When writing test for new 0 11 0 `toIO ` mentioned today in

Question

When writing test for new 0 11 0 `toIO ` mentioned today in < C5UPMM0A0|arrow> I saw that `attempt` is like ```fun attempt IO lt E Either lt Throwable A gt gt ``` While in Scala Cats there is ```val x IO Either Int String = IO Either Int String Left 1 val y IO Either Throwable Either Int String = x attempt``` ```def attempt IO Either Throwable A ``` Won t be more intuitive to type it like this Also in actual form I am unable to call `IO attempt unsafeRunSync ` because `unsafeRunSync` is like ```fun lt A gt IOOf lt Nothing A gt unsafeRunSync A```

Bob Glamm · Answer

That must be the new Bifunctor IO on 0 11 0 Arrow has `IO lt E A gt ` whereas cats effect has `IO A ` and `E` is implicitly fixed to `Throwable`

Bob Glamm · Answer

`IO lt E A gt attempt IO lt E Either lt Throwable A gt gt ` enables translation of `Throwable` into a custom error hierarchy `E`

Bob Glamm · Answer

No such translation can be done in cats effect

Bob Glamm · Answer

There is some debate I think as to whether the complexity of Bifunctor IO is justified over Functor IO

Hiosdra · Answer

No i did not meant that I meant that in cats `IO Either E A ` is nearly nested mapping the same as arrow 0 11 0 `IO lt E A gt ` Before BIO implementation allowed ` attempt unsafeRunSync ` chain but BIO does not And I wish why `attempt` result goes to right value of IO but not to error side Wouldn t it be less confusing to `IO lt E A gt attemt IO lt Throwable Either lt E A gt gt `

Bob Glamm · Answer

I have to think that `IO lt E A gt gt IO lt Throwable Either lt E A gt gt ` breaks some monad laws but I m not enough of a category theorist to say for sure

Jannis · Answer

No laws apply here so it does not matter It s purely a style choice Handling the inner `IO` s failure case is by far `IO lt E A gt ` s weakest point It has two valid `MonadError` instances one for `Throwable` and one for `E` and both have valid use cases The instance for `Throwable` is especially important when catching unhandled errors which will eventually appear The instance for `E` is perfect for handling domain errors and expected error cases So anyway I like the idea of going `IO lt E A gt attempt IO lt Throwable Either lt E A gt gt ` it neatly makes the unexpected unhandled error visible but it still requires multiple steps to get to the `Throwable` But since that is probably expected anyway shrug