# Why does new "when" syntax (in Kotlin 1.3M1) need the "val"?

**URL:** <https://discuss.kotlinlang.org/t/why-does-new-when-syntax-in-kotlin-1-3m1-need-the-val/8893>\
**Category:** Language Design\
**Created:** [August 5, 2018, 3:54pm UTC](https://discuss.kotlinlang.org/t/why-does-new-when-syntax-in-kotlin-1-3m1-need-the-val/8893 "2018-08-05T15:54:22Z")\
**Posts on this page:** 7\
**Page:** 1

<div class="post-metadata">

**Author:** ![zhengkeli](https://sea1.discourse-cdn.com/flex019/user_avatar/discuss.kotlinlang.org/zhengkeli/32/3974_2.png) [@zhengkeli](https://discuss.kotlinlang.org/u/zhengkeli)\
**Post date:** [August 5, 2018, 3:54pm UTC](https://discuss.kotlinlang.org/t/why-does-new-when-syntax-in-kotlin-1-3m1-need-the-val/8893/1 "2018-08-05T15:54:22Z")

</div>

I’ve read the [blog](https://blog.jetbrains.com/kotlin/2018/07/see-whats-coming-in-kotlin-1-3-m1/) about Kotlin 1.3M1 and its new features.  
I noticed this new syntax:

```
fun Request.getBody() =
when (val response = executeRequest()) {
    is Success -> response.body
    is HttpError -> throw HttpException(response.status)
}

```

It’s a good feature. But I’m thinking that - why we need the modifier “val”?  
After all, in “for” syntax, the indexing variable “i” has not such modifier:

```
for(i in 0..10){
    println(i);
}

```

why not just leave it alone without “val” like this ↓ ?

```
fun Request.getBody() =
when (response = executeRequest()) {
    is Success -> response.body
    is HttpError -> throw HttpException(response.status)
}

```

---

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**Author:** ![Wasabi375](https://sea1.discourse-cdn.com/flex019/user_avatar/discuss.kotlinlang.org/wasabi375/32/4741_2.png) [@Wasabi375](https://discuss.kotlinlang.org/u/Wasabi375)\
**Post date:** [August 5, 2018, 10:08pm UTC](https://discuss.kotlinlang.org/t/why-does-new-when-syntax-in-kotlin-1-3m1-need-the-val/8893/2 "2018-08-05T22:08:49Z")

</div>

There is a discussion in the corresponding [keep](https://github.com/Kotlin/KEEP/issues/120) about it.  
TL;DR:  
`x = foo()` looks similar to an expression `x == foo()`. Also you are declaring a new variable.  
But what about `for` you ask? `for` always declares a new variable. This is different to `when` where it is optional.

This is the argument I could find. I would have preferred it without the `val` keyword as well. But it’s definitely a step in the right direction.

---

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**Author:** ![gidds](https://sea1.discourse-cdn.com/flex019/user_avatar/discuss.kotlinlang.org/gidds/32/10865_2.png) [@gidds](https://discuss.kotlinlang.org/u/gidds)\
**Post date:** [August 6, 2018, 8:41am UTC](https://discuss.kotlinlang.org/t/why-does-new-when-syntax-in-kotlin-1-3m1-need-the-val/8893/3 "2018-08-06T08:41:27Z")

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I don’t like the new syntax, because even with `val` it has the same readability problems as assignment expressions in other languages — the problems that Kotlin has been so good at avoiding!

So I’d like to suggest an alternative syntax:

```auto
fun Request.getBody() =
when (executeRequest()) as response {
    is Success -> response.body
    is HttpError -> throw HttpException(response.status)
}

```

Here the new variable is moved away from the expression, to avoid any confusion. And it reads much more like English. Plus, there’s a nice symmetry: the expression is on the left-hand side, and the new variable on the right, just as in the conditions below.

It’s not perfect, as `as` is normally used for casting, not assignment. But the `when` syntax is already special, and I think the greater readability would justify it.

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<div class="post-metadata">

**Author:** ![pdvrieze](https://sea1.discourse-cdn.com/flex019/user_avatar/discuss.kotlinlang.org/pdvrieze/32/1882_2.png) [@pdvrieze](https://discuss.kotlinlang.org/u/pdvrieze)\
**Post date:** [August 8, 2018, 2:56pm UTC](https://discuss.kotlinlang.org/t/why-does-new-when-syntax-in-kotlin-1-3m1-need-the-val/8893/4 "2018-08-08T14:56:30Z")

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The problem with `as` is that it means casting. It would be very confusing where using `val x =` has a very intuitive non-mistakable meaning. Assignment syntax is clearer to me, and if val is used to make clear that it introduces a new name, and to prevent comparison vs assignment issues that is fine.

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**Author:** ![ahmadalsharif.dev](https://avatars.discourse-cdn.com/v4/letter/a/bbe5ce/32.png) [@ahmadalsharif.dev](https://discuss.kotlinlang.org/u/ahmadalsharif.dev)\
**Post date:** [September 1, 2018, 9:26am UTC](https://discuss.kotlinlang.org/t/why-does-new-when-syntax-in-kotlin-1-3m1-need-the-val/8893/5 "2018-09-01T09:26:00Z")

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Your example about the for-loop is potentially invalid. In for-loops, the declared variable is mutated between iterations. However the new syntax, explicitly says that the defined variable is immutable and is limited to the `when` scope

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**Author:** ![roryaburks](https://avatars.discourse-cdn.com/v4/letter/r/67e7ee/32.png) [@roryaburks](https://discuss.kotlinlang.org/u/roryaburks)\
**Post date:** [September 5, 2018, 9:03pm UTC](https://discuss.kotlinlang.org/t/why-does-new-when-syntax-in-kotlin-1-3m1-need-the-val/8893/6 "2018-09-05T21:03:03Z")

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In for loops it rarely if ever makes sense to have no reference to the thing in the loop. Putting a val in there is just extra boiler plate (and technically a bit confusing since it’s not a single val, but several different vals over the course of the loop). For when statements, it is very often that you don’t want to assign the condition to any value. So not only does it make sense to explicitly state that you want the value to be stored, but the val CAN only be implied by checking to see if the condition is a valid assignment statement.

And even if you should probably be using

```auto
val a = if( x == y) 1 
        else 2

```

Rather than

```auto
val a = when( x == y) {
   true -> 1
   false -> 2
}

```

The latter is nonetheless valid and we don’t want to get into any = vs == territory.

End of the day there were a handful of different syntaxes proposed and a rather large discussion over it and this one was chosen.

---

<div class="post-metadata">

**Author:** ![zhengkeli](https://sea1.discourse-cdn.com/flex019/user_avatar/discuss.kotlinlang.org/zhengkeli/32/3974_2.png) [@zhengkeli](https://discuss.kotlinlang.org/u/zhengkeli)\
**Post date:** [September 6, 2018, 7:08am UTC](https://discuss.kotlinlang.org/t/why-does-new-when-syntax-in-kotlin-1-3m1-need-the-val/8893/7 "2018-09-06T07:08:51Z")

</div>

In my opinion, this syntax is also not clear.

```
when (val response = executeRequest()) {
    is Success -> response.body
    is HttpError -> throw HttpException(response.status)
}

```

Such syntax as below can lead me to think that the assignment `val response = executeRequest()` can return something. But actually not, such syntax only works for `when`. Believe me, such rules can be very unfriendly for the beginners.

I’d like not to add this feature unless we have a better solution.