X-Git-Url: http://git.megacz.com/?p=ghc-hetmet.git;a=blobdiff_plain;f=compiler%2Ftypecheck%2FTcRnTypes.lhs;h=2ea26a82457f387aa4efa71611a625c8fbdfec4c;hp=6c8d648c50d928cffc067d93dbe24f2e3b6ad6f1;hb=6d2b0ae3ae3296cb6cdd496cbf85b897c7ce150b;hpb=17b297d97d327620ed6bfab942f8992b2446f1bf

diff --git a/compiler/typecheck/TcRnTypes.lhs b/compiler/typecheck/TcRnTypes.lhs
index 6c8d648..2ea26a8 100644
--- a/compiler/typecheck/TcRnTypes.lhs
+++ b/compiler/typecheck/TcRnTypes.lhs
@@ -3,11 +3,11 @@
 % (c) The GRASP Project, Glasgow University, 1992-2002
 %
 \begin{code}
-{-# OPTIONS_GHC -w #-}
+{-# 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/WorkingConventions#Warnings
+--     http://hackage.haskell.org/trac/ghc/wiki/Commentary/CodingStyle#Warnings
 -- for details
 
 module TcRnTypes(
@@ -470,7 +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)))
@@ -635,6 +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 {
@@ -704,23 +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