I have a question about type inference in the example below

Question

I have a question about type inference in the example below Given this method ``` fun lt R Any gt fromAttr prop KProperty1 lt T R gt transformer AttributeValue gt R transformers put constructorParams getValue prop name transformer ``` If I don t supply the type argument I am able to pass a `prop` with a different `R` than `transformer` For instance ``` u fromAttr Widget count false ``` Where `Widget count` is a `Long` I have to use the form `u fromAttr lt Long gt Widget count false ` to get a compiler error I assume this is happening because of type erasure so Kotlin is inferring `R` to be `Any`

diesieben07 · Answer

It s not because of type erasure it is because ` AttributeValue gt R` can be thought of as `Function1 lt AttributeValue out R gt ` And that is assignable to `Function1 lt AttributeValue out Any gt ` so `Any` is inferred

Shawn A · Answer

Thanks So it seems unavoidable that I must use `u fromAttr lt gt `

diesieben07 · Answer

Depends on your use case What do you use this `transformers` map for

Shawn A · Answer

It maps AWS dynamoDB results to an object Here is the full class ```class Unmarshaller lt T Any gt private val cls KClass lt T gt private val transformers = mutableMapOf lt KParameter AttributeValue gt Any gt private val constructorParams = cls primaryConstructor parameters map it name to it toMap fun lt R Any gt fromAttr prop KProperty1 lt T R gt transformer AttributeValue gt R transformers put constructorParams getValue prop name transformer fun invoke item Map lt String AttributeValue gt T val params = transformers mapValues k func gt func item getValue k name return cls primaryConstructor callBy params ```

Shawn A · Answer

``` Test fun `unmarshall an object from a map of name to AttributeValue` val item = mapOf name to strAttr foo count to longAttr 55 val u = Unmarshaller Widget class u fromAttr Widget name it s u fromAttr Widget count it n toLong val widget = u invoke item ```

diesieben07 · Answer

What you could do is something like this ``` class WrappedProperty lt T R gt val property KProperty1 lt T R gt fun lt R Any gt fromAttr prop WrappedProperty lt T R gt transformer AttributeValue gt R ```

diesieben07 · Answer

Not as elegant but unfortunately there is no way that I know of to override the declaration site variance `out` of the `KProperty` here

Shawn A · Answer

hmm ya I ll probably just stick with having to supply the type argument Thanks though

Shawn A · Answer

though if you supply the type and return the correct type from the lambda intellij tells you its not needed

diesieben07 · Answer

Well yes because it is indeed not needed

Shawn A · Answer

haha right

Shawn A · Answer

I was just hoping to get more help from the compiler but that won t happen unless I adopt your wrapper approach

diesieben07 · Answer

There is something the Stdlib has for its own usage for this problem ` OnlyInputTypes` But it s internal