Implemented form validation in an Android App using Arrow s

Question

Implemented form validation in an Android App using Arrow s Either + Raise as described in the new tutorial and also used Arrow Optics to dig into composite error structures and this feels awesome Such a succinct code and it does a lot of work which would be very verbose otherwise I did this many times Maybe my Folds + lens compositions will scare away some junior dev but I feel like this wouldn t be to hard to explain understand especially considering how good Arrow s documentation is Cheers to the devs pray P S I will post a little sample in the thread in case someone is interested

dimsuz · Answer

``` state is a screen s state to render RegisterCredentials state email text state password text fold ifLeft = errors gt val emailError = Fold nonEmptyList lt RegisterCredentialsValidationError gt compose RegisterCredentialsValidationError invalidEmail compose RegisterCredentialsValidationError InvalidEmail errors compose Fold nonEmptyList firstOrNull errors val passwordError = Fold nonEmptyList lt RegisterCredentialsValidationError gt compose RegisterCredentialsValidationError invalidPassword compose RegisterCredentialsValidationError InvalidPassword errors compose Fold nonEmptyList firstOrNull errors state copy ViewState nullableEmailError set emailError ViewState nullablePasswordError set passwordError ViewState privacyPolicyErrorVisible set state privacyPolicyAccepted ifRight = state ```

Alejandro Serrano Mena · Answer

in theory it should be possible to make it even shorter if you use the accessors generated by Arrow Optics In principle instead of ```Fold nonEmptyList lt RegisterCredentialsValidationError gt compose RegisterCredentialsValidationError invalidEmail compose RegisterCredentialsValidationError InvalidEmail errors compose Fold nonEmptyList ``` you should be able to write ```Every nonEmptyList lt RegisterCredentialsValidationError gt invalidEmail errors nonEmptyList ```

dimsuz · Answer

wow this almost works except I can t use final `nonEmptyList` from your example It will take me some time to wrap my head around the types not that familiar with Every yet But here s the type I get ```val e Every lt NonEmptyList lt UserDetailsValidationError gt NonEmptyList lt NameValidationError gt gt = Every nonEmptyList lt UserDetailsValidationError gt invalidName errors```

dimsuz · Answer

my goal is to extract the first error from `Nel lt NameValidationError gt `

dimsuz · Answer

I can do this with `e firstOrNull errors first ` of course but maybe there s some more optics magic I can do to be less verbose

Alejandro Serrano Mena · Answer

there are a few optics you can use there

Alejandro Serrano Mena · Answer

`index` <https apidocs arrow kt io arrow optics arrow optics dsl index html> can be used to access any element as Optional so you could use `index 0 ` to access the first element

Alejandro Serrano Mena · Answer

you also have `Cons` <https apidocs arrow kt io arrow optics arrow optics typeclasses cons companion index html> which is especifically for getting the first element of things

Alejandro Serrano Mena · Answer

my personal choice in this case would be ```Every nonEmptyList lt UserDetailsValidationError gt invalidName errors index Index nonEmptyList 0 ```

dimsuz · Answer

I ve read about Cons but didn t fully understand how to apply it here Your final example works thanks I needed to add a verbose cast to Fold though otherwise it didn t compile due to overload ambiguity index is defined on Traversal Setter Fold and Every is all of those ```val error = Every nonEmptyList lt UserDetailsValidationError gt invalidName errors as Fold lt NonEmptyList lt UserDetailsValidationError gt NonEmptyList lt NameValidationerror gt gt index Index nonEmptyList 0 firstOrNull errors ```

Alejandro Serrano Mena · Answer

gt otherwise it didn t compile due to overload ambiguity this is one of the things we ve fixed in Arrow 2 0 but we decided to not include it in 1 2 0 because it s a breaking change

dimsuz · Answer

Great to know thanks