I want to handle exceptions in a `map` lambda by s...

# functional

[javaru]

I want to handle exceptions in a map lambda by simply not mapping a value for when the exception happens? I"m currently adding null to the stream, then filtering out the nulls at the end. Is this the best and most idomatic way to handle this case?

fun parseList(strings: Collection<String>): List<Foo>
{
    return strings.stream().map { 
        try {
            parse(it)
        } catch (e: Exception) {
            null
        }
    }.toList().filterNotNull()
}

[Shawn]

probably just

return strings.mapNotNull {
    try {
        parse(it)
    } catch (ignored: Exception) {
        null
    }
}

You don’t need to call stream() and mapNotNull {} will simplify the whole toList/filter bit

[Shawn]

optimally you’d want parse() to return R? to get rid of the need to try/catch at this call site but if you don’t own it I think this is about as well as you can do

[Shawn]

well, at least, as well as you can do without introducing Try<T> or a similar kind of data type

[javaru]

Thanks @Shawn - I had not seen the mapNotNull function. That seems better. (I'm still getting a handle on Functional programming after 25 years in OOP land)
And yeah, I don't own the parse method, so I need to deal with the exception.

[enhan]

I'd do something like this :

val l = listOf("1", "e", "42")
l.flatMap { s -> try {
     listOf(Integer.decode(s))
   } catch(e: Exception){
      emptyList<Int>()
   }
 }
// Output : kotlin.collections.List<kotlin.Int> = [1, 42]