Robustify lookupFamInstEnv

[ghc-hetmet.git] / compiler / types / FamInstEnv.lhs

diff --git a/compiler/types/FamInstEnv.lhs b/compiler/types/FamInstEnv.lhs
index 783ee13..65c2539 100644 (file)
--- a/compiler/types/FamInstEnv.lhs
+++ b/compiler/types/FamInstEnv.lhs
@@ -23,7 +23,6 @@ module FamInstEnv (
 #include "HsVersions.h"
 
 import InstEnv
-import TcType
 import Unify
 import Type
 import TypeRep
@@ -219,8 +218,25 @@ desugared to
 
 we return the matching instance '(FamInst{.., fi_tycon = :R42T}, Int)'.
 
+Note [Over-saturated matches]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+It's ok to look up an over-saturated type constructor.  E.g.
+     type family F a :: * -> *
+     type instance F (a,b) = Either (a->b)
+
+The type instance gives rise to a newtype TyCon (at a higher kind
+which you can't do in Haskell!):
+     newtype FPair a b = FP (Either (a->b))
+
+Then looking up (F (Int,Bool) Char) will return a FamInstMatch 
+     (FPair, [Int,Bool,Char])
+
+The "extra" type argument [Char] just stays on the end.
+
+
 \begin{code}
 type FamInstMatch = (FamInst, [Type])           -- Matching type instance
+  -- See Note [Over-saturated matches]
 
 lookupFamInstEnv :: FamInstEnvs
                 -> TyCon -> [Type]             -- What we are looking for
@@ -229,13 +245,23 @@ lookupFamInstEnv (pkg_ie, home_ie) fam tys
   | not (isOpenTyCon fam) 
   = []
   | otherwise
-  = home_matches ++ pkg_matches
+  = ASSERT( n_tys >= arity )   -- Family type applications must be saturated
+    home_matches ++ pkg_matches
   where
     rough_tcs    = roughMatchTcs tys
     all_tvs      = all isNothing rough_tcs
     home_matches = lookup home_ie 
     pkg_matches  = lookup pkg_ie  
 
+    -- See Note [Over-saturated matches]
+    arity = tyConArity fam
+    n_tys = length tys
+    extra_tys = drop arity tys
+    (match_tys, add_extra_tys) 
+       | arity > n_tys = (take arity tys, \res_tys -> res_tys ++ extra_tys)
+       | otherwise     = (tys,            \res_tys -> res_tys)
+                -- The second case is the common one, hence functional representation
+
     --------------
     lookup env = case lookupUFM env fam of
                   Nothing -> []        -- No instances for this class
@@ -256,8 +282,8 @@ lookupFamInstEnv (pkg_ie, home_ie) fam tys
       = find rest
 
         -- Proper check
-      | Just subst <- tcMatchTys tpl_tvs tpl_tys tys
-      = (item, substTyVars subst (tyConTyVars tycon)) : find rest
+      | Just subst <- tcMatchTys tpl_tvs tpl_tys match_tys
+      = (item, add_extra_tys $ substTyVars subst (tyConTyVars tycon)) : find rest
 
         -- No match => try next
       | otherwise
@@ -314,17 +340,11 @@ lookupFamInstEnvUnify (pkg_ie, home_ie) fam tys
                )
                -- 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 subst -> let rep_tys = substTyVars subst (tyConTyVars tycon)
                           in
                           ((item, rep_tys), subst) : find rest
            Nothing    -> find rest
-
--- See explanation at @InstEnv.bind_fn@.
---
-bind_fn :: TyVar -> BindFlag
-bind_fn tv | isTcTyVar tv && isExistentialTyVar tv = Skolem
-          | otherwise                             = BindMe
 \end{code}
 
 %************************************************************************