Hi, Is it possible to have function which has two generic types and explicitly define only one type ...

# announcements

[Antanas A.]

Hi,
Is it possible to have function which has two generic types and explicitly define only one type in call sites:

`
        suspend fun <T, U> DataSource.mustHave(block: suspend T.() -> U): U =
            ((this as? T) ?: throw RuntimeException("Operation is not supported")).block()
`

and use it: dataSource.mustHave<DataSource.HasAddOperation> { add(command) }

Logically it should be able to infer U type automatically, but if I'm specifying first type T compiler requires me to specify also U type.
eg. dataSource.mustHave<DataSource.HasAddOperation, AddResult> { add(command) }

Is it possible to somehow declare that extension function in a way that U type be inferred automatically?

[ribesg]

I don’t think you can have partial content in <>

[Czar]

Currently this is not possible in Kotlin, but I remember there was an issue or two on Youtrack, try searching for it. I don't remember what was the decision on this, but it was definitely discussed at some point.

[Pavlo Liapota]

I would recommend:

fun <T> DataSource.mustHave(): T =
    (this as? T) ?: throw RuntimeException("Operation is not supported")

And then you can write

dataSource
.mustHave<DataSource.HasAddOperation>()
.add(command)

[Pavlo Liapota]

or following, if you want to call several methods:

dataSource
.mustHave<DataSource.HasAddOperation>()
.run { add(command) }

[Antanas A.]

thank you all for answers

[Antanas A.]

Kotlin could implement "infer" keyword like in TypeScript, and this could be written as:
suspend fun <T, infer U> DataSource.mustHave(block: suspend T.() -> U): U

[Antanas A.]

@Pavlo Liapota thanks for a solution, haven't thought about this