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digitalmars.D - Need runtime reflection?

reply lijie <cpunion gmail.com> writes:
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 =3D new Foo;
f.foo(3, "abc", 2.2);

f =3D 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 =3D NewB(10)
p2, ok :=3D p.(IReadWriter)
--

Seems [p.(IReadWriter)] dynamically build a virtual table *[3]*=EF=BC=8Cbec=
ause 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 =3D 1;
p.foo(4, "ccc", 6.6);
--
In theory no problem.

[2] example from
https://github.com/xushiwei/gobook/blob/master/dive-into/interface/03/inter=
face.go
[3] /path/of/go/src/pkg/runtime/iface.c: static Itab* itab(InterfaceType
*inter, Type *type, int32 canfail)


Best regards,

-- Li Jie
Jul 16 2012
parent reply "David Piepgrass" <qwertie256 gmail.com> writes:
 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?
Jul 17 2012
next sibling parent reply lijie <cpunion gmail.com> writes:
On Tue, Jul 17, 2012 at 11:08 PM, David Piepgrass <qwertie256 gmail.com>wrote:
 I, too, was enamored with Go Interfaces and implemented them for .NET:

 http://www.codeproject.com/**Articles/87991/Dynamic-**
 interfaces-in-any-NET-language<http://www.codeproject.com/Articles/87991/Dynamic-interfaces-in-any-NET-language>

Interesting, good article and project, thanks. 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.)

D has powerful template and CTFE, seems we have more choices, fat pointer or thin pointer. I didn't think seriously about the performance differences. About every cast creates a heap-allocated wrapper, I think we can try to use lower cost way in D. In my code: -- void opAssign(V)(GoInterface!(V) v) if (is(V == interface)) { //pragma(msg, "assign for GoInterface"); static if (isImplicitlyConvertible!(V, T)) { //pragma(msg, V.stringof ~ " can implicitly convert to " ~ T.stringof); m_impl = v.m_impl; } else static if (isImplicitlyConvertible!(T, V)) { //pragma(msg, T.stringof ~ " can implicitly convert to " ~ V.stringof ~ ", try dynamic cast"); if (v.m_impl is null) { m_impl = null; } else { m_impl = cast(T)(v.m_impl); if (m_impl is null) { // dynamic cast failed, try dynamic proxy m_impl = buildDynamicProxy!(V)(v); } } } else { //pragma(msg, "cannot implicitly between " ~ V.stringof ~ " and " ~ T.stringof); static if (isInterfaceConvertible!(V, T)) { //pragma(msg, "generate static proxy to convert " ~ V.stringof ~ " to " ~ T.stringof); m_impl = new StaticProxy!(V)(v.m_impl); } else { //pragma(msg, V.stringof ~ " not compatible " ~ T.stringof ~ ", must dynamic build call proxy"); m_impl = buildDynamicProxy!(V)(v); } } } -- Some cases we can directly do assignment, it also can be optimized to reduce heap allocation.
 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?

Free functions support is hard, with runtime cast, I just think compile time. I am trying to support free functions at compile time, is also hard, since the generator in gointerface module, but other modules are only visible in the module that used these modules. I have an ugly implementation used compile time string mixin, trying to simplify it. Fighting.
Jul 17 2012
parent reply Jacob Carlborg <doob me.com> writes:
On 2012-07-18 06:10, lijie wrote:

 Free functions support is hard, with runtime cast, I just think compile
 time.

It's possibly to implement runtime reflection by loading the running executable and inspecting the symbol table. It's an ugly hack but it should work. http://flectioned.kuehne.cn/
 I am trying to support free functions at compile time, is also hard,
 since the generator in gointerface module, but other modules are only
 visible in the module that used these modules. I have an ugly
 implementation used compile time string mixin, trying to simplify it.
 Fighting.

I think you can pass a module to a template via an alias parameter. Then the template should be able to inspect all free functions using something like __traits(allMembers). -- /Jacob Carlborg
Jul 17 2012
next sibling parent reply lijie <cpunion gmail.com> writes:
On Wed, Jul 18, 2012 at 2:59 PM, Jacob Carlborg <doob me.com> wrote:

 On 2012-07-18 06:10, lijie wrote:

  Free functions support is hard, with runtime cast, I just think compile
 time.

It's possibly to implement runtime reflection by loading the running executable and inspecting the symbol table. It's an ugly hack but it should work. http://flectioned.kuehne.cn/

It is an optional way. I want to do all thing in D code.
  I am trying to support free functions at compile time, is also hard,
 since the generator in gointerface module, but other modules are only
 visible in the module that used these modules. I have an ugly
 implementation used compile time string mixin, trying to simplify it.
 Fighting.

I think you can pass a module to a template via an alias parameter. Then the template should be able to inspect all free functions using something like __traits(allMembers).

I didn't know this usage, I will try it. Thanks.
Jul 18 2012
parent Jacob Carlborg <doob me.com> writes:
On 2012-07-18 09:26, lijie wrote:

 It is an optional way. I want to do all thing in D code.

You can do this in D. -- /Jacob Carlborg
Jul 18 2012
prev sibling parent reply lijie <cpunion gmail.com> writes:
On Wed, Jul 18, 2012 at 2:59 PM, Jacob Carlborg <doob me.com> wrote:

 I think you can pass a module to a template via an alias parameter. Then
 the template should be able to inspect all free functions using something
 like __traits(allMembers).

Have a problem. -- // file: A.d module A; // functions and classes -- -- // file: testtraits.d import std.stdio; import A; void main() { writeln(__traits(allMembers, A)); } -- There is a compilation error: testtraits.d(6): Error: import A has no members If module is under a package name, it is OK, is that a bug?
Jul 18 2012
parent Jacob Carlborg <doob me.com> writes:
On 2012-07-18 17:10, lijie wrote:
 On Wed, Jul 18, 2012 at 2:59 PM, Jacob Carlborg <doob me.com
 <mailto:doob me.com>> wrote:

     I think you can pass a module to a template via an alias parameter.
     Then the template should be able to inspect all free functions using
     something like __traits(allMembers).


 Have a problem.

 --
 // file: A.d

 module A;

 // functions and classes
 --

 --
 // file: testtraits.d

 import std.stdio;
 import A;

 void main() {
    writeln(__traits(allMembers, A));
 }
 --

 There is a compilation error:
 testtraits.d(6): Error: import A has no members

 If module is under a package name, it is OK, is that a bug?

Seems like it. http://d.puremagic.com/issues/ -- /Jacob Carlborg
Jul 18 2012
prev sibling parent "Paulo Pinto" <pjmlp progtools.org> writes:
On Tuesday, 17 July 2012 at 15:08:18 UTC, David Piepgrass wrote:
 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

Interesting article, but why not just make use of "dynamic" interfaces? -- Paulo
Jul 17 2012