Kotlin’s way to make DSLs and many standard library functions work
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Kotlin’s way to make DSLs and many standard library functions work
Kotlin Function Literals with Receiver — The basis for DSLs and many Library Functions
As we know, Kotlin makes heavy use of functions that take other functions as their argument. This is one of two types of functions we call higher-order function. Related to this, Kotlin also comes with first-class support for passing functions around using function literals. There are two types of function literals: lambdas and anonymous functions. The entire standard library wouldn’t be half as powerful if it wasn’t using any higher-order functions.
Typical examples of higher-order functions in Kotlin are candidates such as map
, filer
or fold
as can be used for collections.
On top of that, there’s a special type of higher-order function that adds a very important tool to the language: Function literals passed to other functions can work with a so called receiver to improve both the call and the definition side. In this article, I will be explaining how to identify, write and use those function literals in your code. A popular example of this kind of function is used with the apply
scope function that is shown in the following example:
Kotlin apply function example
Isn’t it interesting that age
can be accessed without naming the object as in person.age
? How is this structure made possible?
The entire concept of function literals with receiver is what makes Kotlin a great choice for designing Domain Specific Languages.
Same basics
Kotlin, other than Java, has proper function types, which means that variables can represent a type such as a function accepting an integer and returning a string:
(Int) -> String // a function type
We can use those function types as parameters for other functions. We call those functions "higher-order functions".
Definition of a higher order function
To call the depicted function as a client, we pass a lambda, sometimes also referred to as function literal, to the function:
Calling a higher-order function
Next level: Function Literals with Receiver
As seen in the previous part, function literals are used as arguments to other functions, which is already an awesome feature by itself.
Kotlin goes one step further and provides support for a concept called function literals with receivers. This feature enables the developer to call methods on the receiver of the function literal in its body without any specific qualifiers. This is quite similar to extension functions, as they also make it possible to access members of the extension’s receiver object inside the extension code. Let’s see what these function literals look like:
Definition of a function literal with receiver type
We define a variable of type String.() -> Unit
that represents a function type () -> Unit
with String
as the receiver. All methods of this receiver can be accessed in the method body without the use of an additional qualifier. If we need to refer to the receiver explicitly, we do so using the this
as shown in the example. The caller has two possible ways to invocate this function:
calling a function literal with receiver type
With these basics in mind, let’s go through an example.
Scope functions in action
As already mentioned in the beginning of this article, Kotlin’s standard library contains multiple scope functions, one of which is apply
. It is defined as shown here:
Apply function definition
The apply
function is defined as an extension function to all types, denoted by the generic type T
, and it expects a function literal with a generic receiver of the very same generic type T
. The implementation is pretty straightforward: the function literal argument is called before the receiver of apply
is returned to the caller. The apply function, though looking very simple, is extremely powerful. One of the things you can do with it is the initialization of objects as shown here:
Apply function in action
In this, an object of type Bar
is created and apply
called on it. The new object becomes the receiver of apply
. At the same time, the lambda passed to apply
works on the same receiver which results in unqualified access to foo1
and foo2
which are both properties of type Bar
.
If the function parameter taken by apply
didn’t define a receiver, we’d have to qualify access to the Bar
object using it.foo1
(it
being the implicit lambda argument name which can also be changed into an arbitrary name). Thanks to function literals with receiver types, this becomes more easy.
It’s important to be aware of this structure because it’s essential when trying to understand more complicated constructs in Kotlin.
A quick detour to DSLs
As promoted earlier in this article, the concept of function literals with receiver is the foundation for more complicated structures such as Domain Specific Languages (DSLs). Here’s a short example of what this looks like:
Kotlin DSL example
If you want to find out more about DSLs, have a look at the official documentation here.
Enjoy!
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