I have a `List<String>` and a function `f: String -> T?` and would like to apply `f` to ea...

# announcements

[Ellen Spertus]

I have a List<String> and a function f: String -> T? and would like to apply f to each element s of the list in order until f(s) is non-null, returning that f(s). If f(s) is null for all values, null should be returned. Here’s an ugly implementation:

fun g(list: List<String>): String? =
    if (list.isEmpty()) {
        null
    } else {
        f(list.first()) ?: g(list.subList(1, list.size))
    }

Can someone suggest a more elegant way?

Here’s the code, if you’d like to play with it. https://pl.kotl.in/mByy6tkph

[Michael de Kaste]

Yours is functional.

This is what I would do

list.firstOrNull{ f(it) != null }

[Michael de Kaste]

Codefoematter doesnt seem to work on phone

[Ellen Spertus]

Thanks, @Michael de Kaste!

[Ellen Spertus]

Actually @Michael de Kaste, that isn’t working for me. firstOrNull expects a predicate. My function f is not a predicate.

[Ellen Spertus]

This works but repeats a calculation:

list.firstOrNull { f(it) != null }?.let { f(it) }

[parth]

why not just list.firstOrNull { f(it) != null } ?

[Ellen Spertus]

Thanks @parth but I need the return value from f.

[parth]

ah, I understand now.

honestly I think a for-loop might be the most readable option 😄

[Ellen Spertus]

I was afraid of that. As you may have been able to tell from my original code, I think like a Scheme programmer.

[parth]

for (s : list){
  val result = f(s)
  if (result == null) continue
  return result
}

[Ellen Spertus]

Thanks! Would this work?

for( s : list){
  val result = f(s)
  if (result != null) {
    return result
  }
}

[parth]

I did SML way back in school, your code is familiar 😉

[parth]

yeah totally; my last job was big on “early return” to avoid nesting logic, but your above code is synonymous

[Ruckus]

for (s in list) return f(s) ?: continue

(Haven't tested it, and arguably far from readable)

[Ellen Spertus]

Ooh!

[Ellen Spertus]

Very nice!

[Ellen Spertus]

I had to add a return null after the for loop.

[araqnid]

list.asSequence().map(f).firstOrNull { it != null }

I think this dtrt too. tmtowtdi 🙂

[Michael de Kaste]

list.mapNotNull(::f).firstOrNull()

(sorry I went to bed)

[Roger Home-Martinsen]

@Michael de Kaste has a nice solution. This is equivalent to

fun g(list: List<String>) = list.mapNotNull { str -> f(str) }.firstOrNull()

[araqnid]

It will evaluate f against the whole list, though, rather than stopping after the first non-null

[Roger Home-Martinsen]

Good point.

[Michael de Kaste]

Just add an asSequence before it?

[Roger Home-Martinsen]

Yes, as Steve has done. https://kotlinlang.slack.com/archives/C0922A726/p1590028211398000?thread_ts=1590013332.391300&cid=C0922A726

[Ellen Spertus]

I tried using @araqnid’s elegant version but couldn’t because g is a suspend function (something I didn’t mention). I ended up sticking with @Ruckus’s version. Thanks everyone! That was fun, and I learned from it.