Comment on what an "enumeration" type is

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

diff --git a/compiler/types/TyCon.lhs b/compiler/types/TyCon.lhs
index e7f8507..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 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
     }
 
   -- | Information about those 'TyCon's derived from a @newtype@ declaration

@@ -580,6 +580,16 @@ data CoTyConDesc
   | CoUnsafe 
 \end{code}
 
   | 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)
 Note [Newtype coercions]
 ~~~~~~~~~~~~~~~~~~~~~~~~
 The NewTyCon field nt_co is a a TyCon (a coercion constructor in fact)

@@ -983,6 +993,7 @@ isGadtSyntaxTyCon _                                    = False
 
 -- | Is this an algebraic 'TyCon' which is just an enumeration of values?
 isEnumerationTyCon :: TyCon -> Bool
 
 -- | 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
 isEnumerationTyCon (AlgTyCon {algTcRhs = DataTyCon { is_enum = res }}) = res
 isEnumerationTyCon (TupleTyCon {tyConArity = arity}) = arity == 0
 isEnumerationTyCon _                                                   = False