Comments only

diff --git a/compiler/types/TyCon.lhs b/compiler/types/TyCon.lhs
index 9595759..144a670 100644 (file)
--- a/compiler/types/TyCon.lhs
+++ b/compiler/types/TyCon.lhs
@@ -267,19 +267,23 @@ visibleDataCons (NewTyCon{ data_con = c })    = [c]
 -- structure (ie, the class or family from which they derive) using a type of
 -- the following form.
 --
-data AlgTyConParent = -- An ordinary type constructor has no parent.
-                     NoParentTyCon
-
-                     -- Type constructors representing a class dictionary.
-                   | ClassTyCon    Class       
-
-                     -- Type constructors representing an instances of a type
-                     -- family.
-                   | FamilyTyCon   TyCon       -- the type family
-                                   [Type]      -- instance types
-                                   TyCon       -- a *coercion* identifying
-                                               -- the representation type
-                                               -- with the type instance
+data AlgTyConParent 
+  = NoParentTyCon      -- An ordinary type constructor has no parent.
+
+  | ClassTyCon         -- Type constructors representing a class dictionary.
+       Class   
+
+  | FamilyTyCon                -- Type constructors representing an instance of a type
+       TyCon           --   The type family
+       [Type]          --   Instance types
+       TyCon           --   A CoercionTyCon identifying the representation 
+                       --     type with the type instance family.  
+                       --      c.f. Note [Newtype coercions]
+       -- E.g.  data intance T [a] = ...
+       -- gives a representation tycon:
+       --      data T77 a = ...
+       --      axiom co a :: T [a] ~ T77 a
+       -- with T77's algTcParent = FamilyTyCon T [a] co
 
 data SynTyConRhs
   = OpenSynTyCon Kind  -- Type family: *result* kind given
@@ -316,7 +320,7 @@ and then when we used CoT at a particular type, s, we'd say
        CoT @ s
 which encodes as (TyConApp instCoercionTyCon [TyConApp CoT [], s])
 
-But in GHC we instead make CoT into a new piece of type syntax
+But in GHC we instead make CoT into a new piece of type syntax, CoercionTyCon,
 (like instCoercionTyCon, symCoercionTyCon etc), which must always
 be saturated, but which encodes as
        TyConApp CoT [s]