A slight inconvenience but in an `either ` block I wanted to

Question

A slight inconvenience but in an `either ` block I wanted to instead of ensuring that something is not null something like `ensureNull` which doesn t exist of course smile My use case was that I do this ```val userError Error = response userError ensure userError == null ErrorMessage userError message not getting smart cast to userError being non null here in order to be able to access `message` without a ` ` or a ` ` ``` There s no smart contract that `ensure` can use to ensure not only that the condition is true if the function returns but also that the condition is not true if the function goes inside the `raise` block correct

Stylianos Gakis · Answer

Of course trivial to make your own ``` OptIn ExperimentalContracts class public inline fun lt Error B Any gt Raise lt Error gt ensureNull value B raise B gt Error B contract callsInPlace raise InvocationKind AT MOST ONCE returns implies value == null return if value == null value else raise raise value ``` Where you can propagate the right non null `B` into the `raise` block by changing the signature of it but then you gotta use that B which comes as the received of the lambda and don t get the nice smart casting Also you gotta add this new function and place it in the right spot etc of course At that point maybe my best and most easy to the eye approach is to just do ```val userError Error = response userError if userError = null raise ErrorMessage userError message ``` And I m good I get the right casting inside the if lambda and the right assumption after the lambda is over that userError is in fact nullable there

Alejandro Serrano.Mena · Answer

the problem here is that nullability contracts are treated in a special way by the compiler you cannot just say if this returns this general condition is true this is why we need a special `ensureNotNull` on top of `ensure`

Alejandro Serrano.Mena · Answer

we could in fact include `ensureNull` but given that it s not as commonly used as `ensureNotNull` we decided it would pollute the namespace more than help but we re open to discussion about this if several people find this function useful

Stylianos Gakis · Answer

Yup as I was playing with this I did realize that the simple if check is just convenient enough especially considering it s not that often that I need this The reason I started with this is that we have an API that responds with ```data class Foo data Bar userError Baz ``` And I want to short circuit if userError is not null and then I also can do ensureNotNull on `data` I m not sure it s useful enough to pollute the API of arrow here so I would not be one to suggest to add it but I m leaving my use case here just in case other people have their own opinions on it too

simon.vergauwen · Answer

Funnily enough working in a client codebase I came across a use case for `ensureNull` today too sweat smile I think it s a different way of modelling I think when we designed these APIs we expected `fun validate str String Either lt ParsingFailure Unit gt ` but I m encountering `fun validate str String ParsingFailure ` returns `null` if successfull So it s using `null` as a success value kind of unexpected sweat smile Not sure if it s similar to your use case < Stylianos Gakis> but it sounds familiar It s also quite typical for a Java SDK to return `data class Result val success Response val failure Exception ` as you ve shown above

CLOVIS · Answer

I often use `ensure foo == null ` but I rarely do anything in the message more than printing it so I ve never really felt the need for a dedicated function

simon.vergauwen · Answer

So far `ensure foo == null ` was sufficient in my case I slightly refactored the code

Stylianos Gakis · Answer

> I often use `ensure foo == null ` but I rarely do anything in the message more than printing it so I ve never really felt the need for a dedicated function Yes but our error was a type and then we did want to do `userError message` in the raise block so we d have to do `userError message` to make it compile in our case Something like that

Alejandro Serrano.Mena · Answer

to be honest in this case I would use an explicit `raise` to remove the ` ` it doesn t add so much noise ```val userError Error = response userError if userError = null raise ErrorMessage userError message ```

Stylianos Gakis · Answer

Exactly what I did in the end wink <https github com HedvigInsurance android blob 0372f5172bed4397af2640b5b59e1b27019655c5 app feature feature changeaddress src main kotlin com hedvig android feature changeaddress data ChangeAddressRepository kt L58 L61|https github com HedvigInsurance android blob 0372f5172bed4397af2640b5b59e1b2701 g android feature changeaddress data ChangeAddressRepository kt>

simon.vergauwen · Answer

< Alejandro Serrano Mena> < Stylianos Gakis> we should probably get that repo on the website as a large real world example

Stylianos Gakis · Answer

I d be super happy to see that happen Being a full app does make it somewhat hard perhaps for some folks to navigate the app but at the same time that s maybe a good thing it shows the flexibility of arrow regarding using it in the places where it fits and not necessarily something that every single thing in your entire app has to use We do use arrow in pretty much 100 of the UseCase Repository classes basically things that our Presenters ViewModels depend on And I say pretty much 100 since I know that s the case for all code that has been written in the past ~1 2 years ever since I brought Arrow in to the repo but there are still some outliers which I would migrate in an instant if we ever touch them sweat smile

simon.vergauwen · Answer

I think that s great There is plenty of small examples already out there but a larger example is harder to find Especially one that has been going on for several years

Stylianos Gakis · Answer

Lmk if you need any help from my side would be happy to help

simon.vergauwen · Answer

< Alejandro Serrano Mena> I think we should introduce a new page for this on the website WDYT Perhaps under Community we can add an Example Projects or Examples section We can also add Ktor Arrow Examples your Weather App HedvigInsurance App and others

Alejandro Serrano.Mena · Answer

there s a page for this in Design <https arrow kt io learn design projects >