Deriving for indexed data types

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

diff --git a/compiler/types/FamInstEnv.lhs b/compiler/types/FamInstEnv.lhs
index 9b49f5c..5ff0139 100644 (file)
--- a/compiler/types/FamInstEnv.lhs
+++ b/compiler/types/FamInstEnv.lhs
@@ -12,7 +12,7 @@ module FamInstEnv (
        FamInstEnv, emptyFamInstEnv, extendFamInstEnv, extendFamInstEnvList, 
        famInstEnvElts, familyInstances,
 
-       lookupFamInstEnv, lookupFamInstEnvUnify
+       lookupFamInstEnvExact, lookupFamInstEnv, lookupFamInstEnvUnify
     ) where
 
 #include "HsVersions.h"
@@ -174,7 +174,7 @@ extendFamInstEnv inst_env ins_item@(FamInst {fi_fam = cls_nm, fi_tcs = mb_tcs})
     add (FamIE items tyvar) _ = FamIE (ins_item:items)
                                      (ins_tyvar || tyvar)
     ins_tyvar = not (any isJust mb_tcs)
-\end{code}                   
+\end{code}
 
 %************************************************************************
 %*                                                                     *
@@ -182,6 +182,50 @@ extendFamInstEnv inst_env ins_item@(FamInst {fi_fam = cls_nm, fi_tcs = mb_tcs})
 %*                                                                     *
 %************************************************************************
 
+@lookupFamInstEnvExact@ looks up in a @FamInstEnv@ using an exact match.
+This is used when we want the @TyCon@ of a particular family instance (e.g.,
+during deriving classes).
+
+\begin{code}
+lookupFamInstEnvExact :: (FamInstEnv           -- External package inst-env
+                        ,FamInstEnv)           -- Home-package inst-env
+                     -> TyCon -> [Type]        -- What we are looking for
+                     -> Maybe FamInst
+lookupFamInstEnvExact (pkg_ie, home_ie) fam tys
+  = home_matches `mplus` 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 -> 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 -> Nothing
+                    | otherwise                   -> find insts
+
+    --------------
+    find [] = Nothing
+    find (item@(FamInst { fi_tcs = mb_tcs, fi_tys = tpl_tys }) : rest)
+       -- Fast check for no match, uses the "rough match" fields
+      | instanceCantMatch rough_tcs mb_tcs
+      = find rest
+
+        -- Proper check
+      | tcEqTypes tpl_tys tys
+      = Just item
+
+        -- No match => try next
+      | otherwise
+      = find rest
+\end{code}
+
 @lookupFamInstEnv@ looks up in a @FamInstEnv@, using a one-way match.
 Multiple matches are only possible in case of type families (not data
 families), and then, it doesn't matter which match we choose (as the