-- (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
-- | 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:
--
-- | 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
| 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)
-- | 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