Hey all. I have a problem I’m stumped by. I have a block of code that I invoke once, but it runs tw...

# arrow

[Erik Dreyer]

Hey all.

I have a problem I’m stumped by. I have a block of code that I invoke once, but it runs twice.
If anyone has some guidance/intuition/experience with a similar problem, I’d love the assist.

Code is here:

suspend fun <E: Event> dispatch(command: Command): Effect<WorkflowError, E> {
    log.debug("BEFORE EFFECT")
    return effect {
      log.debug("START EFFECT")
      ensureNotNull(commandHandlers[command::class]) {
        MissingHandler("No handler for command $command")
      }.let {
        it.invoke(command) as E
      }
    }
  }

The signature of the invoke method, if this helps:

context(EffectScope<WorkflowError>)
  final override suspend fun invoke(request: R): E {

Log output:

2023-03-22T21:07:07.106-04:00 DEBUG 38879 --- [-5 @coroutine#7] i.l.common.workflow.WorkflowDispatcher   : BEFORE EFFECT
2023-03-22T21:07:07.107-04:00 DEBUG 38879 --- [-5 @coroutine#7] i.l.common.workflow.WorkflowDispatcher   : START EFFECT
...
2023-03-22T21:07:07.175-04:00 DEBUG 38879 --- [-3 @coroutine#7] i.l.common.workflow.WorkflowDispatcher   : START EFFECT

[Youssef Shoaib [MOD]]

Where is the code that consumes the effect though? I think you might be invoking the effect itself twice

[Erik Dreyer]

Oh interesting. I’ll investigate that. Thx for the insight

[Erik Dreyer]

OK, so I refactored a return type from Either to Effect and a side effect (no pun intented) of that is that fold was being called twice. For Either this is fine, but for Effect it runs the process twice.

[Youssef Shoaib [MOD]]

Oh yeah, that makes sense. Effect "basically" returns an Either, and so what happened is that you were recomputing that either twice, thus producing side effects.

[Erik Dreyer]

Thx for the assist!