Method Chaining (enhancing Java syntax)
Maurizio Cimadamore
maurizio.cimadamore at oracle.com
Tue Jun 13 10:51:05 UTC 2023
Hi,
the topic you mention is sometimes known, in type-system literature, as
“self types”. The topic of adding self types to Java was reasearched few
years ago (see [1, 2]). While adding self-types increases the
expressiveness of the language (e.g. allowing support for so called
“binary methods” - e.g. typing methods such as Object::equals
correctly), such extension makes the language unsound. Given this
declaration (which I borrow from the cited paper):
|class Node { ... public void setNext(ThisClass next) { ... } } |
And given the following generic method declaration:
|static void setNextStatic(Node node1, Node node2) {
node1.setNext(node2); // whooops } |
Here we have an issue, because it is now possible to pass two instances
of Node with different “self types”, and this program would still
type-check. In other words, subtyping here works against us - because I
can declare some subclass of Node, e.g. MyColoredNode <: Node, and then
call |setNextStatic(new Node(), new MyColoredNode())|, which clearly
violates the specification of Node::setNext, but the type system is not
powerful enough to detect that.
Typically this tension is resolved by adding support for so called
“exact types” (denoted as @T). A variable with an exact type guarantees
that it can only ever hold values that are the same as its static type.
So, e.g. assigning a MyColoredNode object to a variable of type @Node
would fail. Equipped with this new weapon, we can go back and change the
declaration of setNext to:
|public void setNext(@ThisClass next) { ... } |
Which makes things sound again.
This is a common tale when coming up with new language ideas - e.g. we
start from a relatively constrained problems (e.g. how to better express
covariant returns, in a way that doesn’t force us to re-override a
method in all subclasses?) and we find something that works. Then we see
what happens if we use the same construct in places we did not
anticipate (in this case, usage of |this| type in parameter position)
and see what happens. Almost every non-trivial type-system extension
I’ve been working with in the past 15 years starts showing some kind of
subtle interactions when looked at it that way. And “this” is no exception.
At which point the question becomes: is the feature important enough to
either (a) swallow the cost of any other dependent feature it brings
about (e.g. exact @ types), or (b) restrict the usage of the feature
only in the places where it is well behaved (e.g. return position) ? The
former has a complexity cost (introduction of a new kind of type),
whereas the latter has an irregularity cost (e.g. the new types can only
be used in few selected places). My (subjective) judgment is that, in
this case the feature you propose is not worth paying any of these
costs, given that there are some alternatives to achieve the same thing.
On that subject, one alternative you have left out in your description
is to use the generic type system to implement a poor man “self type” - e.g.
|class Foo<T extends Foo<T>> { T get(); } class SubFoo extends
Foo<SubFoo> { } |
This is not perfect (especially when working with complex class
hierarchies) but I have seen this technique used with success many times
(e.g. in the implementation of the Stream API itself).
Cheers
Maurizio
[1] - https://link.springer.com/chapter/10.1007/978-3-540-24851-4_18
[2] - https://www.sciencedirect.com/science/article/pii/S0167642313000038
On 09/06/2023 17:35, Tomáš Bleša wrote:
> //* I sent the following to discuss@ mailing list yesterday. (wrong
> list for the topic) I hope this will be more appropriate mailing list *//
>
> Hi all,
>
> this is a request for feedback on a topic that has been on my mind for
> a few weeks. I have written a short document in JEP format and would
> like to ask you to comment if you find the described proposal useful.
>
> Thanks,
> Tomas Blesa
> __________________________________
>
> Summary
> -------
> Enhance the Java language syntax to better support the method chaining
> (named parameter idiom) programming pattern.
>
> Goals
> -----
> The primary goal is to remove unnecessary boilerplate code in class
> methods designed for type-safe chained calls, especially when combined
> with inheritance.
>
> Motivation
> ----------
> [Method chaining](https://en.wikipedia.org/wiki/Method_chaining) is a
> widely used and popular programming pattern, particularly in creating
> libraries (APIs) or configuration objects. Programmers can easily
> create a method that returns `this` with a method signature that
> specifies the returning type of the containing class.
>
> ```java
> class Shape {
> public Shape scale(double ratio) {
> // recalculate all points
> return this;
> }
> }
> ```
>
> The problem arises when we combine this pattern with inheritance. We
> can lose type information when calling the method on a subclass. For
> example, let's create two subclasses of the `Shape` superclass:
>
> ```java
> class Rectangle extends Shape {
> public Rectangle roundCorners(double pixels) {
> // ...
> return this;
> }
> }
>
> class Circle extends Shape {
> }
> ```
>
> Now, imagine the following piece of code using the mini-library above:
>
> ```java
> var myRect = new Rectangle().scale(1.2).roundCorners(10);
> ```
>
> The code won't compile because `scale()` returns the type `Shape`,
> which doesn't have the `roundCorners` method. There is also a problem
> even without the final `roundCorners()` call:
>
> ```java
> var myRect = new Rectangle().scale(1.2);
> ```
>
> The inferred type of `myRect` is `Shape` and not `Rectangle`, so the
> following line will also be invalid:
>
> ```java
> myRect.roundCorners(10);
> ```
>
> Straightforward solutions to the problem could be:
>
> 1) Override the `scale()` method in all subclasses and change the
> return type:
>
> ```java
> class Rectangle extends Shape {
> // ...
> @Override
> public Rectangle scale(double ratio) {
> super.scale(ratio);
> return this;
> }
> }
> ```
>
> 2) Split object construction and method calls:
>
> ```java
> var myRect = new Rectangle();
> myRect.scale(1.2);
> myRect.roundCorners(10);
> ```
>
> 3) Partial solution - reorder chained calls (if possible):
>
> ```java
> var myRect = new Rectangle();
> myRect.roundCorners(10).scale(1.2); // roundCorners called first
> ```
>
> All of these solutions add unnecessary lines of code, and as the
> library of shapes grows, keeping the desired return type will
> introduce more and more boilerplate code.
>
> Description
> -----------
> The proposed solution to the problem described in the previous section
> is to extend the Java syntax for the returned type in method signatures:
>
> ```java
> class Shape {
> public this scale(double ratio) { // <=== returns this
> // recalculate all points
> return this;
> }
> }
> ```
>
> Methods declared or defined as returning `this` can only return the
> instance on which they are called. The following code will be
> type-safe and perfectly valid:
>
> ```java
> var myRect = // inferred Rectangle type
> new Rectangle() // returns Rectangle instance
> .scale(1.2) // returns Rectangle instance
> .roundCorners(10); // returns Rectangle instance
> ```
>
> The constructed type `Rectangle` is preserved throughout the entire
> call chain.
>
> It is possible to override methods returning `this`, but the subclass'
> implementation must also be declared with the `this` keyword instead
> of a concrete returning type.
>
> It is even possible to remove the explicit return statement altogether:
>
> ```java
> class Shape {
> public this scale(double ratio) {
> // recalculate all points
> }
> }
> ```
>
> Or simply remove the value `this` from the return statement:
>
> ```java
> class Shape {
> public this scale(double ratio) {
> // recalculate all points
> if (condition) return; // <== automatically returns this
> // do something else
> }
> }
> ```
>
> In fact, methods returning `this` can be compiled to the same bytecode
> as methods returning `void`. This is because the instance reference
> (and the returned value) is already known to the caller, eliminating
> the need to pass that value back through the call stack. As a result,
> both CPU cycles and memory are saved.
>
> In the Java world, it is common to create getters and setters
> according to the Java Beans specification in the form of
> `getProperty`/`setProperty` pairs or `isProperty`/`setProperty`.
> Setters are defined as returning `void`. These setters can be more
> useful if defined as returning `this`:
>
> ```java
> class Customer {
> public this setFirstname() { ... }
> public this setSurname() { ... }
> public this setEmail() { ... }
> }
> ```
>
> This allows for more concise usage when constructing and configuring
> an instance without adding more code:
>
> ```java
> customers.add(
> new Customer()
> .setFirstname(resultSet.getString(1))
> .setSurname(resultSet.getString(2))
> .setEmail(resultSet.getString(3))
> );
> ```
>
> It is also possible to declare an interface with methods returning `this`:
>
> ```java
> interface Shape {
> this scale(double ratio);
> }
> ```
>
> In this case, all implementing classes must define the method as
> returning `this`.
>
> The proposed syntax is a bit less useful for enums or records, as
> neither of them allows for inheritance. But enums and records can also
> implement interfaces and for this reason and for overall consistency,
> "return this" syntax should be allowed for enums and records.
>
> To accommodate the syntax with the Java Reflection API, it will
> probably be required to create a special final placeholder class
> `This` (with an uppercase "T"), similar to `java.lang.Void`.
>
> Alternatives
> ------------
> It is probably possible to help auto-generate overriding methods in
> subclasses using annotation processing, but this option wasn't fully
> explored. However, such an approach would add extra unnecessary code
> to compiled subclasses and go against the primary goal of reducing
> boilerplate.
>
> Risks and Assumptions
> ---------------------
> The proposed syntax is likely to break the compatibility of
> library-dependent code whose author decides to switch to the "return
> this" syntax between versions.
>
> Older code that looks like this:
>
> ```java
> class MyUglyShape extends Shape {
> @Override
> public MyUglyShape scale(double ratio) {
> return this;
> }
> }
> ```
>
> will have to be rewritten as:
>
> ```java
> class MyUglyShape extends Shape {
> @Override
> public this scale(double ratio) { // signature change
> // optional removal of the return this statement
> }
> }
> ```
>
> or
>
> ```java
> class MyUglyShape extends Shape {
> // override removed
> }
> ```
>
> This problem can be mitigated with the help of smart IDEs
> automatically suggesting such refactoring.
>
> Another possible risk is breaking old code that relies on the
> Reflection API for scanning the returning types of methods.
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