Type inference failed: Not enough information to infer parameter Please specify it explicitly

Question

I'm trying to write a Vaadin application in Kotlin. For data binding, Vaadin 8 now provides a possibility for type safe data binding. In Kotlin I would have expected work like that:

class LoginModel {
    var username: String = ""
    var password: String = ""
}

class LoginView : FormLayout() {
  val name = TextField("name")
  val password = TextField("password")
  val loginButton = Button("login")

  init {
      val binder = Binder<LoginModel>()
      binder.forField(name).bind(
          { it.username }, 
          { bean, value -> bean.username = value })

     //... 
  }
}

I'm getting the following error message here:

Error:(23, 31) Kotlin: Type inference failed: Not enough information to infer parameter BEAN in fun <BEAN : Any!, TARGET : Any!, BEAN : Any!> Binder.BindingBuilder<BEAN#1 (type parameter of bind), TARGET>.bind(p0: ((BEAN#1!) -> TARGET!)!, p1: ((BEAN#1!, TARGET!) -> Unit)!): Binder.Binding<BEAN#1!, TARGET!>!
Please specify it explicitly.

I tried by explicit specifying the type parameters:

binder.forField(name).bind<LoginView, String, LoginView>(
    { it.username }, 
    { bean, value -> bean.username = value })

but that leads to the error message (and other sytax errors, so I did not follow that approach)

Error:(23, 35) Kotlin: No type arguments expected for fun bind(p0: ValueProvider<LoginModel!, String!>!, p1: Setter<LoginModel!, String!>!): Binder.Binding<LoginModel!, String!>! defined in com.vaadin.data.Binder.BindingBuilder

My second approach was trying to pass the Kotlin property accessors directly, but the error message was the same as the first one:

binder.forField(name).bind(LoginModel::username.getter, LoginModel::username.setter)

The last approeach was trying to use an extension method and make everything as explicit as possible:

fun <BEAN, TARGET> Binder.BindingBuilder<BEAN, TARGET>.bind(property: KMutableProperty1<BEAN, TARGET>) {

    fun set(bean: BEAN): TARGET = property.get(bean)
    fun get(bean: BEAN, value: TARGET): Unit = property.set(bean, value)
    this.bind(::set, ::get)
}

But it leads still to the same error message as the first one

Karlykarlyn

Answer 1

I have tried your example and it compiles just fine with my Intellij 2017.1.4 and Kotlin 1.1.2-5. Probably you have discovered a bug in older version of a Kotlin plugin?

The method bind takes no generic parameters so it cannot be generified. The forField method takes one generic parameter, so perhaps you can try

    binder.forField<String>(name).bind(
            { it.username },
            { bean, value -> bean.username = value })

But first, please make sure that you have the newest version of the Kotlin plugin, and/or perhaps try it out with Intellij Community Edition.

However, I urge you to use bind(String) since other bind methods won't work with JSR303 validations. You can also define the following extension method:

fun <BEAN, FIELDVALUE> Binder.BindingBuilder<BEAN, FIELDVALUE>.bind(prop: KMutableProperty1<BEAN, FIELDVALUE?>): Binder.Binding<BEAN, FIELDVALUE> =
        bind(prop.name)

And call it as follows from your code binder.forField(name).bind(LoginModel::username). Please see here for more examples: https://github.com/mvysny/karibu-dsl/blob/master/example-v8/src/main/kotlin/com/github/vok/karibudsl/example/form/FormView.kt

Subastral

I had no issues on Kotlin 1.1.2-5 as well. – Notions

Answer 2

You must be using API level 26+. This version has changed the signature of View.findViewById() - see here https://developer.android.com/preview/api-overview.html#fvbi-signature

The result of findViewById is ambiguous, you need to supply the type:

For example:

val myTextView = findViewById(R.id.list) as TextView

becomes

val myTextView = findViewById<TextView>(R.id.myTextView)

Agro

The question was related to Vaadin, it is not related to Android at all. – Subastral

Answer 3

Since your model is fairly simple, you can complete the binding by connecting to the property with the given name via the Binder#bind(String propertyName) method

val binder = Binder<LoginModel>()
binder.forField(name).bind("username");

Notions

That would work, but I'd like to use the type safe data binding and understand what exactly the problem with the type inference is. – Karlykarlyn

This is actually a good advice, for the following reason: if you ever try to use BeanValidationBinder with JSR-303 validations with the bind(ValueProvider, Setter) method, it will skip the JSR-303 validations! You actually need to use bind(String) so that the validation annotations can be picked from the field itself. – Subastral

@MartinVysny I agree. However, the the OP wants to do it kotlin style. And I can appreciate that. – Notions

@Notions I agree. That's why I'm not using bind(String) directly; but instead I'm using the abovementioned bind(KMutableProperty1). – Subastral