Overlap check for family instances def'd in current module

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

diff --git a/compiler/types/FamInstEnv.lhs b/compiler/types/FamInstEnv.lhs
index c44c200..9b49f5c 100644 (file)
--- a/compiler/types/FamInstEnv.lhs
+++ b/compiler/types/FamInstEnv.lhs
@@ -11,13 +11,15 @@ module FamInstEnv (
 
        FamInstEnv, emptyFamInstEnv, extendFamInstEnv, extendFamInstEnvList, 
        famInstEnvElts, familyInstances,
-       lookupFamInstEnv
+
+       lookupFamInstEnv, lookupFamInstEnvUnify
     ) where
 
 #include "HsVersions.h"
 
 import InstEnv
 import Unify
+import TcGadt
 import TcType
 import Type
 import TyCon
@@ -210,6 +212,7 @@ lookupFamInstEnv (pkg_ie, home_ie) fam tys
                     | otherwise                   -> find insts
 
     --------------
+    find [] = []
     find (item@(FamInst { fi_tcs = mb_tcs, fi_tvs = tpl_tvs, 
                          fi_tys = tpl_tys, fi_tycon = tycon }) : rest)
        -- Fast check for no match, uses the "rough match" fields
@@ -224,3 +227,60 @@ lookupFamInstEnv (pkg_ie, home_ie) fam tys
       | otherwise
       = find rest
 \end{code}
+
+While @lookupFamInstEnv@ uses a one-way match, the next function
+@lookupFamInstEnvUnify@ uses two-way matching (ie, unification).  This is
+needed to check for overlapping instances.
+
+For class instances, these two variants of lookup are combined into one
+function (cf, @InstEnv@).  We don't do that for family instances as the
+results of matching and unification are used in two different contexts.
+Moreover, matching is the wildly more frequently used operation in the case of
+indexed synonyms and we don't want to slow that down by needless unification.
+
+\begin{code}
+lookupFamInstEnvUnify :: (FamInstEnv, FamInstEnv) -> TyCon -> [Type]
+                     -> [(TvSubst, FamInst)]
+lookupFamInstEnvUnify (pkg_ie, home_ie) fam tys
+  = 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  
+
+    --------------
+    lookup env = case lookupUFM env fam of
+                  Nothing -> []        -- No instances for this class
+                  Just (FamIE insts has_tv_insts)
+                      -- Short cut for common case:
+                      --   The thing we are looking up is of form (C a
+                      --   b c), and the FamIE has no instances of
+                      --   that form, so don't bother to search 
+                    | all_tvs && not has_tv_insts -> []
+                    | otherwise                   -> find insts
+
+    --------------
+    find [] = []
+    find (item@(FamInst { fi_tcs = mb_tcs, fi_tvs = tpl_tvs, 
+                         fi_tys = tpl_tys, fi_tycon = tycon }) : rest)
+       -- Fast check for no match, uses the "rough match" fields
+      | instanceCantMatch rough_tcs mb_tcs
+      = find rest
+
+      | otherwise
+      = ASSERT2( tyVarsOfTypes tys `disjointVarSet` tpl_tvs,
+                (ppr fam <+> ppr tys <+> ppr all_tvs) $$
+                (ppr tycon <+> ppr tpl_tvs <+> ppr tpl_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
+           Just subst -> (subst, item) : find rest
+           Nothing    -> find rest
+
+-- See explanation at @InstEnv.bind_fn@.
+--
+bind_fn tv | isTcTyVar tv && isExistentialTyVar tv = Skolem
+          | otherwise                             = BindMe
+\end{code}