shithub: MicroHs

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Fix a bug in instance selection.

--- a/src/MicroHs/TypeCheck.hs

+++ b/src/MicroHs/TypeCheck.hs

@@ -930,6 +930,7 @@

 tcExpand dst = withTypeTable $ do

   dsc <- concat <$> mapM expandClass dst       -- Expand all class definitions

   dsf <- concat <$> mapM expandField dsc       -- Add HasField instances

+--  traceM $ showEDefs dsf

   dsd <- concat <$> mapM doDeriving  dsf       -- Add derived instances

 --  traceM $ showEDefs dsd

   dsi <- concat <$> mapM expandInst  dsd       -- Expand all instance definitions

@@ -1545,7 +1546,6 @@

 failureFree _ = False

 eSetFields :: EField -> Expr -> Expr

---eSetFields ([i], e) r = eSetField (i, e) r

 eSetFields (EField is e) r =

   let loc = getSLoc is

       eCompose = EVar $ mkIdentSLoc loc "composeSet"

@@ -2382,6 +2382,7 @@

 solveGen fds insts loc iCls cts = do

 --  traceM ("solveGen " ++ showEType ct)

   let matches = getBestMatches $ findMatches loc fds insts cts

+--  traceM ("matches " ++ showListS show (findMatches loc fds insts cts))

 --  traceM ("matches " ++ showListS showMatch matches)

   case matches of

     []              -> return Nothing

@@ -2513,12 +2514,13 @@

         Just t' -> matchType [] t' t

 -- Get the best matches.  These are the matches with the smallest substitution.

+-- Always prefer arguments rather than global instances.

 getBestMatches :: [(Int, (Expr, [EConstraint], [Improve]))] -> [(Expr, [EConstraint], [Improve])]

 getBestMatches [] = []

 getBestMatches ams =

-  let (args, insts) = partition (\ (_, (ei, _, _)) -> (adictPrefix ++ uniqIdentSep) `isPrefixOf` unIdent (unvar ei)) ams

-      unvar (EVar i) = i

-      unvar e = impossibleShow e

+  let (args, insts) = partition (\ (_, (ei, _, _)) -> isDictArg ei) ams

+      isDictArg (EVar i) = (adictPrefix ++ uniqIdentSep) `isPrefixOf` unIdent i

+      isDictArg _ = True

       pick ms =

         let b = minimum (map fst ms)         -- minimum substitution size

         in  [ ec | (s, ec) <- ms, s == b ]   -- pick out the smallest

-- 

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