X-Git-Url: http://git.megacz.com/?p=ghc-hetmet.git;a=blobdiff_plain;f=compiler%2Ftypecheck%2FTcRnTypes.lhs;h=eb1cd0427111f1af107c0e4c3a5f65231bed1566;hp=4138c00a187ee1ab84b5f3f2dfe2ce4350201563;hb=3f1b316d7035c55cd712cd39a9981339bcef2e8c;hpb=5822cb8d13aa3c05d2b46b4510c13d94b902eb21

diff --git a/compiler/typecheck/TcRnTypes.lhs b/compiler/typecheck/TcRnTypes.lhs
index 4138c00..eb1cd04 100644
--- a/compiler/typecheck/TcRnTypes.lhs
+++ b/compiler/typecheck/TcRnTypes.lhs
@@ -1,8 +1,15 @@
-%
+
 % (c) The University of Glasgow 2006
 % (c) The GRASP Project, Glasgow University, 1992-2002
 %
 \begin{code}
+{-# OPTIONS -w #-}
+-- The above warning supression flag is a temporary kludge.
+-- While working on this module you are encouraged to remove it and fix
+-- any warnings in the module. See
+--     http://hackage.haskell.org/trac/ghc/wiki/Commentary/CodingStyle#Warnings
+-- for details
+
 module TcRnTypes(
 	TcRnIf, TcRn, TcM, RnM,	IfM, IfL, IfG, -- The monad is opaque outside this module
 	TcRef,
@@ -145,7 +152,7 @@ data TcGblEnv
 		-- (Ids defined in this module start in the local envt, 
 		--  though they move to the global envt during zonking)
 
-	tcg_type_env_var :: TcRef TypeEnv,	
+	tcg_type_env_var :: TcRef TypeEnv,
 		-- Used only to initialise the interface-file
 		-- typechecker in initIfaceTcRn, so that it can see stuff
 		-- bound in this module when dealing with hi-boot recursions
@@ -362,7 +369,7 @@ type ThLevel = Int
 	-- Incremented when going inside a bracket,
 	-- decremented when going inside a splice
 	-- NB: ThLevel is one greater than the 'n' in Fig 2 of the
-	--     original "Template meta-programmign for Haskell" paper
+	--     original "Template meta-programming for Haskell" paper
 
 impLevel, topLevel :: ThLevel
 topLevel = 1	-- Things defined at top level of this module
@@ -463,7 +470,7 @@ data RefinementVisibility
 					-- GADT refinement
 
 instance Outputable TcTyThing where	-- Debugging only
-   ppr (AGlobal g)      = ppr g
+   ppr (AGlobal g)      = pprTyThing g
    ppr elt@(ATcId {})   = text "Identifier" <> 
 			  ifPprDebug (brackets (ppr (tct_id elt) <> dcolon <> ppr (tct_type elt) <> comma
 				 <+> ppr (tct_level elt) <+> ppr (tct_co elt)))
@@ -628,6 +635,17 @@ In addition to the basic Haskell variants of 'Inst's, they can now also
 represent implication constraints 'forall tvs. (reft, given) => wanted'
 and equality constraints 'co :: ty1 ~ ty2'.
 
+NB: Equalities occur in two flavours:
+
+  (1) Dict {tci_pred = EqPred ty1 ty2}
+  (2) EqInst {tci_left = ty1, tci_right = ty2, tci_co = coe}
+
+The former arises from equalities in contexts, whereas the latter is used
+whenever the type checker introduces an equality (e.g., during deferring
+unification).
+
+I am not convinced that this duplication is necessary or useful! -=chak
+
 \begin{code}
 data Inst
   = Dict {
@@ -643,7 +661,7 @@ data Inst
 				    -- Includes coercion variables
 				    --   mentioned in tci_reft
 	tci_reft   :: Refinement,
-	tci_given  :: [Inst],	    -- Only Dicts
+	tci_given  :: [Inst],	    -- Only Dicts and EqInsts
 				    --   (no Methods, LitInsts, ImplicInsts)
 	tci_wanted :: [Inst],	    -- Only Dicts and ImplicInsts
 				    --   (no Methods or LitInsts)
@@ -697,23 +715,22 @@ data Inst
 
   | EqInst {			  -- delayed unification of the form 
 				  --  	co :: ty1 ~ ty2
-	tci_left  :: TcType,      -- ty1
-	tci_right :: TcType,      -- ty2
+	tci_left  :: TcType,      -- ty1    -- both types are...
+	tci_right :: TcType,      -- ty2    -- ...free of boxes
 	tci_co    :: Either    	  -- co
-			TcTyVar	  --	a wanted equation, with a hole, to be 
-				  --      filled with a witness for the equality
-                                  --        for equation generated by the 
-                                  --        unifier, 'ty1' is the actual and
-                                  --        'ty2' the expected type
-			Coercion, --    a given equation, with a coercion
-				  --	  witnessing the equality
-				  --         a coercion that originates from a
-				  --         signature or a GADT is a CoVar, but
-                                  --         after normalisation of coercions,
-				  --         they can be arbitrary Coercions 
-                                  --         involving constructors and 
-                                  --         pseudo-constructors like sym and 
-                                  --         trans.
+			TcTyVar	  --  - a wanted equation, with a hole, to be 
+				  --    filled with a witness for the equality;
+                                  --    for equation arising from deferring
+                                  --    unification, 'ty1' is the actual and
+                                  --    'ty2' the expected type
+			Coercion, --  - a given equation, with a coercion
+				  --	witnessing the equality;
+				  --    a coercion that originates from a
+				  --    signature or a GADT is a CoVar, but
+                                  --    after normalisation of coercions, they
+				  --    can be arbitrary Coercions involving
+                                  --    constructors and pseudo-constructors 
+                                  --    like sym and trans.
 	tci_loc   :: InstLoc,
 
 	tci_name  :: Name	-- Debugging help only: this makes it easier to