- = do { -- In case of a type instance, we need to invent a new name for the
- -- instance type, as `tc_name' is the family name.
- ; uniq <- newUnique
- ; (final_name, parent) <-
- case mb_family of
- Nothing -> return (tc_name, NoParentTyCon)
- Just family ->
- do { final_name <- newImplicitBinder tc_name (mkLocalOcc uniq)
- ; return (final_name, FamilyTyCon family)
- }
- ; let { tycon = mkAlgTyCon final_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 }
+ = 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
+ { (final_name, parent) <- maybeComputeFamilyInfo mb_family tycon_rec
+ ; let { tycon = mkAlgTyCon final_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 have to perform three extra tasks:
+ --
+ -- (1) The instance tycon (representing the family at a particular type
+ -- instance) need to get a new, derived name - we may not reuse the
+ -- family name.
+ -- (2) 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.
+ -- (3) Produce a `AlgTyConParent' value containing the parent and coercion
+ -- information.
+ --
+ maybeComputeFamilyInfo Nothing rep_tycon =
+ return (tc_name, NoParentTyCon)
+ maybeComputeFamilyInfo (Just (family, instTys)) rep_tycon =
+ do { -- (1) New, derived name for the instance tycon
+ ; uniq <- newUnique
+ ; final_name <- newImplicitBinder tc_name (mkInstTyTcOcc uniq)
+
+ -- (2) Create the coercion.
+ ; co_tycon_name <- newImplicitBinder tc_name (mkInstTyCoOcc uniq)
+ ; let co_tycon = mkDataInstCoercion co_tycon_name tvs
+ family instTys rep_tycon
+
+ -- (3) Produce parent information.
+ ; return (final_name, FamilyTyCon family instTys co_tycon)
+ }
+