-- because the latter is part of a knot, whereas the former is not.
mkNewTyConRhs tycon_name tycon con
= do { co_tycon_name <- newImplicitBinder tycon_name mkNewTyCoOcc
- ; let co_tycon = mkNewTypeCoercion co_tycon_name tycon etad_rhs
+ ; let co_tycon = mkNewTypeCoercion co_tycon_name tycon etad_tvs etad_rhs
cocon_maybe | all_coercions || isRecursiveTyCon tycon
= Just co_tycon
| otherwise
= Nothing
; return (NewTyCon { data_con = con,
nt_rhs = rhs_ty,
- nt_etad_rhs = etad_rhs,
- nt_co = cocon_maybe,
+ nt_etad_rhs = (etad_tvs, etad_rhs),
+ nt_co = cocon_maybe,
-- Coreview looks through newtypes with a Nothing
-- for nt_co, or uses explicit coercions otherwise
nt_rep = mkNewTyConRep tycon rhs_ty }) }
-- Instantiate the data con with the
-- type variables from the tycon
- etad_rhs :: ([TyVar], Type)
- etad_rhs = eta_reduce (reverse tvs) rhs_ty
-
+ etad_tvs :: [TyVar] -- Matched lazily, so that mkNewTypeCoercion can
+ etad_rhs :: Type -- return a TyCon without pulling on rhs_ty
+ -- See Note [Tricky iface loop] in LoadIface
+ (etad_tvs, etad_rhs) = eta_reduce (reverse tvs) rhs_ty
+
eta_reduce :: [TyVar] -- Reversed
-> Type -- Rhs type
-> ([TyVar], Type) -- Eta-reduced version (tyvars in normal order)
; let { clas_kind = mkArrowKinds (map tyVarKind tvs) liftedTypeKind
; tycon = mkClassTyCon tycon_name clas_kind tvs
- rhs rec_clas tc_isrec
+ rhs rec_clas tc_isrec
-- A class can be recursive, and in the case of newtypes
-- this matters. For example
-- class C a where { op :: C b => a -> b -> Int }