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digitalmars.D.learn - Recursive templated structs disallowed?

reply "Simen kjaeraas" <simen.kjaras gmail.com> writes:
struct bar( T ) {
     auto baz( U )( U arg ) {
         bar!( typeof( this ) ) tmp;
         return tmp;
     }
}

void main( ) {
     bar!int n;
     n.baz( 3 );
}

This code fails with
Error: recursive template expansion for template argument bar!(int)
Now, I agree it is recursive, but it is not infinitely recursive, so
there shouldn't really be a problem.
Is this a bug? Is there a workaround?

-- 
Simen
Aug 04 2010
next sibling parent reply "Steven Schveighoffer" <schveiguy yahoo.com> writes:
On Wed, 04 Aug 2010 15:37:32 -0400, Simen kjaeraas  
<simen.kjaras gmail.com> wrote:


 struct bar( T ) {
      auto baz( U )( U arg ) {
          bar!( typeof( this ) ) tmp;
          return tmp;
      }
 }

 void main( ) {
      bar!int n;
      n.baz( 3 );
 }

 This code fails with
 Error: recursive template expansion for template argument bar!(int)
 Now, I agree it is recursive, but it is not infinitely recursive, so
 there shouldn't really be a problem.
 Is this a bug? Is there a workaround?


It's lazily recursive.  Meaning, it *is* infinitely recursive, but the  
compiler does not have to evaluate all the recursions until they are used.

I'm not sure it should be disallowed, but it's definitely on the edge.  Do  
you have some real-world case for this?  If you want to get something like  
this fixed, you'll need a good example, not an academic one.

-Steve
Aug 04 2010
next sibling parent "Simen kjaeraas" <simen.kjaras gmail.com> writes:
Steven Schveighoffer <schveiguy yahoo.com> wrote:


 On Wed, 04 Aug 2010 15:37:32 -0400, Simen kjaeraas  
 <simen.kjaras gmail.com> wrote:


 struct bar( T ) {
      auto baz( U )( U arg ) {
          bar!( typeof( this ) ) tmp;
          return tmp;
      }
 }

 void main( ) {
      bar!int n;
      n.baz( 3 );
 }

 This code fails with
 Error: recursive template expansion for template argument bar!(int)
 Now, I agree it is recursive, but it is not infinitely recursive, so
 there shouldn't really be a problem.
 Is this a bug? Is there a workaround?


 It's lazily recursive.  Meaning, it *is* infinitely recursive, but the  
 compiler does not have to evaluate all the recursions until they are  
 used.

 I'm not sure it should be disallowed, but it's definitely on the edge.   
 Do you have some real-world case for this?  If you want to get something  
 like this fixed, you'll need a good example, not an academic one.


I sorta do. I have a struct that keeps track of the operations performed
on it, to create a template stack for calculating stuff at the end.
Basically, I build a function by performing the operations on the
struct. Not sure if this is an expression template, but possibly related.

-- 
Simen
Aug 04 2010
prev sibling parent Philippe Sigaud <philippe.sigaud gmail.com> writes:
On Wed, Aug 4, 2010 at 22:06, Steven Schveighoffer <schveiguy yahoo.com>wrote:


 On Wed, 04 Aug 2010 15:37:32 -0400, Simen kjaeraas <simen.kjaras gmail.com>
 wrote:

  struct bar( T ) {

     auto baz( U )( U arg ) {
         bar!( typeof( this ) ) tmp;
         return tmp;
     }
 }

 void main( ) {
     bar!int n;
     n.baz( 3 );
 }

 This code fails with
 Error: recursive template expansion for template argument bar!(int)
 Now, I agree it is recursive, but it is not infinitely recursive, so
 there shouldn't really be a problem.
 Is this a bug? Is there a workaround?


 It's lazily recursive.  Meaning, it *is* infinitely recursive, but the
 compiler does not have to evaluate all the recursions until they are used.

 I'm not sure it should be disallowed, but it's definitely on the edge.  Do
 you have some real-world case for this?  If you want to get something like
 this fixed, you'll need a good example, not an academic one.

 -Steve


I could get this to work, using a factory function:

import std.stdio;

struct Bar( T ) {
   auto baz( U )( U arg ) {
       return bar(this);
   }
}

Bar!T bar(T)(T t)
{
    return Bar!T();
}

void main( ) {
   Bar!int n;
   auto nn = n.baz( 3 );
   writeln(typeof(n).stringof);
   writeln(typeof(nn).stringof);
}

It's... fragile. Changes a few things here and there, and it's back to
infinite recursion.



Philippe
Aug 04 2010
prev sibling parent Don <nospam nospam.com> writes:
Simen kjaeraas wrote:

 struct bar( T ) {
     auto baz( U )( U arg ) {
         bar!( typeof( this ) ) tmp;
         return tmp;
     }
 }
 
 void main( ) {
     bar!int n;
     n.baz( 3 );
 }
 
 This code fails with
 Error: recursive template expansion for template argument bar!(int)
 Now, I agree it is recursive, but it is not infinitely recursive, so
 there shouldn't really be a problem.
 Is this a bug? Is there a workaround?
 


Duplicate of bug 3869.
Aug 05 2010