digitalmars.D.learn - How to assign a delegate to a var ?

bioinfornatics (26/26) Jun 02 2014 Hi,

Meta (5/32) Jun 02 2014 The problem is that you're missing the type of a in "(a) => a ==

bioinfornatics (11/59) Jun 03 2014 with this i got an error even if i do public alias Predicate in

bioinfornatics (44/105) Jun 03 2014 To have same code

=?UTF-8?B?QWxpIMOHZWhyZWxp?= (8/56) Jun 03 2014 A delegate has a context pointer that it uses when executing at

bioinfornatics (14/90) Jun 03 2014 The only reason that is because i do nor know how to do:

bioinfornatics (29/39) Jun 03 2014 i got it with function, thx Ali
Jesse Phillips (9/11) Jun 03 2014 D will decide the type based on if context is needed. If you look

"bioinfornatics" <bioinfornatics fedoraproject.org> writes:

Hi,

I would like store the delegate to another var but when i try i
get:


testTraitsWithDelegate.d(13): Error: expression template
__lambda2 is void and has no value

I do not want to run it only to save the «function» somewhere.


----------- CODE ------------
import std.stdio;
import std.typecons : Tuple;

struct attribute( alias Pred )
{
      alias Predicate = Pred;
}



struct Data
{
       attribute!( (a) => a == 42 )
      int x;
}



void main()
{

      bool delegate( int ) tmp;

      pragma( msg, __traits(getAttributes, Data.x)[0].Predicate( 42
) );

      tmp = __traits(getAttributes, Data.x)[0].Predicate;
}
----------- END CODE ------------

Jun 02 2014

"Meta" <jared771 gmail.com> writes:

On Monday, 2 June 2014 at 22:18:39 UTC, bioinfornatics wrote:
 Hi,

 I would like store the delegate to another var but when i try i
 get:


 testTraitsWithDelegate.d(13): Error: expression template
 __lambda2 is void and has no value

 I do not want to run it only to save the «function» somewhere.


 ----------- CODE ------------
 import std.stdio;
 import std.typecons : Tuple;

 struct attribute( alias Pred )
 {
      alias Predicate = Pred;
 }



 struct Data
 {
       attribute!( (a) => a == 42 )
      int x;
 }



 void main()
 {

      bool delegate( int ) tmp;

      pragma( msg, __traits(getAttributes, Data.x)[0].Predicate( 
 42
 ) );

      tmp = __traits(getAttributes, Data.x)[0].Predicate;
 }
 ----------- END CODE ------------

The problem is that you're missing the type of a in "(a) => a == 
42". Without the type, it's actually a template lambda, not a 
lambda function. Try changing it to "(int a) => a == 42".
auto

Jun 02 2014

"bioinfornatics" <bioinfornatics fedoraproject.org> writes:

On Monday, 2 June 2014 at 23:27:03 UTC, Meta wrote:
 On Monday, 2 June 2014 at 22:18:39 UTC, bioinfornatics wrote:
 Hi,

 I would like store the delegate to another var but when i try i
 get:


 testTraitsWithDelegate.d(13): Error: expression template
 __lambda2 is void and has no value

 I do not want to run it only to save the «function» somewhere.


 ----------- CODE ------------
 import std.stdio;
 import std.typecons : Tuple;

 struct attribute( alias Pred )
 {
     alias Predicate = Pred;
 }



 struct Data
 {
      attribute!( (a) => a == 42 )
     int x;
 }



 void main()
 {

     bool delegate( int ) tmp;

     pragma( msg, __traits(getAttributes, Data.x)[0].Predicate( 
 42
 ) );

     tmp = __traits(getAttributes, Data.x)[0].Predicate;
 }
 ----------- END CODE ------------

 The problem is that you're missing the type of a in "(a) => a 
 == 42". Without the type, it's actually a template lambda, not 
 a lambda function. Try changing it to "(int a) => a == 42".
 auto

with this i got an error even if i do public alias Predicate in
struct attribute:
Error: struct testTraitsWithDelegate.attribute!(function (int a)
=> a == 42).attribute member __lambda2 is not accessible

and i fail to do same with

struct attribute
{
    public:
      static bool delegate( int ) Predicate;
}

Jun 03 2014

"bioinfornatics" <bioinfornatics fedoraproject.org> writes:

On Tuesday, 3 June 2014 at 12:27:45 UTC, bioinfornatics wrote:
 On Monday, 2 June 2014 at 23:27:03 UTC, Meta wrote:
 On Monday, 2 June 2014 at 22:18:39 UTC, bioinfornatics wrote:
 Hi,

 I would like store the delegate to another var but when i try 
 i
 get:


 testTraitsWithDelegate.d(13): Error: expression template
 __lambda2 is void and has no value

 I do not want to run it only to save the «function» somewhere.


 ----------- CODE ------------
 import std.stdio;
 import std.typecons : Tuple;

 struct attribute( alias Pred )
 {
    alias Predicate = Pred;
 }



 struct Data
 {
     attribute!( (a) => a == 42 )
    int x;
 }



 void main()
 {

    bool delegate( int ) tmp;

    pragma( msg, __traits(getAttributes, Data.x)[0].Predicate( 
 42
 ) );

    tmp = __traits(getAttributes, Data.x)[0].Predicate;
 }
 ----------- END CODE ------------

 The problem is that you're missing the type of a in "(a) => a 
 == 42". Without the type, it's actually a template lambda, not 
 a lambda function. Try changing it to "(int a) => a == 42".
 auto

 with this i got an error even if i do public alias Predicate in
 struct attribute:
 Error: struct testTraitsWithDelegate.attribute!(function (int a)
 => a == 42).attribute member __lambda2 is not accessible

 and i fail to do same with

 struct attribute
 {
    public:
      static bool delegate( int ) Predicate;
 }




To have same code

------------- code -----------
import std.stdio;
import std.typecons : Tuple;
//~ import std.functional : toDelegate;
//~ struct attribute( alias Pred )
//~ {
//~     public:
//~     alias Predicate = Pred;
//~ }

struct attribute
{
     public bool delegate( int ) predicate;
     public this(  bool delegate( int ) pred )
     {
         predicate = pred;
     }
}

struct Data
{
      attribute( (int a) => a == 42 )
     int x;
}



void main()
{

     bool delegate( int ) tmp;

     pragma( msg, __traits(getAttributes, Data.x)[0] );
     pragma( msg, __traits(getAttributes, Data.x)[0].predicate( 42 
) );

     pragma( msg, __traits(getAttributes, 
Data.x)[0].predicate.stringof );
     tmp = __traits(getAttributes, Data.x)[0].predicate;
     writeln( tmp(42 ) );
     //~tmp = toDelegate(__traits(getAttributes, 
Data.x)[0].Predicate);
}
----------------------------------




give at compile time this:

attribute(function (int a) => a == 42)
true
(attribute __ctmp1474;
  , __ctmp1474).this(function (int a) => a == 42).predicate


And segfault at run-time

Jun 03 2014

=?UTF-8?B?QWxpIMOHZWhyZWxp?= <acehreli yahoo.com> writes:

On 06/03/2014 05:57 AM, bioinfornatics wrote:

 On Tuesday, 3 June 2014 at 12:27:45 UTC, bioinfornatics wrote:

 I would like store the delegate to another var but when i try i
 get:


 testTraitsWithDelegate.d(13): Error: expression template
 __lambda2 is void and has no value

 I do not want to run it only to save the «function» somewhere.




 ------------- code -----------
 import std.stdio;
 import std.typecons : Tuple;
 //~ import std.functional : toDelegate;
 //~ struct attribute( alias Pred )
 //~ {
 //~     public:
 //~     alias Predicate = Pred;
 //~ }

 struct attribute
 {
      public bool delegate( int ) predicate;
      public this(  bool delegate( int ) pred )
      {
          predicate = pred;
      }
 }

 struct Data
 {
       attribute( (int a) => a == 42 )
      int x;
 }



 void main()
 {

      bool delegate( int ) tmp;

      pragma( msg, __traits(getAttributes, Data.x)[0] );
      pragma( msg, __traits(getAttributes, Data.x)[0].predicate( 42 ) );

      pragma( msg, __traits(getAttributes, 

Data.x)[0].predicate.stringof );
      tmp = __traits(getAttributes, Data.x)[0].predicate;
      writeln( tmp(42 ) );
      //~tmp = toDelegate(__traits(getAttributes, Data.x)[0].Predicate);
 }
 ----------------------------------




 give at compile time this:

 attribute(function (int a) => a == 42)
 true
 (attribute __ctmp1474;
   , __ctmp1474).this(function (int a) => a == 42).predicate


 And segfault at run-time

A delegate has a context pointer that it uses when executing at 
run-time. However, there can't be a run-time context of a delegate that 
is created at compile-time. I think that is why the segfault.

Is there a reason why it needs to be a delegate?

Replacing every 'delegate' with 'function' makes your code work.

Ali

Jun 03 2014

"bioinfornatics" <bioinfornatics fedoraproject.org> writes:

On Tuesday, 3 June 2014 at 15:00:05 UTC, Ali Çehreli wrote:
 On 06/03/2014 05:57 AM, bioinfornatics wrote:

 On Tuesday, 3 June 2014 at 12:27:45 UTC, bioinfornatics wrote:

 I would like store the delegate to another var but when i




 try i
 get:


 testTraitsWithDelegate.d(13): Error: expression template
 __lambda2 is void and has no value

 I do not want to run it only to save the «function»




 somewhere.

 ------------- code -----------
 import std.stdio;
 import std.typecons : Tuple;
 //~ import std.functional : toDelegate;
 //~ struct attribute( alias Pred )
 //~ {
 //~     public:
 //~     alias Predicate = Pred;
 //~ }

 struct attribute
 {
      public bool delegate( int ) predicate;
      public this(  bool delegate( int ) pred )
      {
          predicate = pred;
      }
 }

 struct Data
 {
       attribute( (int a) => a == 42 )
      int x;
 }



 void main()
 {

      bool delegate( int ) tmp;

      pragma( msg, __traits(getAttributes, Data.x)[0] );
      pragma( msg, __traits(getAttributes,

 Data.x)[0].predicate( 42 ) );
      pragma( msg, __traits(getAttributes,

 Data.x)[0].predicate.stringof );
      tmp = __traits(getAttributes, Data.x)[0].predicate;
      writeln( tmp(42 ) );
      //~tmp = toDelegate(__traits(getAttributes,

 Data.x)[0].Predicate);
 }
 ----------------------------------




 give at compile time this:

 attribute(function (int a) => a == 42)
 true
 (attribute __ctmp1474;
   , __ctmp1474).this(function (int a) => a == 42).predicate


 And segfault at run-time

 A delegate has a context pointer that it uses when executing at 
 run-time. However, there can't be a run-time context of a 
 delegate that is created at compile-time. I think that is why 
 the segfault.

 Is there a reason why it needs to be a delegate?

 Replacing every 'delegate' with 'function' makes your code work.

 Ali


The only reason that is because i do nor know how to do:

   attribute( (int a) => a == 42 )
  int x;

without using a delegate, with a function that will not work as  
(int a) => a == 42 is a delegate no ?

struct attribute
{
     public bool function( int ) predicate;
     public this(  bool function( int ) pred )
     {
         predicate = pred;
     }
}

Jun 03 2014

"bioinfornatics" <bioinfornatics fedoraproject.org> writes:

delegate has a context pointer that it uses when executing
 at run-time. However, there can't be a run-time context of a 
 delegate that is created at compile-time. I think that is why 
 the segfault.

 Is there a reason why it needs to be a delegate?

 Replacing every 'delegate' with 'function' makes your code 
 work.

 Ali


i got it with function, thx Ali

-------------- code ----------
import std.stdio;
import std.typecons : Tuple;

struct attribute
{
     public bool function( int ) predicate;
     public this(  bool function( int ) pred )
     {
         predicate = pred;
     }
}

struct Data
{
      attribute( (int a) => a == 42 )
     int x;
      attribute( (int a) => a == 8 )
     int y;
}



void main()
{

     bool function( int ) tmp;

     pragma( msg, __traits(getAttributes, Data.x)[0] );
     pragma( msg, __traits(getAttributes, Data.x)[0].predicate( 42 
) );
     tmp = __traits(getAttributes, Data.x)[0].predicate;
     writeln( tmp(42 ) );
}

Jun 03 2014

"Jesse Phillips" <Jesse.K.Phillips+D gmail.com> writes:

On Tuesday, 3 June 2014 at 15:14:06 UTC, bioinfornatics wrote:
 without using a delegate, with a function that will not work as
  (int a) => a == 42 is a delegate no ?

D will decide the type based on if context is needed. If you look 
at the output you had:

     , __ctmp1474).this(function (int a) => a == 42).predicate

Inside this, you'll see that your lambda was identified as a 
function. I'm guessing that having the function assigned to a 
delegate has confused the compiler which is why segfault.

I think such should be filed as a bug, likely should not have 
compiled.

Jun 03 2014

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digitalmars.D.learn - How to assign a delegate to a var ?