-buildAlgTyCon tc_name tvs stupid_theta rhs arg_vrcs is_rec want_generics gadt_syn
- = do { let { tycon = mkAlgTyCon tc_name kind tvs arg_vrcs stupid_theta
- rhs fields is_rec want_generics gadt_syn
- ; kind = mkArrowKinds (map tyVarKind tvs) liftedTypeKind
- ; fields = mkTyConSelIds tycon rhs
- }
- ; return tycon }
+buildAlgTyCon tc_name tvs stupid_theta rhs is_rec want_generics gadt_syn
+ mb_family
+ = do { -- We need to tie a knot as the coercion of a data instance depends
+ -- on the instance representation tycon and vice versa.
+ ; tycon <- fixM (\ tycon_rec -> do
+ { parent <- parentInfo mb_family tycon_rec
+ ; let { tycon = mkAlgTyCon tc_name kind tvs stupid_theta rhs
+ fields parent is_rec want_generics gadt_syn
+ ; kind = mkArrowKinds (map tyVarKind tvs) liftedTypeKind
+ ; fields = mkTyConSelIds tycon rhs
+ }
+ ; return tycon
+ })
+ ; return tycon
+ }
+ where
+ -- If a family tycon with instance types is given, the current tycon is an
+ -- instance of that family and we need to
+ --
+ -- (1) create a coercion that identifies the family instance type and the
+ -- representation type from Step (1); ie, it is of the form
+ -- `Co tvs :: F ts :=: R tvs', where `Co' is the name of the coercion,
+ -- `F' the family tycon and `R' the (derived) representation tycon,
+ -- and
+ -- (2) produce a `AlgTyConParent' value containing the parent and coercion
+ -- information.
+ --
+ parentInfo Nothing rep_tycon =
+ return NoParentTyCon
+ parentInfo (Just (family, instTys)) rep_tycon =
+ do { -- Create the coercion
+ ; co_tycon_name <- newImplicitBinder tc_name mkInstTyCoOcc
+ ; let co_tycon = mkDataInstCoercion co_tycon_name tvs
+ family instTys rep_tycon
+ ; return $ FamilyTyCon family instTys co_tycon
+ }
+