try package:polysemy

try :: Member (Error e) r => Sem r a -> Sem r (Either e a)
polysemy Polysemy.Error
Similar to catch, but returns an Either result which is (Right a) if no exception of type e was thrown, or (Left ex) if an exception of type e was thrown and its value is ex.
tryJust :: Member (Error e) r => (e -> Maybe b) -> Sem r a -> Sem r (Either b a)
polysemy Polysemy.Error
A variant of try that takes an exception predicate to select which exceptions are caught (c.f. catchJust). If the exception does not match the predicate, it is re-thrown.
tryMembership :: forall e r . (Typeable e, KnownRow r) => Maybe (ElemOf e r)
polysemy Polysemy.Internal.Union Polysemy.Membership
Extracts a proof that e is an element of r if that is indeed the case; otherwise returns Nothing.