Hi. I have the following code

val version = service.getAllTemplates()
  .find { ... }
  ?.versions // returns a list
  ?.single()
  ?: error("no template found matching the given predicate")

I’m using the

find{}

on purpose so I could handle its failure with a proper error message, I want to do the same with

single()

. I want to throw an exception with my own error message if the list contains more than one element. I tried

singleOrNull()

but still can’t provide a separate error message for it. Looks like the last

error(…)

handles both nulls.

You could add an extension method that calls

singleOrNull()

and handles the error. Or possibly add the first

error

after

find

with some parentheses.

Or possibly add the first

error

after

find

with some parentheses.

What do you mean by that? Can’t understand how to make it 🤔

You could try getOrElse()

val version = (service.getAllTemplates().find { ... } ?: error("no template found matching that given predicate")
  .versions // returns a list
  .singleOrNull()
  ?: error("no template found matching the given predicate")

But I would probably go with

val template = service.getAllTemplates().find { } ?: error("no template found matching that given predicate")
val version = template
  .versions // returns a list
  .singleOrNull()
  ?: error("no template found matching the given predicate")

Something like that, I haven't tested it.

getOrElse

is probably also a useful solution.

IMO the clearest thing to do would be to split out the separate error cases, e.g.

val template = service.getAllTemplates().find { ... }
    ?: error("no template ...")
val version = template.versions.singleOrNull()
    ?: error("...")

Thank you all! I think splitting is a good option. I will go with it