I have a function signature similar to this ```fun...

# getting-started

[David Kubecka]

I have a function signature similar to this

fun <T> test(param: T, func: () -> T) {}

Is there any way to enforce that param and lambda return types are the same? Current version allows calling the function like this

test(1) { "1" }

The reason is that the inferred type is the closest common type. So explicitly specifying the generic type would help but I would like to avoid that if possible.

[ephemient]

https://youtrack.jetbrains.com/issue/KT-13198/Expose-OnlyInputTypes-annotation-to-allow-proper-checks-of-parameters-of-generic-methods-same-as-those-used-Collection.contains

[Joshua Hansen]

It seems that exposing the first parameter as an argument to the second will enforce it. Not sure if that's what you're looking for but I think for now it's the best you can do.

fun <T> test(param: T, func: (T) -> T) {
    val funcResult = func(param)
}

val test = test(1) { "1" } // Error

[David Kubecka]

@Joshua Hansen That's a nice and acceptable workaround. Thanks!

[David Kubecka]

Sadly, the workaround isn't so nice once you have multiple type params because then this trick slightly leaks to the callers via the specified but ignored lambda arguments:

fun <T, R> test(param1: T, param2: R, func: (T, R) -> Pair<T, R>) {
    val funcResult = func(param1, param2)
}

val test = test(1, 2) { _, _ -> "1" } // Error

[David Kubecka]

Hmm, I suppose even this can be fixed. It looks more and more hacky, though 🙂

fun <T, R> test(param1: T, param2: R, func: (Pair<T, R>) -> Pair<T, R>) {
    val funcResult = func(param1 to param2)
}

[David Kubecka]

The workaround doesn't work with Unit for some reason, i.e. this compiles fine:

fun <T> test(param: T, func: (T) -> T) {
    val funcResult = func(param)
}

val test = test(Unit) { "1" } // compiles ok

Why is that? I thought that Unit has no special place in the type hierarchy.

[Youssef Shoaib [MOD]]

Lambdas that need to return Unit don't have to explicitly return it. Hence, the "1" expression is ignored, and Unit is returned. IntelliJ should probably report an error that "1" is unused

[Klitos Kyriacou]

In that code, { "1" } is treated as a function of type () -> Unit with an expression whose result is not used. Unused results are very common, for example Set.add returns a Boolean but we are allowed to ignore it, so in the same way "1" returns a String but we ignore it. IntelliJ reports a warning (it's not an error).

[David Kubecka]

Makes sense. Unfortunately, Intellij doesn't report anything.

[David Kubecka]

I mean it doesn't report anything in my particular case where of course I don't have a constant in the lambda but a function call (which can have side effects). I guess there's nothing I can do about it.