OK, here's one for you that sounds like it ought to be easy, but I don't immediately see how to do it in a pretty way.

Given a type parameter T of a template:
If T is an integral type, I want to declare a variable 'widest' of type ulong;
If T is a floating-point type, I want to declare a variable 'widest' of type double.
And it has to be prettier than my solution.  :)

        static if (is (T: ulong))
            ulong widest = 0;
        else if (is (T: double))
            double widest = 0.0;
        else
            static assert (false, "Unimplemented type " ~ T.stringof) ;

Now, I thought this sounds like a great job for a mixin:

template Widen (T, alias varname)
{
    static if (is (T: ulong))
        ulong varname = 0;
    else if (is (T: double))
        double varname = 0.0;
    else
        static assert (false, "Unimplemented type " ~ T.stringof) ;
}

mixin Widen!(T, widest);

...but alas, "Declaration expected, not 'if'".

Help?

Doctor J wrote:
> OK, here's one for you that sounds like it ought to be easy, but I don't immediately see how to do it in a pretty way.
> 
> Given a type parameter T of a template:
> If T is an integral type, I want to declare a variable 'widest' of type ulong;
> If T is a floating-point type, I want to declare a variable 'widest' of type double.
> And it has to be prettier than my solution.  :)
> 
>         static if (is (T: ulong))
>             ulong widest = 0;
>         else if (is (T: double))
>             double widest = 0.0;
>         else
>             static assert (false, "Unimplemented type " ~ T.stringof) ;
> 
> Now, I thought this sounds like a great job for a mixin:
> 
> template Widen (T, alias varname)
> {
>     static if (is (T: ulong))
>         ulong varname = 0;
>     else if (is (T: double))
>         double varname = 0.0;
>     else
>         static assert (false, "Unimplemented type " ~ T.stringof) ;
> }
> 
> mixin Widen!(T, widest);
> 
> ....but alas, "Declaration expected, not 'if'".
> 
> Help?

The error tells you everything you need to know if you read it.

Actually, you have two problems: you're trying to use "if" where you should be using "static if", and you can't alias a symbol name then use it in a declaration.  Here's a fixed, expanded version.

template Widen (T, char[] varname)
{
    static if (is (T: ulong))
    {
        mixin(`ulong `~varname~` = 0;`);
    }
    else
    {
        static if (is (T: double))
        {
            mixin(`double `~varname~` = 0.0`);
        }
        else
        {
            static assert (false, "Unimplmented type " ~ T.stringof);
        }
    }
}

You can remove those braces, I just wanted to point out that putting "static" out the front of an "if" doesn't magically make the "else" branch static as well.

  -- Daniel

On Thu, Apr 16, 2009 at 10:11 PM, Doctor J <nobody@nowhere.com> wrote:

> template Widen (T, alias varname)
> {
>    static if (is (T: ulong))
>        ulong varname = 0;
>    else if (is (T: double))

else *static* if(is(T: double))

Hello Daniel,

> You can remove those braces, I just wanted to point out that putting
> "static" out the front of an "if" doesn't magically make the "else"
> branch static as well.
> 

I known what you are saying but that didn't read right to me: An if as a static if's else clause is not magically a static if. 

> -- Daniel
> 

On Fri, 17 Apr 2009 06:11:00 +0400, Doctor J <nobody@nowhere.com> wrote:

> OK, here's one for you that sounds like it ought to be easy, but I don't immediately see how to do it in a pretty way.
>
> Given a type parameter T of a template:
> If T is an integral type, I want to declare a variable 'widest' of type ulong;
> If T is a floating-point type, I want to declare a variable 'widest' of type double.
> And it has to be prettier than my solution.  :)
>
>         static if (is (T: ulong))
>             ulong widest = 0;
>         else if (is (T: double))
>             double widest = 0.0;
>         else
>             static assert (false, "Unimplemented type " ~ T.stringof) ;
>
> Now, I thought this sounds like a great job for a mixin:
>template Widen (T, alias varname)
> {
>     static if (is (T: ulong))
>         ulong varname = 0;
>     else if (is (T: double))
>         double varname = 0.0;
>     else
>         static assert (false, "Unimplemented type " ~ T.stringof) ;
> }
>
> mixin Widen!(T, widest);
>
> ...but alas, "Declaration expected, not 'if'".
>
> Help?
>
>

I would avoid mixin in such situation and use template instead:

template Widen(T)
{
   static if (is(T : ulong)) {
       alias ulong Widen;
   } else static if (is(T : double)) {
       alias double Widen;
   } else {
       static assert (false, "Unimplemented type " ~ T.stringof) ;
   }
}

Widen!(T) widest;

Doctor J:
> Given a type parameter T of a template:
> If T is an integral type, I want to declare a variable 'widest' of type ulong;
> If T is a floating-point type, I want to declare a variable 'widest' of type double.
> And it has to be prettier than my solution.  :)

A solution using my dlibs:

import std.stdio: writefln;
import d.templates: IsIntegral, IsType, If;

template Widened(T) {
    alias If!(IsIntegral!(T), ulong, If!(IsType!(T, float, double, real), double, void)) Widened;
}

void main() {
    writefln(typeid(Widened!(int))); // prints: ulong
    writefln(typeid(Widened!(byte))); // prints: ulong
    writefln(typeid(Widened!(real))); // prints: double
    writefln(typeid(Widened!(float))); // prints: double
    writefln(typeid(Widened!(double))); // prints: double

    writefln(typeid(Widened!(char))); // prints: void
    writefln(typeid(Widened!(string))); // prints: void
}

Note that real=>double and char=>void.
If you want char=>ulong, then you have to use another isType!(T, ...) instead of IsIntegral!(T).

dlibs (soon to be updated again to improve the apply()):
http://www.fantascienza.net/leonardo/so/libs_d.zip

Bye,
bearophile