Hi! Why is type inference failing here? To my eye...

# compiler

[dimsuz]

Hi!

Why is type inference failing here? To my eyes it has all the info it needs to infer the type...

class Builder {
  fun <R> build(action: (R) -> R): Builder = this
}

fun main() {
  val value = 3
  Builder()
    .build { value } // compiler error: not enough information to infer type variable R
}

That is: lambda is returning Int therefore R is Int

UPD: this is kotlin 1.5.0

[Ivan Krylov]

Because “value” is not explicitly “Int”, it can be at least a Double or Float as well

[dimsuz]

no, it's Int. that's kotlin's default, it has no ambiguity in this case. If I add val value: Int = 3 it highlights : Int as "redundant" and the error still stays. Even if I replace int with string, or whatever other type, it stays, so that's not it.

[russhwolf]

I’m guessing it’s because there’s not enough info to infer the input parameter of the lambda. It seems to work if you change action to be () -> R or if you do Builder().build { _: Int -> value }

[dimsuz]

Indeed! But I wonder why is it so? I've reduced it to a simple function:

fun <R> build(action: (R) -> R) = Unit

fun main() {
  val value: Int = 3
  build { value } // not enough information to infer type variable R
}

But here's an equivalent Haskell program and it compiles and runs perfectly. So it seems like a type inference limitation...

build :: (a -> a) -> ()
build action = ()

main = do
  let value = 3
  return $ build (\_ -> value)

[ephemient]

Kotlin has more type inference than Java but not as much as a full H-M system like Haskell (none of which work freely with polymorphism, so don't expect that)

[lhwdev]

It seems .build<Int> { value } looks cleaner to me.
There is BuilderInference, but I don't know if it can be applied here; afaik it infers type from value arguments.

[ephemient]

for this case, yes. .build { _: Int -> value } is sometimes preferable if there are multiple type parameters and you only need to hint a single one (because <> forces you to write all of them)

[dimsuz]

I wonder can this be done at all in kotlin compiler, maybe I should file a feature request.

[ephemient]

I think you're asking for https://youtrack.jetbrains.com/issue/KT-8138
although as a QoL improvement I'd like https://youtrack.jetbrains.com/issue/KT-13394 too

[dimsuz]

Thanks, subscribed! I'll add my case to the second one.

However. No, it doesn't look exactly like the second one. Will try to report a new one if I won't find existing issue.