I’m trying to figure out a more elegant way to wri...

# announcements

[Ellen Spertus]

I’m trying to figure out a more elegant way to write:

if (intent.getStringExtra(SUGGESTION_KEY) != null) {
    suggestionButton.text = intent.getStringExtra(SUGGESTION_KEY)
    suggestionButton.visibility = View.VISIBLE
} else {
    suggestionButton.visibility = View.INVISIBLE
}

I can’t write the following, because one arm returns a String and the other Unit :

intent.getStringExtra(SUGGESTION_KEY)?.let { suggestion ->
    suggestionButton.text = suggestion
    suggestionButton.visibility = View.VISIBLE
} ?: suggestionButton.visibility = View.INVISIBLE

(I know there is a function intent.hasStringExtra(), but I’d like a general solution not specific to intents.)

How should I rewrite the original block of code, or is it best as is?

[Casey Brooks]

You can use run { } for the second arm (?.let { ... } ?: run { ... }). Though I think the original example is more clear (maybe just drop intent.getStringExtra into a temporary variable to avoid the double lookup)

[Ellen Spertus]

Thanks, @Casey Brooks. Why does let ?: run do the trick? (I’ll stick with the original code but want to understand.)

[Casey Brooks]

The whole statement is evaluated as an expression. Each branch, both the let and ?: need an expression to return a value. Variable assignment is not an expression and cannot return a value, which is why the compiler chokes on it. Wrapping that assignment in run { } turns it into a Unit-returning expression

[Ellen Spertus]

So if one path has run, the compiler knows the overall expression should return Unit (even if one path returns a value)?

[bezrukov]

run can return any type as well

[bezrukov]

also consider isInvisible extension from ktx, in this case you can avoid branches.

val suggestion = intent.getStringExtra(SUGGESTION_KEY)
suggestionButton.text = suggestion
suggestionButton.isInvisible = suggestion == null

[Ellen Spertus]

Thanks, @bezrukov, it’s good to know about isInvisible, although there’s more code in the then-body than I showed.

[Ellen Spertus]

I’d still like to know why using `let`/`run` solves the problem.

[bezrukov]

According to your initial question why it didn't work without run - it's because assignment in kotlin is not an expression (it is in Java, so you can write a = b = c).

[bezrukov]

And when you write some code ?: some more code both parts must be expression

[Ellen Spertus]

I see. I assumed that the value of statements was of type Unit, but they don’t have a value at all. Thank you!

[bezrukov]

When you wrap the assignment into run block, it knows there is no return statement and adds return Unit to it

[louiscad]

FYI, it's possible to make intent specs to provide type-safe and key-safe usage: https://github.com/LouisCAD/Splitties/tree/master/modules/intents