How do people decide when/whether to use .run or .let? They do the same thing, other than how you re...

# getting-started

[Travis Griggs]

How do people decide when/whether to use .run or .let? They do the same thing, other than how you reference the object under operation (parameter - it vs context - this). But what's the compelling reason to use on over the other? I'm particular interested in using it as a transformer on an optional variable (e.g. thing?.let .. vs thing?.run)

[Vampire]

This has some hints: https://kotlinlang.org/docs/scope-functions.html

[Jeff Lockhart]

Usually it's a matter of whether the block is calling functions on the variable, e.g.:

thing?.run {
   foo()
}

or passing the variable as a parameter to other functions, e.g.:

thing?.let {
    foo(it)
}

[Travis Griggs]

Yeah, I've read a bunch of these that explain WHAT they are. run/let return the transform, also/apply return the original. run/apply change this to the receiver (vs the calling scope), let/also keep this in place and use it instead. For many cases one of these axes doesn't really matter. So then you end up wondering which to use. If you're an overthinker and easily prone to analysis paralysis, it's particularly annoying 😄

[Jeff Lockhart]

There's definitely a good amount of overlap in practice, as the block may be performing multiple operations that could be satisfied by either function. I've also used let at times when I'm using functions and properties with it as a receiver, but need to disambiguate those calls from the parent this scope. It really just comes down to which best fits the particular code in each scenario. There's not always a right or wrong answer.

[CLOVIS]

I tend to avoid the run overload with a receiver. I use let for transformations, and the receiver-less run for semantic scoping:

val foo = bar ?: run {
    log.info("something")
    return
}

[ephemient]

I'm more likely to use with(foo) { ... } than foo.run { ... }, but will use ?.run { ... } sometimes if that's what's works better

[Klitos Kyriacou]

It's also possible to use both let and run to disambiguate between two different things without having to use specific names instead of `it`:

foo().let {
    bar().run {
        // Here, you can refer to foo()'s returned value as "it" and bar()'s as "this"
    }
}

Having said that, I've never actually done this in real code.