Re: Some problems with std.typecons.Nullable. It automaticaly calls get and fails (when null)

[Simen Kjærås]

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