# Lambda DSLs and infinite loops

**URL:** <https://discuss.kotlinlang.org/t/lambda-dsls-and-infinite-loops/17739>\
**Category:** Support\
**Created:** [May 18, 2020, 10:57am UTC](https://discuss.kotlinlang.org/t/lambda-dsls-and-infinite-loops/17739 "2020-05-18T10:57:00Z")\
**Posts on this page:** 3\
**Page:** 1

<div class="post-metadata">

**Author:** ![1blustone](https://sea1.discourse-cdn.com/flex019/user_avatar/discuss.kotlinlang.org/1blustone/32/6885_2.png) [@1blustone](https://discuss.kotlinlang.org/u/1blustone)\
**Post date:** [May 18, 2020, 10:57am UTC](https://discuss.kotlinlang.org/t/lambda-dsls-and-infinite-loops/17739/1 "2020-05-18T10:57:00Z")

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I’m not sure if this is a bug, but it’s quite odd.

If I do

```kotlin
fun foo(): Int {
    while (true) {
        if (doSomeWork()) return 42
    }
}

```

this compiles fine, because when (if) this function returns, it returns an int.

However if I try to do this in a lambda body

```kotlin
fun foo(): Int {
    return run<Int> {
        while (true) {
            if (doSomeWork()) return@run 42
        } // Type mismatch. Required: Int; Found: Unit
    }
}

```

the compiler complains that the body returns `Unit` instead of `Int`.

Throwing an exception after this loop fixes the problem, but the compiler will now (rightfully) complain that the exception is never thrown… which implies that it should’ve compiled in the first place.

```kotlin
fun foo(): Int {
    return run<Int> {
        while (true) {
            if (doSomeWork()) return@run 42
        }
        throw IllegalStateException() // Unreachable code; Throwable instance 'IllegalStateException' is not thrown 
    }
}

```

This is highly simplified; I’m actually running into this problem with coroutine builders, but the issue is the same.

(Kotlin 1.3.72)

---

<div class="post-metadata">

**Author:** ![tieskedh](https://sea1.discourse-cdn.com/flex019/user_avatar/discuss.kotlinlang.org/tieskedh/32/2816_2.png) [@tieskedh](https://discuss.kotlinlang.org/u/tieskedh)\
**Post date:** [May 18, 2020, 2:33pm UTC](https://discuss.kotlinlang.org/t/lambda-dsls-and-infinite-loops/17739/2 "2020-05-18T14:33:41Z")

</div>

ah, kotlin doesn’t realise `while true` doesn’t return…  
But, we can create or own functions…

If you want to say that something doesn’t return, you use `Nothing`.  
Therefor, you can create a function `runForever`

```
inline fun runForEver(
    lamb: ()->Unit
) : Nothing {
    while(true) lamb()
}

fun foo(): Int {
    return run {
        var i = 0;
        runForever{
            if (i == 10) return@run i
            i++
        }
    }
}
fun main() = println(foo()) // this prints 10

```

This is an oportunity for the type inference to improve a bit 🙂

Found the topic on youtrack: [https://youtrack.jetbrains.com/issue/KT-27970](https://youtrack.jetbrains.com/issue/KT-27970)

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<div class="post-metadata">

**Author:** ![1blustone](https://sea1.discourse-cdn.com/flex019/user_avatar/discuss.kotlinlang.org/1blustone/32/6885_2.png) [@1blustone](https://discuss.kotlinlang.org/u/1blustone)\
**Post date:** [May 18, 2020, 3:25pm UTC](https://discuss.kotlinlang.org/t/lambda-dsls-and-infinite-loops/17739/3 "2020-05-18T15:25:44Z")

</div>

I would agree, but the thing is, it _does_ realize that it never returns, because it works fine when not in a lambda expression as mentioned in the linked issue.

I guess I’ll refactor this into an extra function as mentioned for now.