I have colleagues that use `.let { ... }` all the ...

# announcements

[jmfayard]

I have colleagues that use .let { ... } all the time and don't really understand the need for the others.
I know there are multiple articles on when to use
let, also, with, apply, run, takeIf, ...
But which one did you find the most useful?

[Stephan Schroeder]

each of them have different use cases. This video is a good introduction:

https://www.youtube.com/watch?v=YxOTU9F_YX4&t=1262▾

Personally, I started of using let a lot but by now it’s (in order of use) mostly: apply, let and also. I’m not sure takeIf belongs with the others though 🤔

[maxmello]

For me, it often comes down to avoiding variable declaration, so I mostly use let, also (side effects) and apply (additional operations on created object that are not part of constructor).

[jmfayard]

Shameless plug:
one great use case for with is to extract constants in an object
with(ConfigObject) { ... }
https://pl.kotl.in/4I3H2kfPt
https://dev.to/jmfayard/with-configobject-language-kotlin-issparkingjoy-ic4

[jmfayard]

@Stephan Schroeder to be honest I have never understood the use case for takeIf

[Stephan Schroeder]

I can relate, takeIf seems a bit indulgent (/code golfing) to me https://medium.com/@elye.project/using-kotlin-takeif-or-takeunless-c9eeb7099c22

[jmfayard]

Oh really nice. My TLDR is that it make sense if you use the object in the conditionnal and you do a quick return after it.
?: error("Error") or ?: return false

[Vishnu Haridas]

I always use ?.let { ... } to perform something when the receiver is not null. Example,

val response = getResponse(url)
response?.data?.userId?.let { 
    PreferenceHelper.userId = it
}

[streetsofboston]

Maybe you'll like my article 😀
“Let’s also apply a run with Kotlin on our minds” by Anton Spaans https://link.medium.com/wRHcV8OQm1

[Stephan Schroeder]

@Vishnu Haridas that looks more like a sideeffect to me (you don’t use the return-value of let), so also would be more idiomatic:

val response = getResponse(url)
response?.data?.userId?.also { 
    PreferenceHelper.userId = it
}

On the other hand assuming that PreferenceHelper.userId is null before you set it and therefore nullable, why not use a simple assignment?!

PreferenceHelper.userId = response?.data?.userId

[Vishnu Haridas]

@Stephan Schroeder Oops! 🤦 My actual intent was to have a block of code (obviously with more lines of code) run only when the userId is not null. Anyway, curious to know why you said .also is more idiomatic than .let 🤔

[Stephan Schroeder]

because let returns the last expression it computes, which can mess up your code, if that’s not what you’re intending (because if-else is also an expression and suddenly starts complaining that you need an else as well). So if you’re not using the value returned by let, it’s better to use also (which returns Unit and is therefore safe(r)).
I see let mostly as the “single value version of `map`“.

[pavi2410]

I always refer to this chart https://encrypted-tbn0.gstatic.com/images?q=tbn%3AANd9GcQINfXqC7a64YtL6CKS_aQzuGLrSg_mnKk833eMaSGBCm9j5aIJ
From https://medium.com/androiddevelopers/kotlin-demystified-scope-functions-57ca522895b1

[maxmello]

@Stephan Schroeder Small correction, “also” returns the value it is called on, not Unit. Which is still safer than let, as you still produce no unintended effects by calling it to the surrounding code.

[jmfayard]

I like what T.run { ... } does differently than T.let { ... }
but I don't like its name.
I don't find it clear at all.
Anybody has a better name?

[louiscad]

In order, I like also, with, let and apply.

[gildor]

Almost never use with, only when want to use member extension function
Also think that local variable and if check is better than let for null check. A lot more explicit and more readable

[gildor]

Use also and apply for object configuration
run as analogue of map, convert one type to another (or let, depends on use case)
takeIf is quite different than other scope funxtiobs, but like to use it in some cases where I want to validate object (often use for files or strings)

[louiscad]

After saying my message above, I just used takeIf 😅

[jmfayard]

T.run as analogue of map

... yes but I don't find the name run clear at all that it is doing this
I would use it more if it had a better name
Not that I have a better suggestion mind you.

[maxmello]

Maybe mapSingle then? But to be consistent with the it vs this, let would have to be named that way.

[jmfayard]

@Wiebe Elsinga Why not
T.mapIt { } (copy of let) and
T.mapThis { } (copy of run)
?

[gildor]

It and this version are really strange. Also it's not like map, it also changes receiver scope

[gildor]

let is closer to map than run

[jmfayard]

Really?
My mental model was
- <T>.let is map for a single value called it
- <T>.run { .. } (not run { ... }) is like let but instead of the single value being it, it is this

[gildor]

I mean signature of let is closer to map, as you said both have receiver as argument

[jmfayard]

that's correct yes