I’m a bit skeptical of the difficulty ascribed to doing something like resumable conditions in a Java system.
You could have a per-thread singleton stack of handler objects. To raise a resumable condition you call a method on the stack that searches outward for an active handler to ask for a restart option and just returns it. If none is found an exception is thrown to unwind the call stack.
(Common Lisp has restartable conditions native to the language and this is essentially how it works. Conditions just don’t unwind the stack until no restart is available.)
Haskell monads can easily model other interesting control flow structures but this is basically just a syntax nicety of the do notation that lets you avoid writing nested lambdas to represent continuations. If you’re okay with heavy lambda nesting in Java you can do it all by just implementing whatever monad you like in Java—although the type system isn’t as glorious when it comes to higher-order polymorphism and inference.
You could have a per-thread singleton stack of handler objects. To raise a resumable condition you call a method on the stack that searches outward for an active handler to ask for a restart option and just returns it. If none is found an exception is thrown to unwind the call stack.
(Common Lisp has restartable conditions native to the language and this is essentially how it works. Conditions just don’t unwind the stack until no restart is available.)
Haskell monads can easily model other interesting control flow structures but this is basically just a syntax nicety of the do notation that lets you avoid writing nested lambdas to represent continuations. If you’re okay with heavy lambda nesting in Java you can do it all by just implementing whatever monad you like in Java—although the type system isn’t as glorious when it comes to higher-order polymorphism and inference.