Re: Default Implementation For an Interface
On Wed, 15 Feb 2012 22:01:51 -0500, Kevin kevincox...@gmail.com wrote:
I was implementing a framework and I found that I wanted two things.
- A strong set of interfaces so that I can get what I want from a
variety of sources.
- Some basic implementations of these interfaces.
For example, say I was writing a database class. I could either name
the interface Database and call the class DatabaseImplementation or
something but that is ugly. If I call the interface IDatabase, the
Database class looks nice but I need to convince users to write
functions that take IDatabases not Databases.
I was wondering if there was any way to implement a default
implementation. This way, I could create my Database interface and
classes could implement that but if you called `new Database()` you
would still get a basic database.
Aside from what has been said already, if you wish to have methods that
are not static defined in the interface, final methods currently work:
interface I
{
void foo();
final void callFoo() {writeln(about to call foo); foo();
writeln(ok, I called foo);}
}
This isn't exactly a default implementation, since you can't override it.
Note that template methods are supposed to work (And also are implicitly
final), but this doesn't currently work.
http://d.puremagic.com/issues/show_bug.cgi?id=4174
-Steve
Re: Default Implementation For an Interface
I wasn't looking to implement meathods in the interface, I was looking to
have a default class that implements the interface that would be created if
you called `new Interface();` I don't think this is possible. and now that
I think about it I think that it is for a good reason.
On Fri, Feb 17, 2012 at 12:21 PM, Jacob Carlborg d...@me.com wrote:
On 2012-02-17 18:04, Steven Schveighoffer wrote:
On Wed, 15 Feb 2012 22:01:51 -0500, Kevin kevincox...@gmail.com wrote:
I was implementing a framework and I found that I wanted two things.
- A strong set of interfaces so that I can get what I want from a
variety of sources.
- Some basic implementations of these interfaces.
For example, say I was writing a database class. I could either name
the interface Database and call the class DatabaseImplementation or
something but that is ugly. If I call the interface IDatabase, the
Database class looks nice but I need to convince users to write
functions that take IDatabases not Databases.
I was wondering if there was any way to implement a default
implementation. This way, I could create my Database interface and
classes could implement that but if you called `new Database()` you
would still get a basic database.
Aside from what has been said already, if you wish to have methods that
are not static defined in the interface, final methods currently work:
interface I
{
void foo();
final void callFoo() {writeln(about to call foo); foo(); writeln(ok,
I called foo);}
}
This isn't exactly a default implementation, since you can't override
it.
But you could have one final method, the implementation and one virtual,
the one you would override. It's an idea, I don't know if it's a good one.
--
/Jacob Carlborg
Re: Default Implementation For an Interface
On 02/16/2012 04:01 AM, Kevin wrote:
I was implementing a framework and I found that I wanted two things.
- A strong set of interfaces so that I can get what I want from a
variety of sources.
- Some basic implementations of these interfaces.
For example, say I was writing a database class. I could either name the
interface Database and call the class DatabaseImplementation or
something but that is ugly. If I call the interface IDatabase, the
Database class looks nice but I need to convince users to write
functions that take IDatabases not Databases.
I was wondering if there was any way to implement a default
implementation. This way, I could create my Database interface and
classes could implement that but if you called `new Database()` you
would still get a basic database.
I thought about doing it in different modules but it just gets messier
as you have to fully qualify all of the names so both look ugly. I
though about overloading the new operator for the interface but it would
have to be final and that would mess everything up. I though about a
meathod like `Database.newDefault()` but that is messy and has no
meaning in a derived class.
I couldn't find anything about this so I was wondering what you would
recommend. Should I just pick a naming scheme?
As a user (read developer), I'd rather code to the generic interface
when possible. I like that concrete implementations looks rather long
and ugly :)
I don't think you should be worried that your users is using direct
implementations rather than the interface - their problem!
Remember that in D, interfaces can contain implementations that only
uses static methods on the interface:
interface DB {
@property string name();
// interfaces can have implementations
static DB createDefault() { return new GenericDB(); }
}
class GenericDB : DB {
@property string name() {
return generic; }
}
class MySQLDB : DB {
@property string name() {
return mysql; }
}
void main() {
assert(DB.createDefault().name == generic);
assert((new MySQLDB()).name == mysql);
}
Re: Default Implementation For an Interface
On 2012-02-16 04:01, Kevin wrote:
I was implementing a framework and I found that I wanted two things.
- A strong set of interfaces so that I can get what I want from a
variety of sources.
- Some basic implementations of these interfaces.
For example, say I was writing a database class. I could either name the
interface Database and call the class DatabaseImplementation or
something but that is ugly. If I call the interface IDatabase, the
Database class looks nice but I need to convince users to write
functions that take IDatabases not Databases.
I was wondering if there was any way to implement a default
implementation. This way, I could create my Database interface and
classes could implement that but if you called `new Database()` you
would still get a basic database.
You can create an abstract class that implements some parts of the
interface. Then the user (developer) is free to choose to inherit from
the interface or the abstract class.
I thought about doing it in different modules but it just gets messier
as you have to fully qualify all of the names so both look ugly. I
though about overloading the new operator for the interface but it would
have to be final and that would mess everything up. I though about a
meathod like `Database.newDefault()` but that is messy and has no
meaning in a derived class.
I couldn't find anything about this so I was wondering what you would
recommend. Should I just pick a naming scheme?
About the naming scheme I would go for this:
interface Database {}
abstract AbstractDatabase : Database {}
class Implementation : Database {}
class Implementation2 : AbstractDatabase {}
Possibly suffix the implementations with Database.
class ImplementationDatabase : Database {}
class Implementation2Database : AbstractDatabase {}
But it feels a bit redundant with Database in the implementation name.
--
/Jacob Carlborg
Re: Default Implementation For an Interface
On Thursday, February 16, 2012 11:11:20 Jacob Carlborg wrote: You can create an abstract class that implements some parts of the interface. Then the user (developer) is free to choose to inherit from the interface or the abstract class. Which results in a classic problem that you run into in Java all the time when dealing with event-based programming (since it deals with events via interfaces). If you have a class that only really needs to implement a couple of the functions from the interface of an event listener, then you can derive from the class which implements it and gives them all empty bodies. But if you need your class to implement multiple such interfaces, you can only do that with one of them, which gets really annoying, because then you have to create a bunch of empty method bodies yourself. It's one of the classic examples where multiple inheritance would be desirable. The current situation in D is exactly the same (though, since we don't have a swing equivalent, I don't think that it's something that D programmers are frequently running into at the moment). AIUI, Java is going to be adding the ability to give interfaces default implementations such that that implementation is effectively copy-pasted into your class when you implement it and don't provide an implementation yourself (rather than the function in your class overidding it as would be the case with an abstract class). This nicely solves the event listener problem, and D doesn't have that. I assume that that's the sort of thing that the OP is looking for. Now, if you use template mixins, I believe that it's possible to use that to mixin default implementations for the functions in an interface, which should solve the problem for D. So, that's probably good enough for D without having to make it so that interface functions can have default implementations. - Jonathan M Davis
Re: Default Implementation For an Interface
On Thursday, 16 February 2012 at 10:24:44 UTC, Jonathan M Davis
wrote:
On Thursday, February 16, 2012 11:11:20 Jacob Carlborg wrote:
You can create an abstract class that implements some parts of
the
interface. Then the user (developer) is free to choose to
inherit from
the interface or the abstract class.
Which results in a classic problem that you run into in Java
all the time when dealing with event-based programming (since
it deals with events via interfaces). If you have a class that
only really needs to implement a couple of the functions from
the interface of an event listener, then you can derive from
the class which implements it and gives them all empty bodies.
But if you need your class to implement multiple such
interfaces, you can only do that with one of them, which gets
really annoying, because then you have to create a bunch of
empty method bodies yourself. It's one of the classic examples
where multiple inheritance would be desirable.
The current situation in D is exactly the same (though, since
we don't have a swing equivalent, I don't think that it's
something that D programmers are frequently running into at the
moment). AIUI, Java is going to be adding the ability to give
interfaces default implementations such that that
implementation is effectively copy-pasted into your class when
you implement it and don't provide an implementation yourself
(rather than the function in your class overidding it as would
be the case with an abstract class). This nicely solves the
event listener problem, and D doesn't have that. I assume that
that's the sort of thing that the OP is looking for.
Now, if you use template mixins, I believe that it's possible
to use that to mixin default implementations for the functions
in an interface, which should solve the problem for D. So,
that's probably good enough for D without having to make it so
that interface functions can have default implementations.
- Jonathan M Davis
BlackHole from std.typeconst can be used for that purpose.
import std.typecons;
interface A
{
void a();
int b(void* arg);
}
interface B
{
int c(string arg);
}
interface Common : A, B
{
}
class Good : BlackHole!Common
{
override int b(void* arg)
{
return 0;
}
}
void main()
{
auto g = new Good;
g.a();
auto i = g.b(null);
auto j = g.c(Hello);
}
Re: Default Implementation For an Interface
On 2012-02-16 11:23, Jonathan M Davis wrote: On Thursday, February 16, 2012 11:11:20 Jacob Carlborg wrote: You can create an abstract class that implements some parts of the interface. Then the user (developer) is free to choose to inherit from the interface or the abstract class. Which results in a classic problem that you run into in Java all the time when dealing with event-based programming (since it deals with events via interfaces). If you have a class that only really needs to implement a couple of the functions from the interface of an event listener, then you can derive from the class which implements it and gives them all empty bodies. But if you need your class to implement multiple such interfaces, you can only do that with one of them, which gets really annoying, because then you have to create a bunch of empty method bodies yourself. It's one of the classic examples where multiple inheritance would be desirable. Interfaces and abstract classes is the simple solution. If the class hierarchy is quite simple won't be a problem. It looks like it is quite simple in this case. Or one could skip the interface completely perhaps. The current situation in D is exactly the same (though, since we don't have a swing equivalent, I don't think that it's something that D programmers are frequently running into at the moment). AIUI, Java is going to be adding the ability to give interfaces default implementations such that that implementation is effectively copy-pasted into your class when you implement it and don't provide an implementation yourself (rather than the function in your class overidding it as would be the case with an abstract class). This nicely solves the event listener problem, and D doesn't have that. I assume that that's the sort of thing that the OP is looking for. Since D have delegates I would use those for event handling and not listeners. I think they are a much better fit, as long as you don't have to force the user to handle many different events on the same object. Now, if you use template mixins, I believe that it's possible to use that to mixin default implementations for the functions in an interface, which should solve the problem for D. So, that's probably good enough for D without having to make it so that interface functions can have default implementations. - Jonathan M Davis Template mixins cause their own problems. You can't overload methods with template mixins, may it's possible to get around that with aliases, I don't remember. -- /Jacob Carlborg
Re: Re: Default Implementation For an Interface
As a user (read developer), I'd rather code to the generic interface
when possible. I like that concrete implementations looks rather long
and ugly
I don't think you should be worried that your users is using direct
implementations rather than the interface - their problem!
Remember that in D, interfaces can contain implementations that only
uses static methods on the interface:
interface DB {
@property string name();
// interfaces can have implementations
static DB createDefault() { return new GenericDB(); }
}
class GenericDB : DB {
@property string name() {
return generic; }
}
class MySQLDB : DB {
@property string name() {
return mysql; }
}
void main() {
assert(DB.createDefault().name == generic);
assert((new MySQLDB()).name == mysql);
}
I see what you are saying. If it is not possible to have a default I
think I will call the interface Database and the class BasicDatabase as
that sounds intuitive.
Thanks to everyone for the help.
Re: Default Implementation For an Interface
On Thursday, February 16, 2012 13:26:59 Jacob Carlborg wrote: Since D have delegates I would use those for event handling and not listeners. I think they are a much better fit, as long as you don't have to force the user to handle many different events on the same object. Oh, I'm not necessarily arguing that using interfaces far listeners is the way to go (in fact, I agree that delegates would be much better). It's just that that's a prime example of a situation where you want default implementations for interface methods, since with single inheritance, you can't derive a class from multiple classes which give you default implementations for interface methods. Now, if you use template mixins, I believe that it's possible to use that to mixin default implementations for the functions in an interface, which should solve the problem for D. So, that's probably good enough for D without having to make it so that interface functions can have default implementations. Template mixins cause their own problems. You can't overload methods with template mixins, may it's possible to get around that with aliases, I don't remember. I thought that you could, since they can be virtual, unlike templated functions. I don't know though. It's not something that I've really had to worry about - particularly since so few of my D programs need classes, let alone interfaces (because most of my D programs are small). - Jonathan M Davis
Re: Default Implementation For an Interface
On 2012-02-16 20:05, Jonathan M Davis wrote: On Thursday, February 16, 2012 13:26:59 Jacob Carlborg wrote: Since D have delegates I would use those for event handling and not listeners. I think they are a much better fit, as long as you don't have to force the user to handle many different events on the same object. Oh, I'm not necessarily arguing that using interfaces far listeners is the way to go (in fact, I agree that delegates would be much better). It's just that that's a prime example of a situation where you want default implementations for interface methods, since with single inheritance, you can't derive a class from multiple classes which give you default implementations for interface methods. Ok, I see. I got the impression that you preferred listeners. Now, if you use template mixins, I believe that it's possible to use that to mixin default implementations for the functions in an interface, which should solve the problem for D. So, that's probably good enough for D without having to make it so that interface functions can have default implementations. Template mixins cause their own problems. You can't overload methods with template mixins, may it's possible to get around that with aliases, I don't remember. I thought that you could, since they can be virtual, unlike templated functions. I don't know though. It's not something that I've really had to worry about - particularly since so few of my D programs need classes, let alone interfaces (because most of my D programs are small). - Jonathan M Davis Note that I'm saying overload not override. -- /Jacob Carlborg
Re: Default Implementation For an Interface
On Thursday, February 16, 2012 20:17:22 Jacob Carlborg wrote: Note that I'm saying overload not override. Ah, so you did. Yes, that would probably be a problem, though aliases can probably fix it (that's how you deal with having all of the overloads for a function in the same overload set for a derived class when you only override some of them). But I haven't tried it, so I don't know. - Jonathan M Davis
Re: Default Implementation For an Interface
On 2012-02-16 22:08, Jonathan M Davis wrote: On Thursday, February 16, 2012 20:17:22 Jacob Carlborg wrote: Note that I'm saying overload not override. Ah, so you did. Yes, that would probably be a problem, though aliases can probably fix it (that's how you deal with having all of the overloads for a function in the same overload set for a derived class when you only override some of them). But I haven't tried it, so I don't know. - Jonathan M Davis I know it works with derived classes but I haven't tried it with template mixins. -- /Jacob Carlborg
