Is `a?.let{} ?: run{}` idiomatic in Kotlin?

Question

I saw the following comment in a S.O. post, and I'm intrigued:

why don't you use if for null checks? a?.let{} ?: run{} is only appropriate in rare cases, otherwise it is not idiomatic – voddan May 15 '16 at 7:29 best way to null check in kotlin?

Why is that construct "only appropriate in rare cases"?
The lead engineer for Kotlin says,

run allows you to use multiple statements on the right side of an elvis operator https://stackoverflow.com/a/51241983/6656019

although I admit that's not actually endorsing it as idiomatic. Both of these posts seem to be from very well respected S.O. Kotlin contributors.
The post that inspired the original comment mentions that the let part of the expression is important if a is mutable. In that case, you'll need a?.let{} ?: run{} instead of if{} else {}.

I find I like the "let Elvis run" construct. Should I avoid it in most cases?
Thanks for any insight.

Answer 1

It's dangerous to conflate foo?.let { bar(it) } ?: baz() with if (foo != null) bar(foo) else baz().

Say you have a function: fun computeElements(): List<Int>? = emptyList()

Consider this code:

val maxElement = computeElements()?.let { it.max() } ?: return
println("Max element was $maxElement")

Compared to:

val list: List<Int>? = computeElements()
val maxElement = if (list != null) list.max() else return
println("Max element was $maxElement")

You may think these are two equivalent forms. However, if you run both, you'll see that the former does not print anything to stdout!

This is because it.max() returns null for an empty list (because there is no max element), which causes the right-hand side of the Elvis expression to be evaluated, and so the function returns early.

In short, ?.let { ... } ?: ... allows both branches of the "if-else" to be evaluated, which is dangerous. Aside from this form not being readable (if-else is universally understood, while let-run is not), subtle bugs can occur.