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digitalmars.D - Would it be possible (and useful) to introduce declarations like `auto

reply "Roman D. Boiko" <rb d-coding.com> writes:
Is there anything preventing us from adding constraints on the 
auto function return value? I mean, such language extension seems 
to be quite useful.

For example, it would be no longer necessary to provide method 
bodies for functions with auto return values.

In many cases this would eliminate the need for introducing an 
interface.

interface returnsDuckTyped
{
     auto foo() if(isInputRange(auto));
}
May 17 2012
next sibling parent "Roman D. Boiko" <rb d-coding.com> writes:
On Thursday, 17 May 2012 at 11:49:18 UTC, Roman D. Boiko wrote:
 Is there anything preventing us from adding constraints on the 
 auto function return value? I mean, such language extension 
 seems to be quite useful.

 For example, it would be no longer necessary to provide method 
 bodies for functions with auto return values.

 In many cases this would eliminate the need for introducing an 
 interface.

interface returnsDuckTyped { auto foo() if(isInputRange!auto); } (fixed typo)
May 17 2012
prev sibling next sibling parent "Jonathan M Davis" <jmdavisProg gmx.com> writes:
`
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On Thursday, May 17, 2012 13:49:16 Roman D. Boiko wrote:
 Is there anything preventing us from adding constraints on the
 auto function return value? I mean, such language extension seems
 to be quite useful.
 
 For example, it would be no longer necessary to provide method
 bodies for functions with auto return values.
 
 In many cases this would eliminate the need for introducing an
 interface.
 
 interface returnsDuckTyped
 {
 auto foo() if(isInputRange(auto));
 }

It would still be necessary, because the compiler needs to know what the actual return type is. Knowing that the type implements popFront, front, and empty isn't enough. It needs to know the actual, physical layout of the type to generate the proper code. And when dealing with an interface, the return type must be covariant, and unless the types are both classes and one is derived from the other (directly or indirectly), they won't be covariant even if they have all of the same functions. - Jonathan M Davis
May 17 2012
prev sibling next sibling parent "Roman D. Boiko" <rb d-coding.com> writes:
On Thursday, 17 May 2012 at 21:09:10 UTC, Jonathan M Davis wrote:
 It would still be necessary, because the compiler needs to know 
 what the
 actual return type is. Knowing that the type implements 
 popFront, front, and
 empty isn't enough. It needs to know the actual, physical 
 layout of the type
 to generate the proper code. And when dealing with an 
 interface, the return
 type must be covariant, and unless the types are both classes 
 and one is
 derived from the other (directly or indirectly), they won't be 
 covariant even
 if they have all of the same functions.

 - Jonathan M Davis

been implemented already. But couldn't find any myself. Thanks!
May 17 2012
prev sibling parent Jonathan M Davis <jmdavisProg gmx.com> writes:
On Thursday, May 17, 2012 13:49:16 Roman D. Boiko wrote:
 Is there anything preventing us from adding constraints on the
 auto function return value? I mean, such language extension seems
 to be quite useful.
 
 For example, it would be no longer necessary to provide method
 bodies for functions with auto return values.
 
 In many cases this would eliminate the need for introducing an
 interface.
 
 interface returnsDuckTyped
 {
      auto foo() if(isInputRange(auto));
 }

It would still be necessary, because the compiler needs to know what the actual return type is. Knowing that the type implements popFront, front, and empty isn't enough. It needs to know the actual, physical layout of the type to generate the proper code. And when dealing with an interface, the return type must be covariant, and unless the types are both classes and one is derived from the other (directly or indirectly), they won't be covariant even if they have all of the same functions. - Jonathan M Davis
May 17 2012