Tidy up the interface to lookupInstEnv

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

diff --git a/compiler/types/InstEnv.lhs b/compiler/types/InstEnv.lhs
index 9cb68c8..cc0c2dd 100644 (file)
--- a/compiler/types/InstEnv.lhs
+++ b/compiler/types/InstEnv.lhs
@@ -385,20 +385,41 @@ the env is kept ordered, the first match must be the only one.  The
 thing we are looking up can have an arbitrary "flexi" part.
 
 \begin{code}
-lookupInstEnv :: (InstEnv      -- External package inst-env
-                ,InstEnv)      -- Home-package inst-env
-             -> Class -> [Type]                -- What we are looking for
-             -> ([(TvSubst, Instance)],        -- Successful matches
-                 [Instance])                   -- These don't match but do unify
-       -- The second component of the tuple happens when we look up
-       --      Foo [a]
-       -- in an InstEnv that has entries for
-       --      Foo [Int]
-       --      Foo [b]
-       -- Then which we choose would depend on the way in which 'a'
-       -- is instantiated.  So we report that Foo [b] is a match (mapping b->a)
-       -- but Foo [Int] is a unifier.  This gives the caller a better chance of
-       -- giving a suitable error messagen
+type InstTypes = [Either TyVar Type]
+       -- Right ty     => Instantiate with this type
+       -- Left tv      => Instantiate with any type of this tyvar's kind
+
+type InstMatch = (Instance, InstTypes)
+\end{code}
+
+Note [InstTypes: instantiating types]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+A successful match is an Instance, together with the types at which
+       the dfun_id in the Instance should be instantiated
+The instantiating types are (Mabye Type)s because the dfun
+might have some tyvars that *only* appear in arguments
+       dfun :: forall a b. C a b, Ord b => D [a]
+When we match this against D [ty], we return the instantiating types
+       [Right ty, Left b]
+where the Nothing indicates that 'b' can be freely instantiated.  
+(The caller instantiates it to a flexi type variable, which will presumably
+ presumably later become fixed via functional dependencies.)
+
+\begin{code}
+lookupInstEnv :: (InstEnv, InstEnv)    -- External and home package inst-env
+             -> Class -> [Type]        -- What we are looking for
+             -> ([InstMatch],          -- Successful matches
+                 [Instance])           -- These don't match but do unify
+
+-- The second component of the result pair happens when we look up
+--     Foo [a]
+-- in an InstEnv that has entries for
+--     Foo [Int]
+--     Foo [b]
+-- Then which we choose would depend on the way in which 'a'
+-- is instantiated.  So we report that Foo [b] is a match (mapping b->a)
+-- but Foo [Int] is a unifier.  This gives the caller a better chance of
+-- giving a suitable error messagen
 
 lookupInstEnv (pkg_ie, home_ie) cls tys
   = (pruned_matches, all_unifs)
@@ -427,6 +448,12 @@ lookupInstEnv (pkg_ie, home_ie) cls tys
                        -> find [] [] insts
 
     --------------
+    lookup_tv :: TvSubst -> TyVar -> Either TyVar Type 
+       -- See Note [InstTypes: instantiating types]
+    lookup_tv subst tv = case lookupTyVar subst tv of
+                               Just ty -> Right ty
+                               Nothing -> Left tv
+
     find ms us [] = (ms, us)
     find ms us (item@(Instance { is_tcs = mb_tcs, is_tvs = tpl_tvs, 
                                 is_tys = tpl_tys, is_flag = oflag,
@@ -436,7 +463,11 @@ lookupInstEnv (pkg_ie, home_ie) cls tys
       = find ms us rest
 
       | Just subst <- tcMatchTys tpl_tvs tpl_tys tys
-      = find ((subst,item):ms) us rest
+      = let 
+           (dfun_tvs, _) = tcSplitForAllTys (idType dfun)
+       in 
+       ASSERT( all (`elemVarSet` tpl_tvs) dfun_tvs )   -- Check invariant
+       find ((item, map (lookup_tv subst) dfun_tvs) : ms) us rest
 
        -- Does not match, so next check whether the things unify
        -- See Note [overlapping instances] above
@@ -476,8 +507,7 @@ bind_fn tv | isTcTyVar tv && isExistentialTyVar tv = Skolem
        -- on the grounds that the correct instance depends on the instantiation of 'a'
 
 ---------------
-insert_overlapping :: (TvSubst, Instance) -> [(TvSubst, Instance)] 
-                  -> [(TvSubst, Instance)]
+insert_overlapping :: InstMatch -> [InstMatch] -> [InstMatch]
 -- Add a new solution, knocking out strictly less specific ones
 insert_overlapping new_item [] = [new_item]
 insert_overlapping new_item (item:items)
@@ -493,7 +523,7 @@ insert_overlapping new_item (item:items)
     new_beats_old = new_item `beats` item
     old_beats_new = item `beats` new_item
 
-    (_, instA) `beats` (_, instB)
+    (instA, _) `beats` (instB, _)
        = overlap_ok && 
          isJust (tcMatchTys (is_tvs instB) (is_tys instB) (is_tys instA))
                -- A beats B if A is more specific than B, and B admits overlap