How can I check for generic type in Kotlin

I'm trying to test for a generic type in Kotlin.

if (value is Map<String, Any>) { ... }

But the compiler complains with

Cannot check for instance of erased type: jet.Map

The check with a normal type works well.

if (value is String) { ... }

Kotlin 0.4.68 is used.

What am I missing here?

The problem is that type arguments are erased, so you can't check against the full type Map, because at runtime there's no information about those String and Any.

To work around this, use wildcards:

if (value is Map<*, *>) {...}

I think this is more appropriate way

inline fun <reified T> tryCast(instance: Any?, block: T.() -> Unit) {
    if (instance is T) {
        block(instance)
    }
}

Usage

// myVar is nullable
tryCast<MyType>(myVar) {
    // todo with this e.g.
    this.canDoSomething()
}

Another shorter approach

inline fun <reified T> Any?.tryCast(block: T.() -> Unit) {
    if (this is T) {
        block()
    }
}

Usage

// myVar is nullable
myVar.tryCast<MyType> {
    // todo with this e.g.
    this.canDoSomething()
}

JVM removes the generic type information. But Kotlin has reified generics. If you have a generic type T, you can mark type parameter T of an inline function as reified so it will be able to check it at runtime.

So you can do:

inline fun <reified T> checkType(obj: Object, contract: T) {
  if (obj is T) {
    // object implements the contract type T
  }
}