Why is `fun <T> something(): T = TODO()` all...

# language-proposals

[natpryce]

Why is fun <T> something(): T = TODO() allowed, but val <T> something : T get() = TODO() is not? I would think the two are equivalent.

[dmitriy.novozhilov]

Because there is no syntax to specify type arguments for property access
If T used in receiver then it's always possible to infer T from it (because all types in receiver should be inferred before call/access itself)
But if T is used only in return type then there will be cases when compiler can not infer it, like

val x = something // no information to infer T

[Youssef Shoaib [MOD]]

Btw you can technically suppress that error, but you run into a lot of other code-breaking bugs whenever T can't be figured out from the context. It's just not worth it to suppress it at all.

[dmcg]

I don’t think I understand why functions are different here though? Oh, and hi @Youssef Shoaib [MOD] - good to see you here as well as commenting on my YouTube channel!

[Youssef Shoaib [MOD]]

In functions you can specify the type with angle brackets. A call to funSomething would simply tell you to specify a type arg. There's obviously the simple fix of just specifying the type on call site, but in some places that gets messy.

Here's a hack though. You can @Suppress(TYPE_PARAMETER_OF_PROPERTY_NOT_USED_IN_RECEIVER") and then on the call site hint at the type somehow. E.g.:

@Suppress("TYPE_PARAMETER_OF_PROPERTY_NOT_USED_IN_RECEIVER")
val <T> something: T get() = TODO()
fun <T> coerce(x: T): T = x
fun main() {
    //val x = something //Error
    val x2: Int = something 
    val x3 = coerce<String>(something)
    coerce<Boolean>(something) //even works as an unused expression!
    println("Hello, world!!!")
}

Good to see you too Duncan! I've been enjoying the videos as per usual! Keep it up

[Javier]

@dmitriy.novozhilov in the future can be possible to do, for example, property<T> if it is a getter?

The syntax doesn’t collide with functions as they do foo<T>()

[dmcg]

Ah I see, it’s about the call-site syntax. So because we don’t have this ^ the type only makes sense as a receiver. Maybe Type parameter of a property can only be used as its receiver type would be a better message?

[Youssef Shoaib [MOD]]

Maybe Type parameter of a property can only be used as its receiver type would be a better message?

Using it as part of the receiver type is valid too. E.g.:

val <T> List<T>.first: T

[dmitriy.novozhilov]

@Javier There is no such plans. Just use functions