typelevel/cats-effect

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Gavin Bisesi

@Daenyth

whatever you flatMap after that does whatever you tell it

Arnau Abella

@monadplus

I was thinking it the other way :O

Gavin Bisesi

@Daenyth

I'm fairly certain an implementation that would make your example above handle boom would need to end up violating some applicative/functor laws somewhere along the way

I don't feel like writing that all out :P

but anyway - it doesn't work like that

Fabio Labella

@SystemFw

you can actually see why the behaviour your seek is impossible as soon as you have things that don't return Unit

def ints: IO[Int] = 3.pure[IO].handleErrorWith(e => 4.pure[IO])
  // this is handleError
def string: IO[String] = (ints <* boom).map(_.toString) 
  // if handleErrorWith worked here you'd try to return an Int instead of a String

if handleErrorWith installed a handler which is valid for all IOs flatMapped after this, which type should it return?

it can only be IO[Nothing], which can only be something that never terminates (IO cannot emit zero values)

Arnau Abella

@monadplus

Makes sense, the signature also gives some hints avoit it

Fabio Labella

@SystemFw

the behaviour which is closest to what you seek is the release action in Stream or Resource

Arnau Abella

@monadplus

I'll have a look at them tomorrow

Then, what is the purpose of handleErrorWith ?

Running a piece of code that is suspicious of throwing errors ?

Fabio Labella

@SystemFw

handling the errors of the current IO

it's the equivalent of catch

e.g. imagine wanting to return different http responses on success or failure of an operation

or perhaps wanting to retry something when there's an error

like, I don't see why this model is confusing

you have

IO.unit.handleError(...) >> boom, which doesn't handle errors in boom

just like

try {
  thing
} catch {
   ...
}

boom

doesn't handle errors in boom

Arnau Abella

@monadplus

It's not confusing now at all

Fabio Labella

@SystemFw

ah, nice ;)

Arnau Abella

@monadplus

but I couldn't understand it the first time :S

so thanks

Fabio Labella

@SystemFw

heh, that's ok

Arnau Abella

@monadplus

How do you handle situation where you fork then cancelled (e.g. the resource is not available) but there is a join afterwards ?

Fabio Labella

@SystemFw

you are going to have to be a bit more specific

that use of the word "cancelled" doesn't necessarily map to what I would think in the context of a fork

i.e. it's unclear if you are talking about some logical notion of "I don't want to do this anymore", or actual fiber interruption

Arnau Abella

@monadplus

Fiber interruption (e.g. Fiber.cancel)

I mean I dont want to do that anymore as .cancel

Fabio Labella

@SystemFw

who cancels? the thing that would do the join otherwise or something else?

Arnau Abella

@monadplus

The thread you forked from with .start

Fabio Labella

@SystemFw

and who joins?

I think it would be better if you could sketch an actual example. This can be tricky so it's easier that way

Arnau Abella

@monadplus

  (for {
    fiber <- readFile(new File("data/lore.txt")).start
    _     <- IO.sleep(100.millis) *> fiber.cancel
    lore  <- fiber.join // blocks
  } yield lore).map { println(_) }.unsafeRunSync()

@SystemFw

Fabio Labella

@SystemFw

well, that's kinda contrived given that this specific control flow doesn't really make a lot of sense, you alwasy join after cancel has happened

there are three things I'd say

if you just want to put a timeout on something

you should just readFile.timeout(5.seconds) (assuming that your readfile implementation is cancelable)

number 2 would be to only call .join if you haven't called cancel. In the simplest case this is an if, in more complicated cases you might need some coordination between the interrupter and the joiner

number 3 would be to put the result in a Ref + Deferred thing (in this case a simple Deferred), which has some form of default, this simplest case of which would be Deferred[Option[Thing]]

then you need to bracketCase or guaranteeCase the started operation so that it puts None on that Deferred when it gets cancelled

thoughts?

Arnau Abella

@monadplus

All three points are clear

nothing to add

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