From: simonpj@microsoft.com <unknown>
Date: Mon, 18 Sep 2006 00:41:09 +0000 (+0000)
Subject: Comments only
X-Git-Url: http://git.megacz.com/?p=ghc-hetmet.git;a=commitdiff_plain;h=4e9dc3d744df308623463c7caa320d0cad1ae9d0

Comments only
---

diff --git a/compiler/typecheck/TcUnify.lhs b/compiler/typecheck/TcUnify.lhs
index bb97f8d..bd65c46 100644
--- 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