I want to imitate golang's interface in D, to study D's
template. I wrote
some code: https://gist.github.com/3123593
Now we can write code like golang:
--
interface IFoo {
void foo(int a, string b, float c);
}
struct Foo {
void foo(int a, string b, float c) {
writeln("Foo.foo: ", a, ", ", b, ", ", c);
}
}
struct FooFoo {
void foo(int a, string b, float c) {
writeln("FooFoo.foo: ", a, ", ", b, ", ", c);
}
}
GoInterface!(IFoo) f = new Foo;
f.foo(3, "abc", 2.2);
f = new FooFoo;
f.foo(5, "def", 7.7);
--
It is also very naive, does not support some features, like
out/ref
parameters, free functions *[1]* and so on. The biggest problem
is downcast
not supported. In golang, we can write code like*[2]*:
--
var p IWriter = NewB(10)
p2, ok := p.(IReadWriter)
--
Seems [p.(IReadWriter)] dynamically build a virtual table
*[3]*，because the
type of "p" is IWriter, it is *smaller* than IReadWriter, the
cast
operation must search methods and build vtbl at run time.
In D, GoInterface(T).opAssign!(V)(V v) can build a rich runtime
information
to *V* if we need. But if *V* is interface or base class, the
type
information not complete. So, seems like I need runtime
reflection? and how
can I do this in D? I did not find any useful information in
the TypeInfo*.
------
[1] free functions support, e.g.
--
interface IFoo {
void foo(int a, string b, float c);
}
void foo(int self, int a, string b, float c) {
writefln("...");
}
GoInterface!(int) p = 1;
p.foo(4, "ccc", 6.6);
--
In theory no problem.
I, too, was enamored with Go Interfaces and implemented them for
.NET:
http://www.codeproject.com/Articles/87991/Dynamic-interfaces-in-any-NET-language
And I wasn't the only one; later, someone else published another
library for .NET with the exact same goal. This is definitely a
feature I would want to see in D, preferably as a first-class
feature, although sadly that would break any code that relies on
ISomething being pointer-sized; Go uses fat pointers, and we use
a thin-pointer implementation in .NET but it's inefficient (as
every cast creates a heap-allocated wrapper, and
double-indirection is needed to reach the real method.)
Anyway, they say it's possible to build runtime reflection in D
but I've no idea how... has it never been done before?
Of course, runtime template instantiation won't be possible.
Therefore, run-time casting will have to be more limited than
compile-time casting.
Reflection to free functions would be really nice, but it might
be less capable at run-time. Consider if you there is a class A
in third-party module MA that you want to cast to interface I,
but class A is missing a function F() from I. So in your module
(module MB) you define a free function F(B) and now you can do
the cast. I guess realistically this can only happen at
compile-time, since a run-time cast would naturally only look in
module MA, not MB, for functions it could use to perform the
cast. Presumably, it also requires that MA requested a run-time
reflection table to be built, and is it possible to build a
reflection table for a module over which you have no control?
