Hello D community.

This post is about interface design in D.

First, I see good additions compared to what I know from other languages, mostly Java. Interfaces can be used to share function implementations across multiple classes. It's very great that D interfaces can have pre- and post-conditions. This solves the classic readability/comprehensibility problem of interfaces and helps to implement and use interfaces in the intended way instead of misusing interfaces in non-intended ways.

But I also see a few limitations compared with Java. This is not a feature request. I'd only like to know what you think about these 5 points or if you see design-reasons:

• interfaces don't permit runtime constants.

  Not limiting, only weird. Constants are stronly-pure niladic (no-arguments) functions and D syntactically doesn't differentiate between zero-argument function calls and variable accesses. I could define a static niladic function with constant return value in an `interface`. But the constant-member-syntax
  ```D
  static immutable myVar = 0;
  ```
  is easier to write.
  
  Constants are no state. The purpose of an interface is to describe API which is not supposed to change at runtime or during software evolution while not fixing behaviour and ideally not fixing future additions.
  

• interfaces do not permit overriding defaults

  D's interfaces don't allow default implementations of static and non-static functions unlike Java. This makes class implementations more verbose and promotes code duplication because you need to repeat the default implementation in every implementing class, even if you don't care.
  Java uses the `default` keyword as attribute to allow for overriding a default-implemented interface method.
  
  Default methods are most important for the ability to extend interfaces without requiring old classes to implement newly added methods and without introducing stateful behaviour.
  A partial workaround for the extensibility problem is the definition of a new interface which also contains the old one.
  
  If constant declaration syntax (as previously mentioned) would be added to `interface`s, then runtime constants with default value or without any value could be polymorphically overridable within `class`es (overridden with a different constant value). When they are overridable (e.g. using the `default` keyword, opposite to `abstract`), they could be syntactic sugar for niladic functions which return a constant value and **optionally** could do a one-time-computation of the return value when the static-constant-function is called the first time.
  
  Abstract classes are no replacement because, first, you cannot inherit multiple abstract classes and, second, abstract classes implement partial incomplete behaviour while interfaces don't implement behaviour.
  

• Non-polymorphic inheritance exists (alias this in structs or classes) but no non-polymorphic interfaces for structs

  This one is most meaningful.
  
  In my current project in D, I'm working on a low or medium-low level and it's not suitable to use classes (they also need to work in Better-C). I don't need polymorphy. I only like to guarantee a **consistent** interface among my `struct`s. It makes life of users easier and prohibits others from "inheriting" my struct properties in unintended ways.
  
  The current way of creating non-polymorphic `interface`s is cumbersome: create a `mixin template` which mainly instantiates a selfmade trait-template (a predicate) on `this` to check that the environment implements certain function signatures. If I want to use a non-polymorphic `interface` as a type, I'm using type parameters and I use the trait-predicate in the `if`-constraint of the templated entity.
  
  It's more difficult to read and it's more verbose.
  
  ---
  
  Sure, one could avoid explicit template notaton. A solution would be a `static interface` (compile-time-dynamically dispatched methods and constants) which represents a uniform function-layout and a traits-predicate that is generated from the `static interface` (or is at least simulated and implicitly accessible via `is(type : interfaceName)`). Any implementing types must satisfy this predicate.
  Static `interface`s behave this way:
  
  - When used as variable type, `static interfaces` would behave like `auto` and a compile-time assertion of the associated predicate. Any value can be assigned to such a variable as long as it satisfies the predicate of the `static interface`. Covariant types don't need to implement the `static interface` explicitly to satisfy the predicate.
  
  - When used as a function-parameter-type or member-type it would be lowered to an implicit template parameter which is checked against the generated predicate.
  
  A lot of manual traits-checks using `static assert`s and `if`-constraints could be simplified into just a typename, e.g. when using Ranges.
  
  ---
  
  A simpler variant would be a `mixin interface` (purely static dispatch of methods) which only defines required constants and functions to implement for a `class` or `struct` but which cannot be used as a type otherwise except if there is a (default) implementation of all functions. (No templates are created by using the type.) `typeof(this)` would be allowed in `mixin interface`. This essentially behaves like a `mixin template` enhanced with "abstract" functions that must be implemented by the implementing `class` or `struct`.
  

• interfaces can contain type definitions/declarations and alias and can be overloaded, even though it's not documented.

  Runtime-constants are not permitted but surprisingly compile-time constants are and `enum`-blocks and type definitions. They even can be overloaded but *without polymorphy* (dynamic dispatch). It seems this is not documented?
  
  But you should be cautious what to document:
  
  `alias`es can be useful if you'd want to change a function name without breaking old code. Probably there is a way to deprecate `alias`es with an annotation even.
  
  My personal experience however is, that `alias` declarations in `interface`s can be easily abused by using them throughout the entire code of a class, unrelated to the interface functions, which makes it very hard to find the declaration manually.
  
  Currently, one cannot do a lot against bad `alias`es in `interface`s because limiting the scope or use of `alias`-definitions in `interfaces` would be breaking old code. But since it's undocumented, worst case breakage is reduced (nobody said it's supported anyways, right?).
  
  
  BTW, I see benefits in enabling the *pimpl pattern* within interfaces.
  
  ```D
  interface DoodadObtainable {
      class Doodad;    // opaque type of the pimpl pattern
      Doodad obtainDoodad();
      void releaseDoodad(Doodad);
  }
  
  class GadgetGizmo : DoodadObtainable
  {
      class Doodad
      {
          int foo;
          this(int f) { this.foo = f; }
      }
  
      Doodad experienceCounter;
  
      Doodad obtainDoodad()
      {
          return experienceCounter = new Doodad(13);
      }
      void doDoodadThings()
      in (experienceCounter)
      {
          experienceCounter.foo++;
      }
      void releaseDoodad(Doodad d)
      {
          d.destroy();
      }
  }
  ```
  
  An interface-implementation would then implement the declared opaque type from the interface. The opaque type is overriden polymorphically and internally represented by a generic type like `void*` for classes or `void[]...` for structs. Since the type-size is unknown, it wouldn't allow opaque value-types as direct function-parameters, e.g. mere `struct`s; except when they are treated like variadic arguments plus an implicit byte size parameter for copying the data.
  
  A default implementation of an opaque type declarations is imaginable (with `default`) but without default implementation (or: without an abstract constructor method in the `Doodad` class example), opaque types cannot be instantiated outside of the implementation class.
  
  This pimpl pattern is a nice way to avoid template code bloat and to avoid recompilation when something changes in opaque types.
  

• interfaces can contain classes which violate the concept of an interface.

  IMO, this is rather a limitation in persuing the purpose of an `interface`. It can contain a `class` whose behaviour specification defies the purpose of an `interface`. For Java it's the same. If `interface` methods use specific classes, they also can be defined outside of the interface, right? If you need cohesion put them into the same file and make them private. I usually use `template`s for controlling visibility and cohesion and to avoid ugly nestings of definitions:
  
  ```D
  template MyInterface() {
      class MyParameter {
          int something;
      }
      interface MyInterface {
          interface MyInterfaceClass {  // body behaves like an interface itself
              void tell(string);
              string listen();
          }
          MyInterfaceClass do(MyParameter something);
      }
  }
  ```
  
  *Just very annoying: you'd have to write `MyInterface!()` everytime. It would be very useful, if you could omit the `!()` if you'd like to pass zero compile-time arguments to it.*
  
  And you can use `import` statements in `interface`s, if it's located in another file.
  
  For me it seems, classes or structs should not be in an `interface` except when it is a default implementation that can be overridden **polymorphically**.
  
  Making rules more strict is not something, anyone would be able to change without potentially breaking old code.
  

On Sunday, 19 September 2021 at 20:00:11 UTC, Elmar wrote:

You can use mixin template that will contain default implementation of an interface.

You can also declare final functions in interfaces, which can partially serve for first case mentioned.

Best regards,
Alexandru

On Sunday, 19 September 2021 at 20:00:11 UTC, Elmar wrote:

Sounds like what concepts package does. Unfortunately, doesn’t work with BetterC.

On Monday, 20 September 2021 at 08:32:46 UTC, Alexandru Ermicioi wrote:

Thanks, Alexandru. Final functions won't do it however, if you'd like to override the given implementation in classes.

I want to add: after writing the post I found phobos-functions autoImplement, blackHole and whiteHole in std.typecons which automatically implement interface methods in some way.

Providing default implementation support seems not more difficult:

mixin Interface!("MyInterface",
q{
    static immutable PI = 3.141592f;
    default static immutable CONSTANT = 0.0f;

    float toImplement(string s);

    default float isDefaulted()
    {
        return (CONSTANT ^^ PI) % toImplement(r"tau"));
    }
});

Providing default method implementations without code duplication and too much verbosity could work by defining the default method implementations as compile-time token strings. The above would generate

interface MyInterface
{
    final static immutable PI()
    {
        return 3.141592f;
    }
    static immutable CONSTANT();
    enum DEFAULT_CONSTANT = q{{
        return 0.0f;
    }};

    float toImplement(string s);

    float isDefaulted();
    enum DEFAULT_isDefaulted =
    q{{
        return (CONSTANT ^^ PI) % toImplement(r"tau"));
    }};
}

Then people can either mixin (DEFAULT_foo) explicitly (in the class implementation) or use something like mixin DefaultImplement!MyInterface; to automatically implement missing DEFAULT_... token strings.

The solution is artificial but a limited simplistic implementation could use (compile-time) regex for this.

RegEx:
I don't know, whether D's engine would support matching balanced braces but my feeling assumes no. And the documentation description confirms myself. Simplicity over expressivity makes it harder to do the job with Regex and which might be disappointing for programmers from the scripting realm. I'd find a PCRE engine more practical. Worst-Case Performance does not matter if you know how to avoid the worst-case.

On 9/19/21 4:00 PM, Elmar wrote:

Not sure what you are saying here.

this code is valid:

interface I
{
  static immutable myVar = 0;
}

-Steve

On Monday, 20 September 2021 at 11:45:06 UTC, Ogi wrote:

Nice suggestion! @implements looks like solving most parts of my current problem.
But of course, BetterC is a key feature of D and not supporting it (even the standard library) reduces the usability of BetterC.

Although they provide most of the functionality, they don't provide the syntax for it which I find important, more important than compiler message goals stated by the "Concept" package. When I first saw signatures for ranges, I found the long-ish if-constraints unpleasantly verbose for something which really should look just like:

size_t count(InputRange r)
{

}

instead of

size_t count(Range)(Range r)
    if (isInputRange!Range)
{

}

But of course, the main point is the functionality: being able to constraint struct implementations like with an interface for classes. The "Concept" package makes this easier (even though the predicate is still manually defined there).

Even more pleasant would be a type which is treated as interface when applicable and fallbacks to a template otherwise which statically asserts certain members. It would optimize away a lot of template bloat in those cases where it's not required, like with class-types.

bool createWith(out MyObject obj, Allocator alloc)
{

}

The GCAllocator doesn't implement an interface but some can implement IAllocator. It would be useful to fallback to a template for types which provide basic Allocator methods but don't implement IAllocator.

Do only I think, that interface-like type names for some trait-predicates could improve the readability a lot? One could get rid of repetitively defining these predicates manually.

On Monday, 20 September 2021 at 13:35:35 UTC, Steven Schveighoffer wrote:

interface I
{
  static immutable myVar = 0;
}

Wow :-D ! This makes me laugh about my own dumbness. static members in interfaces work.

Is there a way how I can help with documentation?

On Monday, 20 September 2021 at 15:41:53 UTC, Elmar wrote:

There's an "Improve this page" link in the top right of every documentation page if it's a smaller change. For extensive changes, you can fork the repo and make a pull request with your proposed changes:

https://github.com/dlang/dlang.org

On Monday, 20 September 2021 at 13:24:50 UTC, Elmar wrote:

mixin Interface!("MyInterface",
q{
    static immutable PI = 3.141592f;
    default static immutable CONSTANT = 0.0f;

    float toImplement(string s);

    default float isDefaulted()
    {
        return (CONSTANT ^^ PI) % toImplement(r"tau"));
    }
});

You can actually try use mixin templates, not just mixin strings. See https://dlang.org/spec/template-mixin.html .

It has nicer syntax although not as powerful as mixin strings. You can also mix in the template into a dedicated scope inside implementor of interface:

mixin MyDefaultImpl!() myScope;

I didn't try it, but you may be able to mimic default methods using this feature, in conjunction with alias this expression, although I'm pretty skeptic about this due to mangled name generation (i.e haven't tried it).

So in your class you may try and explore whether this works:

mixin MyDefaultImpl!() def;

alias this def;

On Monday, 20 September 2021 at 11:45:06 UTC, Ogi wrote:

What about compile time overhead? Is it lightweight on compilation time and memory?

Thanks,
Alexandru.