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 interface
s 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:
-
interface
s 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.
-
interface
s do not permit overriding defaultsD'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
instruct
s orclass
es) but no non-polymorphicinterface
s for structsThis 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`.
-
interface
s can contain type definitions/declarations andalias
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.
-
interface
s 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.