Kotlin Reified Types in Inline Functions

Kotlin Reified Types in Inline Functions

Kotlin Reified Types in Inline Functions

I’ve noticed that many people haven’t ever heard of reified types or have problems understanding what they are and what they do. Therefore this little post is intended to bring some light into the darkness of Kotlin’s reified types.

Starting situation

fun <T> myGenericFun(c: Class<T>)

In an ordinary generic function like myGenericFun, you can’t access the type T because it is, like in Java, erased at runtime and thus only available at compile time. Therefore, if you want to use the generic type as a normal Class in the function body you need to explicitly pass the class as a parameter like the parameter c in the above example. That’s correct and works fine but makes it a bit unsightly for the caller.

Inlined function with reified to the rescue

If, on the other hand, you use an inline function with a reified generic type T, the value of T can be accessed even at runtime and thus you don’t need to pass the Class<T> additionally. You can work with T as if it was a normal Class, e.g. you might want to check whether a variable is an instance of T, which you can easily do like this: myVar is T.

An inline function with reified type looks like this:

inline fun <reified T> myGenericFun()

Be aware that reified types can only be used in combination with inline functions. Such an inline function makes the compiler copy the function’s bytecode to every place where the function is being called (the function is being “inlined”). When you call an inline function with reified type, the compiler knows the actual type used as a type argument and modifies the generated bytecode to use the corresponding class directly. Therefore calls like myVar is T become myVar is String (if the type argument were String) in the bytecode and at runtime.

Reified in Action

Let’s have a look at an example, where reified is really helpful. We want to create an extension function for String called toKotlinObject, which tries to convert a JSON string to a Kotlin Object, specified by the function’s type T. We can use com.fasterxml.jackson.module.kotlin for this and the first approach is the following:

First approach without reified type

fun <T> String.toKotlinObject(): T {
      val mapper = jacksonObjectMapper()
                                    //does not compile!
      return mapper.readValue(this, T::class.java)
}

The readValue method takes a type that it is supposed to parse the JsonObject to. If we try to get the Class of the type parameter T, the compiler complains: “Cannot use ‘T’ as reified type parameter. Use a class instead.”

Workaround with explicit Class parameter

fun <T: Any> String.toKotlinObject(c: KClass<T>): T {
    val mapper = jacksonObjectMapper()
    return mapper.readValue(this, c.java)
}

As a workaround, we pass the Class of T explicitly, which can directly be used as an argument to readValue. This works and is actually a common pattern in Java code for such scenarios. The function can be called like so:

data class MyJsonType(val name: String)

val json = """{"name":"example"}"""
json.toKotlinObject(MyJsonType::class)

The Kotlin way: reified

Using an inline function with reified type parameter T makes it possible to implement our function as follows:

inline fun <reified T: Any> String.toKotlinObject(): T {
    val mapper = jacksonObjectMapper()
    return mapper.readValue(this, T::class.java)
}

There’s no need to pass the Class of T additionally, T can be used as if it was an ordinary class. For the client the code looks like this:

json.toKotlinObject<MyJsonType>()

Important

Inline reified functions are not callable from Java code, whereas normal inline functions are. That’s probably the reason why not all type parameters used in inline functions are reified by default.

Conclusion

This was just a quick introduction to reified types. In my opinion, the call to a function with reified types looks way better because we can make use of the <>-syntax commonly used whenever generics are relevant. As a result, it’s more readable than the Java approach of passing a Class object as a parameter. More details can be read in this specification document.

If you want to read more about Kotlin’s beautiful features I recommend the book Kotlin in Action to you and also like to direct you to my other articles šŸ™‚

Simon is a software engineer based in Germany with 7 years of experience writing code for the JVM and also with JavaScript. He’s very passionate about learning new things as often as possible and a self-appointed Kotlin enthusiast.

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