+ -- [] means "no information, assume the worst"
+
+data AlgTyConRhs
+ = AbstractTyCon -- We know nothing about this data type, except
+ -- that it's represented by a pointer
+ -- Used when we export a data type abstractly into
+ -- an hi file
+
+ | DataTyCon {
+ data_cons :: [DataCon],
+ -- The constructors; can be empty if the user declares
+ -- the type to have no constructors
+ -- INVARIANT: Kept in order of increasing tag
+ -- (see the tag assignment in DataCon.mkDataCon)
+ is_enum :: Bool -- Cached: True <=> an enumeration type
+ } -- Includes data types with no constructors.
+
+ | NewTyCon {
+ data_con :: DataCon, -- The unique constructor; it has no existentials
+
+ nt_rhs :: Type, -- Cached: the argument type of the constructor
+ -- = the representation type of the tycon
+
+ nt_etad_rhs :: ([TyVar], Type) ,
+ -- The same again, but this time eta-reduced
+ -- hence the [TyVar] which may be shorter than the declared
+ -- arity of the TyCon. See Note [Newtype eta]
+
+ nt_rep :: Type -- Cached: the *ultimate* representation type
+ -- By 'ultimate' I mean that the top-level constructor
+ -- of the rep type is not itself a newtype or type synonym.
+ -- The rep type isn't entirely simple:
+ -- for a recursive newtype we pick () as the rep type
+ -- newtype T = MkT T
+ --
+ -- This one does not need to be eta reduced; hence its
+ -- free type variables are conveniently tyConTyVars
+ -- Thus:
+ -- newtype T a = MkT [(a,Int)]
+ -- The rep type is [(a,Int)]
+ -- NB: the rep type isn't necessarily the original RHS of the
+ -- newtype decl, because the rep type looks through other
+ } -- newtypes.
+
+visibleDataCons :: AlgTyConRhs -> [DataCon]
+visibleDataCons AbstractTyCon = []
+visibleDataCons (DataTyCon{ data_cons = cs }) = cs
+visibleDataCons (NewTyCon{ data_con = c }) = [c]
+\end{code}