"return is not allowed here" when returning inside a kotlin lambda

Question

I use a lambda to handle callbacks from an asynchronous call. I'd like to define the callback outside the calling method to avoid a bulky method, but I can't seem to use early returns within the lambda, which makes the code unnecessarily difficult to read.

I've tried defining the lambda as a variable but returns are not viable inside the lambda.

I've tried defining the lambda inside a function and returning but returns were not viable there either.

For example:

 private fun onDataUpdated(): (Resource<List<Int>>) -> Unit =  {
   if (it.data.isNullOrEmpty()) {          
     // Handle no data callback and return early.
     return@onDataUpdated // This is not allowed   
    }

   // Handle the data update
   }
 }

I've also tried:

 private val onDataUpdated: (Resource<List<Int>>) -> Unit =  {
   if (it.data.isNullOrEmpty()) {          
     // Handle no data callback and return early.
     return // This is not allowed   
    }

   // Handle the data update
   }
 }

I'd like to perform an early return instead of using an else case, to avoid unnecessary indent, but I can't seem to find a way to use returns inside a lambda.

See my answer here: stackoverflow.com/a/74740489/9585130

Binh Ho
– Binh Ho

2022-12-09 08:24:54 +00:00
Commented Dec 9, 2022 at 8:24 — Binh Ho
– Binh Ho, Commented Dec 9, 2022 at 8:24

Julio E. Rodríguez Cabañas · Accepted Answer · 2019-04-29 16:19:32Z

10

You can achieve this by labelling the lambda. For example, if you label it with dataUpdated:

private val onDataUpdated: (Resource<List<Int>>) -> Unit = dataUpdated@ {
    if (it.data.isNullOrEmpty()) {
        // Handle no data callback and return early.
        return@dataUpdated
    }

    // Handle the data update
}

answered Apr 29, 2019 at 16:19

Julio E. Rodríguez Cabañas

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Comments

Dogcatfee · Accepted Answer · 2020-04-20 04:36:19Z

1

Usually you'll just put the "return" at the bottom of the lambda. Here's an example using Dispatchers.IO:

var res = async(Dispatchers.IO){
   var text : String? = null
   if(channel==null) {
      text = networkRequest(url)
   }
   text
}.await()

answered Apr 20, 2020 at 4:36

Dogcatfee

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