In my code I wanted to use std.typecons.Nullable to get advantages in web application when working with database values. I'm writing some struct that can get Nullable!int as input value. But instead of getting struct object itself (in case of overloading) *get* method called and programme fails. This happens when Nullable is null. I'll give an example:

//------
import std.stdio;

import std.typecons;

template isStdNullable(N)
{
	static if( is( N == NullableRef!(TL1), TL1... ) )
		enum bool isStdNullable = true;
	else static if( is( N == Nullable!(TL2), TL2... ) )
		enum bool isStdNullable = true;
	else
		enum bool isStdNullable = false;
}


template getStdNullableType(N)
{
	static if( is( N == NullableRef!(TL2), TL2... ) )
		alias TL2[0] getStdNullableType;
	else static if( is( N == Nullable!(TL2), TL2... ) )
		alias TL2[0] getStdNullableType;
	else
		static assert (0, `Type ` ~ fullyQualifiedName!(N) ~ ` can't be used as Nullable type!!!` );
}



struct Test
{	
	//Uncommenting these lines makes programme fail with assertion
	//string opIndex(int key) const
	//{	return "Not null";	}
	
	//inout(bool) opBinaryRight(string op)(int key) inout
	//{	return true;  }
	
	string opIndex(N)(N key) const
		if( isStdNullable!(N) && is( getStdNullableType!(N) == int )  )
	{	return ( key.isNull() ? "null" : "Not null" );
	
	}	
	
	inout(bool) opBinaryRight(string op, N)(N key) inout
		if(op == "in" && isStdNullable!(N) && is( getStdNullableType!(N) == int ))
	{	return ( key.isNull() ? true : false );  }
	
}


void main()
{
	Test test;
	
	Nullable!(int, 10) val5;
	val5 = 5;
	
	Nullable!(int, 15) valNull;
	
	writeln(test[val5]);
	writeln(test[valNull]);
	
	writeln(val5 in test);
	writeln(valNull in test);
	
}
//--------------------

In this code I want get ability to pass int values and Nullable!int values using overloaded operators. Template "functions" isStdNullable, getStdNullableType help to get information about type of *N*. I think that there is something that I don't understand about order of instantiation for overloaded functions.
How could I solve this problem? Or I need to write my own Nullable variant without "alias get this;"?

The example above is also available at: http://dpaste.dzfl.pl/6790f60e

On 09.12.2013 14:33, Uranuz wrote:
> In my code I wanted to use std.typecons.Nullable to get advantages in
> web application when working with database values. I'm writing some
> struct that can get Nullable!int as input value. But instead of getting
> struct object itself (in case of overloading) *get* method called and
> programme fails. This happens when Nullable is null. I'll give an example:
>
> //------
> import std.stdio;
>
> import std.typecons;
>
> template isStdNullable(N)
> {
>      static if( is( N == NullableRef!(TL1), TL1... ) )
>          enum bool isStdNullable = true;
>      else static if( is( N == Nullable!(TL2), TL2... ) )
>          enum bool isStdNullable = true;
>      else
>          enum bool isStdNullable = false;
> }
>
>
> template getStdNullableType(N)
> {
>      static if( is( N == NullableRef!(TL2), TL2... ) )
>          alias TL2[0] getStdNullableType;
>      else static if( is( N == Nullable!(TL2), TL2... ) )
>          alias TL2[0] getStdNullableType;
>      else
>          static assert (0, `Type ` ~ fullyQualifiedName!(N) ~ ` can't be
> used as Nullable type!!!` );
> }
>
>
>
> struct Test
> {
>      //Uncommenting these lines makes programme fail with assertion
>      //string opIndex(int key) const
>      //{    return "Not null";    }
>
>      //inout(bool) opBinaryRight(string op)(int key) inout
>      //{    return true;  }
>
>      string opIndex(N)(N key) const
>          if( isStdNullable!(N) && is( getStdNullableType!(N) == int )  )
>      {    return ( key.isNull() ? "null" : "Not null" );
>
>      }
>
>      inout(bool) opBinaryRight(string op, N)(N key) inout
>          if(op == "in" && isStdNullable!(N) && is(
> getStdNullableType!(N) == int ))
>      {    return ( key.isNull() ? true : false );  }
>
> }
>
>
> void main()
> {
>      Test test;
>
>      Nullable!(int, 10) val5;
>      val5 = 5;
>
>      Nullable!(int, 15) valNull;
>
>      writeln(test[val5]);
>      writeln(test[valNull]);
>
>      writeln(val5 in test);
>      writeln(valNull in test);
>
> }
> //--------------------
>
> In this code I want get ability to pass int values and Nullable!int
> values using overloaded operators. Template "functions" isStdNullable,
> getStdNullableType help to get information about type of *N*. I think
> that there is something that I don't understand about order of
> instantiation for overloaded functions.
> How could I solve this problem? Or I need to write my own Nullable
> variant without "alias get this;"?

The simple solution is to make your int-specialized functions do their specialization in template constraints:

string opIndex(N)(N key) const if (is(N == int))
{    return "Not null";    }

bool opBinaryRight(string op, N)(N key) const if (is(N == int))
{    return true;  }


I'm not entirely sure why this behavior occurs. One would think not applying alias this would make for a better match than applying it.

btw, in the future you might want to keep discussions like this in digitalmars.D.learn.

--
  Simen

Thanks. It works, though it looks awkward.

Perhaps it's a result of a compiler-bug I reported over here:
https://d.puremagic.com/issues/show_bug.cgi?id=11499