[project @ 2004-10-01 14:25:12 by simonpj]

Comments only

diff --git a/ghc/compiler/typecheck/TcRnTypes.lhs b/ghc/compiler/typecheck/TcRnTypes.lhs
index ea10cce..5903fc0 100644 (file)
--- a/ghc/compiler/typecheck/TcRnTypes.lhs
+++ b/ghc/compiler/typecheck/TcRnTypes.lhs
@@ -278,18 +278,35 @@ data TcLclEnv             -- Changes as we move inside an expression
        tcl_env  :: NameEnv TcTyThing,  -- The local type environment: Ids and TyVars
                                        -- defined in this module
                                        
-       tcl_gadt :: GadtRefinement,     -- The current type refinement for GADTs
-
        tcl_tyvars :: TcRef TcTyVarSet, -- The "global tyvars"
-                       -- Namely, the in-scope TyVars bound in tcl_lenv, 
+                       -- Namely, the in-scope TyVars bound in tcl_env, 
                        -- plus the tyvars mentioned in the types of Ids bound in tcl_lenv
                        -- Why mutable? see notes with tcGetGlobalTyVars
 
-       tcl_lie :: TcRef LIE            -- Place to accumulate type constraints
+       tcl_lie   :: TcRef LIE,         -- Place to accumulate type constraints
+       tcl_gadt  :: GadtRefinement     -- The current type refinement for GADTs
+
+-----------------------------------------------------------
+-- Not yet; it's a new complication and I want to see whether it bites
+--     tcl_given :: [Inst]             -- Insts available in the current context (see Note [Given Insts])
+-----------------------------------------------------------
     }
 
 type GadtRefinement = TvSubstEnv       -- Binds rigid type variables to their refinements
 
+{- Note [Given Insts]
+   ~~~~~~~~~~~~~~~~~~
+Because of GADTs, we have to pass inwards the Insts provided by type signatures 
+and existential contexts. Consider
+       data T a where { T1 :: b -> b -> T [b] }
+       f :: Eq a => T a -> Bool
+       f (T1 x y) = [x]==[y]
+
+The constructor T1 binds an existential variable 'b', and we need Eq [b].
+Well, we have it, because Eq a refines to Eq [b], but we can only spot that if we 
+pass it inwards.
+
+-}
 
 ---------------------------
 -- Template Haskell levels