I am confused as to why `Either.catch()` requires ...

# arrow

[EarthCitizen]

I am confused as to why Either.catch() requires a suspend function

[raulraja]

suspend denotes an effect and exceptions are effects. If they don't run within suspend they will blow up uncontrolled but when they are suspend there is always a continuation that is responsible for error handling in that context.

[EarthCitizen]

So what is the proper way to use Either.catch()?

[raulraja]

The function surrounding it should also be suspend

[raulraja]

You can manually create a function though without suspend that uses try/catch and lifts to left and right before it exits

[raulraja]

If you want an unsafe version of it

[EarthCitizen]

So MonadError.catch() is unsafe?

[EarthCitizen]

If I surround Either.catch() with runBlocking { } is that safe?

[raulraja]

Yes it's unsafe and deprecated because of that https://arrow-kt.io/docs/apidocs/arrow-core-data/arrow.typeclasses/-applicative-error/catch.html

[raulraja]

There is effectCatch that is the suspend version

[raulraja]

If you chain suspend is safe until the point you do runBlocking or similar

[EarthCitizen]

Ok thanks

[raulraja]

The unsafe part is not so much related to the exception potentially blowing up as it is that any effect in the function will run

[raulraja]

Because Either is strict and effects run before they can be placed in left or right

[raulraja]

For lazy evaluation and composition we recommend IO which is pure across the board even with concurrency and async jumps and manages errors with guarantees