Method Chaining (enhancing Java syntax)
Tomáš Bleša
blesa at anneca.cz
Tue Jun 13 17:17:19 UTC 2023
Hi,
thank you for comments and references. I didn’t mean we should create special self-type. The proposed syntax:
public this hello() {}
doesn’t mean “returns an object of the same type as my class”.
but rather
“returns the same instance it is called on”. The fact that it is also the same type is a useful byproduct. Please note that I used lowercase T to emphasize it is the instance not type.
Under the hood nothing has to be returned (on the call stack) and the method should compile to:
public void hello() {}
That’s why using this (ThisClass/MyClass) in method arguments or other places doesn’t make sense to me. (Passing the same instance to method which already has this reference.) It also doesn’t make sense (or little) to use it on static methods like:
public static this hello();
Ad Generics) As you pointed out it doesn’t work well with deep class hierarchy and also reduces readability (adds boilerplate). Imagine for example big library representing UI (like DOM structure):
UIElement // method css(…) here
UILayout
UIGridLayout
…
UIFormElement
UISelect
UIMultiSelect… // has method maxSelection()
// compilation error
var mySelect = new UIMultiSelect().css(„color”, „green”)
.addClass(„attention”).maxSelection(3);
This is basically my use-case. I have 60..100 subclasses.
Ad "Is the feature important enough?")
Technically all language features are just “a syntactic sugar” to the assembler (bytecode). I lived many years without switch-expressions, try-with-resource,… :-)
All the best,
Tomas
> 13. 6. 2023 v 12:51, Maurizio Cimadamore <maurizio.cimadamore at oracle.com>:
>
> 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 <https://link.springer.com/chapter/10.1007/978-3-540-24851-4_18>
> [2] - https://www.sciencedirect.com/science/article/pii/S0167642313000038 <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 <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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