A modern programming language that makes developers happier.

kotlinlang

For extension functions. When do you use the prefix `as` and when do you use the prefix `to`?

I feel like `as` would be some kind of cast while `to` would be a transformation

Use `to` if the function transforms something to another type (like putting all elements of a sequence into a list). Use `as` if it will only provide a view over the original object (like a sequence over the original list).

Yeah there’s also the idea that `to` basically costs more than `as`. It’s more complex.

For me, there's no set rule. It feels like `as` implies that the conversion could be implicit (like when Java upcasts an `int` to  a `double`, because there's no chance of losing information), but `to` implies that you're having to do some fiddling to get it to work and it could conceivably go wrong.

So you think for example `Flowable.asFlow()` should rather be called `Flowable.toFlow()`?

I’m currently reading the OkHttp changelog and they write:

&gt; If the returned value is a view of what created is, we use `as`

With that reasoning they changed `File.toRequestBody()` -&gt; `File.asRequestBody()`

&gt; So you think for example `Flowable.asFlow()` should rather be called `Flowable.toFlow()`
Nope, because under the hood it’s still Flowable, so it should be `as`

I believe the best example of this convention from stdlib are *Array extensions asList() vs toList(), first uses original array under the hood and List just a view of this array (so changed values of array also changed in list), second copies data and return new instance of List

&gt; Yeah there’s also the idea that `to` basically costs more than `as`
Mostly because `as` usually does not copying, but it’s not a rule, it’s more like consequence

Thanks for your input; naming is hard :slightly_smiling_face: