• Piotr Szturmaj (26/26) Dec 30 2010 Hello,
• Simen kjaeraas (19/21) Dec 30 2010 The problem here is that Nullable!T is not a real type. Hence,
• Piotr Szturmaj (16/36) Dec 30 2010 Algebraic is also an alias for VariantN (which is real type):

Piotr Szturmaj <bncrbme jadamspam.pl> writes:

Hello,

I'm using D2.051 and following code:

import std.variant;

template Nullable(T)
{
     alias Algebraic!(T, void*) Nullable;
}

template isNullable(T)
{
     static if (is(T X == Nullable!U, U))
         enum isNullable = true;
     else
         enum isNullable = false;
}

static assert(isNullable!(Nullable!int));

and above static assert fails. Neither this works:

template isNullable(T : Nullable!U, U)
{
     enum isNullable = true;
}

template isNullable(T)
{
     enum isNullable = false;
}

static assert(isNullable!(Nullable!int));

Question is, what I'm doing wrong?

"Simen kjaeraas" <simen.kjaras gmail.com> writes:

Piotr Szturmaj <bncrbme jadamspam.pl> wrote:

 static assert(isNullable!(Nullable!int));

 Question is, what I'm doing wrong?

The problem here is that Nullable!T is not a real type. Hence,  
Nullable!int is actually
Algebraic!(int,void*). Checking for that apparently does not work as  
simply as one might
hope. So, instead you should create a wrapper. This works:


struct Nullable(T) {
     Algebraic!(T, void*) Nullable;
     alias Nullable this;
}

template isNullable(T) {
     static if (is(T X == Nullable!(U), U))
         enum isNullable = true;
     else
         enum isNullable = false;
}

static assert(isNullable!(Nullable!int));

-- 
Simen

Piotr Szturmaj <bncrbme jadamspam.pl> writes:

Simen kjaeraas wrote:
 Piotr Szturmaj <bncrbme jadamspam.pl> wrote:

 static assert(isNullable!(Nullable!int));

 Question is, what I'm doing wrong?

 The problem here is that Nullable!T is not a real type. Hence,
 Nullable!int is actually
 Algebraic!(int,void*). Checking for that apparently does not work as
 simply as one might
 hope.

Algebraic is also an alias for VariantN (which is real type):

template Algebraic(T...)
{
     alias VariantN!(maxSize!(T), T) Algebraic;
}

however this check also yields false:

static if (is(T X == VariantN!(U), U...))

but this works:

static if (is(T == Nullable!int))

so, I was a bit confused...

So, instead you should create a wrapper. This works:
 struct Nullable(T) {
 Algebraic!(T, void*) Nullable;
 alias Nullable this;
 }

 template isNullable(T) {
 static if (is(T X == Nullable!(U), U))
 enum isNullable = true;
 else
 enum isNullable = false;
 }

 static assert(isNullable!(Nullable!int));

Thanks! I've already used wrappers like that, but I though it's possible 
to match aliases directly.

regards,
Piotr