Is there any way how to perform a non local return from a la

Question

Is there any way how to perform a non local return from a lambda passed to a 3rd party function E g ```TransactionTemplate this execute return ``` If not what are my options slightly smiling face Also can anyone explain why that s not technically possible

Sam · Answer

Non local returns are only possible in inline lambdas where the compiler know exactly when and where the lambda body is executed

Sam · Answer

From the compiler s point of view a non inline lambda could end up being executed anywhere even in a different call stack

Sam · Answer

As far as what options you have it depends on what exactly you re trying to achieve Throwing and catching a custom exception while somewhat gross might be the closest you can get to the desired behaviour

Sam · Answer

e g ```try TransactionTemplate this execute throw MySpecialException catch e MySpecialException return ``` I say gross because it has some performance impact and it can make the control flow hard to follow I don t normally like using exceptions for control flow

Joffrey · Answer

< Sam> s solution indeed works as a general way to deal with this if you know the lambda is called in place because as he mentioned too that s not necessarily the case However there are probably better ways depending on the exact function you re using how it s calling the lambda and what you re putting in that lambda In this specific case the special exception approach will most likely NOT be what you want because any exception in this lambda would imply rolling back the transaction as per the documentation of `execute ` One possibly much simpler way would be to use a local return in the lambda Could you please provide a more realistic code and explain what you re trying to do with it here

David Kubecka · Answer

Yeah I can imagine that in concrete cases one can always come up with a tailor made solution workaround as did I My case is basically in transaction lambda collect some objects if there are any objects do something else in txn otherwise stop the processing return return locallly the objects outside transaction do something with collected objects So I ve simply repeated the isObjectsEmpty check outside of the transaction and returned from there

ephemient · Answer

exceptions for control flow can also fail if it s not running in the same stack or not rethrown as is

ephemient · Answer

Scala used to magically translate non local returns into `throw scala runtime NonLocalReturnException` but that s definitely not something Kotlin should emulate

David Kubecka · Answer

Btw what are typical scenarios when the lambda runs on a different stack I can think of threading Anything else

Sam · Answer

+1 asynchronous callbacks on a separate thread or coroutine are the typical example You could also have a callback that s executed higher up the current call stack e g if you have a function that creates and returns a lambda

deive · Answer

If you know that what you run in execute never returns a null then why not use a ` `

ephemient · Answer

I got two threads mixed up

deive · Answer

I think the mixup was mine flushed sorry