Comments only

diff --git a/compiler/typecheck/TcUnify.lhs b/compiler/typecheck/TcUnify.lhs
index bb97f8d..bd65c46 100644 (file)
--- a/compiler/typecheck/TcUnify.lhs
+++ b/compiler/typecheck/TcUnify.lhs
@@ -339,7 +339,7 @@ withBox kind thing_inside
 \begin{code}
 preSubType :: [TcTyVar]                -- Quantified type variables
           -> TcTyVarSet        -- Subset of quantified type variables
-                               -- that can be instantiated with boxy types
+                               --   see Note [Pre-sub boxy]
            -> TcType           -- The rho-type part; quantified tyvars scopes over this
            -> BoxySigmaType    -- Matching type from the context
            -> TcM [TcType]     -- Types to instantiate the tyvars
@@ -348,13 +348,17 @@ preSubType :: [TcTyVar]           -- Quantified type variables
 --     info from the pre-subsumption, if there is any
 --     a boxy type variable otherwise
 --
--- The 'btvs' are a subset of 'qtvs'.  They are the ones we can
--- instantiate to a boxy type variable, because they'll definitely be
--- filled in later.  This isn't always the case; sometimes we have type 
--- variables mentioned in the context of the type, but not the body; 
---             f :: forall a b. C a b => a -> a
--- Then we may land up with an unconstrained 'b', so we want to 
--- instantiate it to a monotype (non-boxy) type variable
+-- Note [Pre-sub boxy]
+--   The 'btvs' are a subset of 'qtvs'.  They are the ones we can
+--   instantiate to a boxy type variable, because they'll definitely be
+--   filled in later.  This isn't always the case; sometimes we have type 
+--   variables mentioned in the context of the type, but not the body; 
+--               f :: forall a b. C a b => a -> a
+--   Then we may land up with an unconstrained 'b', so we want to 
+--   instantiate it to a monotype (non-boxy) type variable
+--
+-- The 'qtvs' that are *neither* fixed by the pre-subsumption, *nor* are in 'btvs',
+-- are instantiated to TauTv meta variables.
        
 preSubType qtvs btvs qty expected_ty
   = do { tys <- mapM inst_tv qtvs