) names
 
+cleanUpDuplicateInstances :: GhcMonad m => m ()
+cleanUpDuplicateInstances = modifySession $ \hscEnv ->
+  let 
+      -- Get all class instancesj
+      ic = hsc_IC hscEnv
+      (clsInsts, famInsts) = ic_instances ic
+      -- Remove duplicates
+      clsInsts' = nubBy instEq clsInsts
+  in hscEnv { hsc_IC = ic { ic_instances = (clsInsts', famInsts) } }
+  where
+    instEq :: ClsInst -> ClsInst -> Bool
+    instEq ClsInst{is_tvs = tpl_tvs,is_tys = tpl_tys} ClsInst{is_tys = tpl_tys'} =
+      let tpl_tv_set = mkVarSet tpl_tvs
+      in isJust $ tcMatchTys tpl_tv_set tpl_tys tpl_tys'
+
+
 -- | Get the type of an expression and convert it to a string.
 getType :: GhcMonad m => String -> m String
 getType expr = do