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digitalmars.D.learn - Self-referencing template parameters

reply Erik Rasmussen <i_am_erik yahoo.com> writes:
One way that I have found java's generics to be useful is in doing 
something like this:

---
public abstract class Animal<T extends Animal<T>>
{
   public abstract T[] procreate(T mate);
}

public class Monkey extends Animal<Monkey>
{
   public Monkey[] procreate(Monkey mate)
   {
      // make sweet monkey love
   }
}
---

Although you're not strictly forced to by the compiler, if you make it a 
policy to always make your template variable the same as your concrete 
subclass, then you have forced any class that extends Animal to provide 
a way to "procreate" with its own kind.

I seem to be unable to implement this pattern in D.  The following 
doesn't work:

---
abstract class Animal(T : Animal!(T))
{
   abstract T[] procreate(T mate);
}
---

It doesn't like that template definition on the first line.  It says, 
"template instance does not match any template declaration".

Any ideas?

Cheers,
Erik
Mar 20 2006
parent reply Oskar Linde <oskar.lindeREM OVEgmail.com> writes:
Erik Rasmussen skrev:

 One way that I have found java's generics to be useful is in doing 
 something like this:
 
 ---
 public abstract class Animal<T extends Animal<T>>
 {
   public abstract T[] procreate(T mate);
 }
 
 public class Monkey extends Animal<Monkey>
 {
   public Monkey[] procreate(Monkey mate)
   {
      // make sweet monkey love
   }
 }
 ---
 
 Although you're not strictly forced to by the compiler, if you make it a 
 policy to always make your template variable the same as your concrete 
 subclass, then you have forced any class that extends Animal to provide 
 a way to "procreate" with its own kind.
 
 I seem to be unable to implement this pattern in D.  The following 
 doesn't work:
 
 ---
 abstract class Animal(T : Animal!(T))
 {
   abstract T[] procreate(T mate);
 }
 ---
 
 It doesn't like that template definition on the first line.  It says, 
 "template instance does not match any template declaration".
 
 Any ideas?



The problem is that this line contains a recursive instantiation:
abstract class Animal(T : Animal!(T))

Animal!(T) has to be instantiated before the compiler knows how to what 
type it is.

But if you place a static assert in a place outside what the compiler 
needs to deduce the type:

class Animal(T) {
	this() {
		static assert(is(T : Animal));
	}
	public abstract T[] procreate(T mate);
}

Things work as they should.


/Oskar
Mar 20 2006
parent reply Erik Rasmussen <i_am_erik yahoo.com> writes:
Oskar Linde wrote:

 The problem is that this line contains a recursive instantiation:
 abstract class Animal(T : Animal!(T))
 
 Animal!(T) has to be instantiated before the compiler knows how to what 
 type it is.
 
 But if you place a static assert in a place outside what the compiler 
 needs to deduce the type:
 
 class Animal(T) {
     this() {
         static assert(is(T : Animal));
     }
     public abstract T[] procreate(T mate);
 }
 
 Things work as they should.
 
 
 /Oskar



Thanks, but...

test.d
---
abstract class Animal(T)
{
   this()
   {
     static assert(is(T : Animal));
   }

   public abstract T[] procreate(T mate);
}

class Monkey : Animal!(Monkey)
{
   public Monkey[] procreate(Monkey mate)
   {
     return new Monkey[2];
   }
}

int main(char[][] args)
{
   new Monkey();
}
---

When I compile, I get:
test.d: variable test.Animal!(Monkey).Animal.this.this abstract cannot 
be applied to variable
test.d:3: variable test.Animal!(Monkey).Animal.this.__result abstract 
cannot be applied to variable

Now what?  Line 3 is the "this()" line.

Erik
Mar 20 2006
parent Erik Rasmussen <i_am_erik yahoo.com> writes:
Erik Rasmussen wrote:

 Oskar Linde wrote:
 

 The problem is that this line contains a recursive instantiation:
 abstract class Animal(T : Animal!(T))

 Animal!(T) has to be instantiated before the compiler knows how to 
 what type it is.

 But if you place a static assert in a place outside what the compiler 
 needs to deduce the type:

 class Animal(T) {
     this() {
         static assert(is(T : Animal));
     }
     public abstract T[] procreate(T mate);
 }

 Things work as they should.


 /Oskar



 
 Thanks, but...
 
 test.d
 ---
 abstract class Animal(T)
 {
   this()
   {
     static assert(is(T : Animal));
   }
 
   public abstract T[] procreate(T mate);
 }
 
 class Monkey : Animal!(Monkey)
 {
   public Monkey[] procreate(Monkey mate)
   {
     return new Monkey[2];
   }
 }
 
 int main(char[][] args)
 {
   new Monkey();
 }
 ---
 
 When I compile, I get:
 test.d: variable test.Animal!(Monkey).Animal.this.this abstract cannot 
 be applied to variable
 test.d:3: variable test.Animal!(Monkey).Animal.this.__result abstract 
 cannot be applied to variable
 
 Now what?  Line 3 is the "this()" line.
 
 Erik



Oops.  The problem was the "abstract" on the very first line.  Bad Java 
habit.

Erik
Mar 20 2006