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digitalmars.D - For the new std.variant

reply Steve Teale <steve.teale britseyeview.com> writes:
Robert,

Maybe you can fix this along the way:

import std.variant;
struct B
{
   int p, q, r, s;
}
typedef B C;

void main()
{
   B b;
   C c;
   b = c;   // ok
   Variant v = c;
   assert(v.convertsTo!(B));   // no dice
}

Steve
Oct 31 2011
next sibling parent reply Jesse Phillips <jessekphillips+D gmail.com> writes:
typedef shouldn't exist anymore, use alias.

Steve Teale Wrote:


 Robert,
 
 Maybe you can fix this along the way:
 
 import std.variant;
 struct B
 {
    int p, q, r, s;
 }
 typedef B C;
 
 void main()
 {
    B b;
    C c;
    b = c;   // ok
    Variant v = c;
    assert(v.convertsTo!(B));   // no dice
 }
 
 Steve
Oct 31 2011
next sibling parent reply Steve Teale <steve.teale britseyeview.com> writes:
On Mon, 31 Oct 2011 11:50:42 -0400, Jesse Phillips wrote:


 typedef shouldn't exist anymore, use alias.



Jesse,

Maybe you want to do something like:

struct X
{
  int a;
  double b;
}
typedef X Y;
alias X Z;

void discriminate(T)(T t)
{
   if (is(T == X))
      writeln("It's an X");
   else
      writeln("No it's not");
}

void main()
{
   Y y;
   Z z;
   discriminate(y);
   discriminate(z);
}

Steve
Oct 31 2011
parent Jesse Phillips <jessekphillips+D gmail.com> writes:
I literally meant it is not supposed to exist. There was a suggestion to even
axe it from 2.056 I believe.

A library solution is intended to be created which will allow for the type to
act as either a parent or child to the type.

http://stackoverflow.com/questions/3576580/does-d-have-newtype/3577246#3577246

So I understand its usage, but you shouldn't use it because it won't be there.
Oct 31 2011
prev sibling next sibling parent Trass3r <un known.com> writes:
 I literally meant it is not supposed to exist. There was a suggestion to  
 even axe it from 2.056 I believe.



At least they prepared to actually deprecate it.



 A library solution is intended to be created which will allow for the  
 type to act as either a parent or child to the type.



http://d.puremagic.com/issues/show_bug.cgi?id=5467
Oct 31 2011
prev sibling parent Steve Teale <steve.teale britseyeview.com> writes:
On Mon, 31 Oct 2011 21:03:39 +0100, Trass3r wrote:


 I literally meant it is not supposed to exist. There was a suggestion
 to even axe it from 2.056 I believe.



 At least they prepared to actually deprecate it.
 
 

 A library solution is intended to be created which will allow for the
 type to act as either a parent or child to the type.



 http://d.puremagic.com/issues/show_bug.cgi?id=5467



OK, my use case is that in the database interface modules, as far as 
possible I would like to infer an SQL type from a D type. There are often 
SQL types for TIME, DATE, DATETIME, and TIMESTAMP, but the latter two 
will usually correspond to D structs with the same members.

I can define two identical structs with the same set of protocol parsing 
and packing methods, but with different names, but to me this seems 
clunky, error-prone, and not a very good advert for the language.

There's was also a temptation to create typedefs of char[] like 
tinyString, mediumString and longString. I have not gone there - just as 
well.

Forgetting typedef, which I should assume does not exist, what is the 
elegant approach to this?

Steve
Oct 31 2011
prev sibling parent "Robert Jacques" <sandford jhu.edu> writes:
On Mon, 31 Oct 2011 06:30:40 -0400, Steve Teale <steve.teale britseyeview.com>
wrote:

 Robert,

 Maybe you can fix this along the way:

 import std.variant;
 struct B
 {
    int p, q, r, s;
 }
 typedef B C;

 void main()
 {
    B b;
    C c;
    b = c;   // ok
    Variant v = c;
    assert(v.convertsTo!(B));   // no dice
 }

 Steve



As others have mentioned, typedef is being deprecated. That said, variant now
supports duck-typing. So 'v.to!B()' works. However, 'convertsTo's mandate is to
test for implicit conversion, which isn't covered as there's no way in __traits
to recover a typedef's base type.
Oct 31 2011