-- (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
| 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.
+
+Moreover, all the data constructor must be enumerations, meaning
+they have type (forall abc. T a b c). GADTs are not enumerations.
+For example consider
+ data T a where
+ T1 :: T Int
+ T2 :: T Bool
+ T3 :: T a
+What would [T1 ..] be? [T1,T3] :: T Int? Easiest thing is to exclude them.
+See Trac #4528.
+
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