Add an extension for GHC's layout-rule relaxations

[ghc-hetmet.git] / compiler / types / TyCon.lhs

diff --git a/compiler/types/TyCon.lhs b/compiler/types/TyCon.lhs
index b552c24..2350973 100644 (file)
--- a/compiler/types/TyCon.lhs
+++ b/compiler/types/TyCon.lhs
@@ -440,7 +440,7 @@ data AlgTyConRhs
                          --      (see the tag assignment in DataCon.mkDataCon)
 
        is_enum :: Bool   -- ^ Cached value: is this an enumeration type? 
-                          --   (See 'isEnumerationTyCon')
+                          --   See Note [Enumeration types]
     }
 
   -- | Information about those 'TyCon's derived from a @newtype@ declaration
@@ -500,7 +500,24 @@ data TyConParent
 
   -- | An *associated* type of a class.  
   | AssocFamilyTyCon   
-        Class                  -- The class in whose declaration the family is declared
+        Class          -- The class in whose declaration the family is declared
+                        -- The 'tyConTyVars' of this 'TyCon' may mention some
+                        -- of the same type variables as the classTyVars of the
+                        -- parent 'Class'.  E.g.
+                        --
+                        -- @
+                        --    class C a b where
+                        --      data T c a
+                        -- @
+                        --
+                        -- Here the 'a' is shared with the 'Class', and that is
+                        -- important. In an instance declaration we expect the
+                        -- two to be instantiated the same way.  Eg.
+                        --
+                        -- @
+                        --    instanc C [x] (Tree y) where
+                        --      data T c [x] = T1 x | T2 c
+                        -- @
 
   -- | Type constructors representing an instance of a type family. Parameters:
   --
@@ -539,12 +556,14 @@ isNoParent _             = False
 
 -- | Information pertaining to the expansion of a type synonym (@type@)
 data SynTyConRhs
-  = SynonymTyCon      -- ^ An ordinary type synony
-       Type          -- ^ The rhs, which mentions head type variables. It acts as a
-                     -- template for the expansion when the 'TyCon' is applied to some
-                     -- types.
+  = -- | An ordinary type synonyn.
+    SynonymTyCon      
+       Type          -- This 'Type' is the rhs, and may mention from 'tyConTyVars'. 
+                     -- It acts as a template for the expansion when the 'TyCon' 
+                      -- is applied to some types.
 
-   | SynFamilyTyCon   -- A type synonym family  e.g. type family F x y :: * -> *
+   -- | A type synonym family  e.g. @type family F x y :: * -> *@
+   | SynFamilyTyCon
 
 --------------------
 data CoTyConDesc
@@ -561,6 +580,16 @@ data CoTyConDesc
   | CoUnsafe 
 \end{code}
 
+Note [Enumeration types]
+~~~~~~~~~~~~~~~~~~~~~~~~
+We define datatypes with no constructors to not be
+enumerations; this fixes trac #2578,  Otherwise we
+end up generating an empty table for
+  <mod>_<type>_closure_tbl
+which is used by tagToEnum# to map Int# to constructors
+in an enumeration. The empty table apparently upset
+the linker.
+
 Note [Newtype coercions]
 ~~~~~~~~~~~~~~~~~~~~~~~~
 The NewTyCon field nt_co is a a TyCon (a coercion constructor in fact)
@@ -964,6 +993,7 @@ isGadtSyntaxTyCon _                                    = False
 
 -- | Is this an algebraic 'TyCon' which is just an enumeration of values?
 isEnumerationTyCon :: TyCon -> Bool
+-- See Note [Enumeration types] in TyCon
 isEnumerationTyCon (AlgTyCon {algTcRhs = DataTyCon { is_enum = res }}) = res
 isEnumerationTyCon (TupleTyCon {tyConArity = arity}) = arity == 0
 isEnumerationTyCon _                                                   = False