Could it be a bug in type inference? Works fine wi...

# announcements

[ghedeon]

Could it be a bug in type inference? Works fine without if but fails otherwise.

Type mismatch error:

sealed class Foo

object Valid : Foo()
object Empty : Foo()

fun doSomething1(): Result<Foo> {
    return Result.failure<Valid>(Exception()).recover { if (true) Empty else Valid }
}

Works fine:

fun doSomething2(): Result<Foo> {
    return Result.failure<Valid>(Exception()).recover { Empty }
}

[StavFX]

Because you tell Result.failure that you will return Valid, but then you return Empty .
This should work (failure<Valid> replaced with failure<Foo>)

fun doSomething1(): Result<Foo> {
      return Result.failure<Foo>(Exception())
         .recover { if (true) Empty else Valid }
   }

[StavFX]

Oh, I see what you mean, why does doSomething2 know how to handle this properly? Not sure actually.. 😅
Somehow it knows to convert to Result<Foo> /shrug

[ghedeon]

yeah, seems to be a bug, because it also works, if you assign the if expression to a variable. It just fails to do it on the fly.

[Mike]

Empty is likely an object of Result<Nothing> and so returning a Result<Nothing> is ok for Result<Foo>.

If you return a Valid, then it's a Result<Valid> rather than a Result<Foo> and Result<Valid> doesn't work.

I don't think it's strictly a failing. I also suspect you could do return Result.failure(Exception())... and the compiler would infer the return type correctly.

[StavFX]

I still think there’s a failure here. There’s no reason this should fail

fun doSomething1(): Result<Foo> {
   return Result.failure<Valid>(Exception())
      .recover {
         val temp = if (true) Empty else Valid
         return@recover temp
      }
}

If this works (just removed return@recover)

fun doSomething1(): Result<Foo> {
   return Result.failure<Valid>(Exception())
      .recover {
         val temp = if (true) Empty else Valid
         temp
      }
}

[ghedeon]

Filled in an issue on youtrack. Btw, seems to be fixed in the new type inference, enabled by -XXLanguage:+NewInference.
https://youtrack.jetbrains.com/issue/KT-35514