Templates considered impressive

[Salih Dincer]

On Tuesday, 1 October 2024 at 17:30:20 UTC, H. S. Teoh wrote:
> On Tue, Oct 01, 2024 at 04:30:27PM +0000, Salih Dincer wrote:
>> Please add this to your MyCon structure:
>> ```d
>> alias value this;
>> // assert(num1 == 3.14);
>> ```
>> And test it like this too, I think it's awesome!
> [...]
>
> IMO it's not a good idea to recommend the use of `alias this`.  
> It's a neat trick that solves the short-term problem...

You may be right about the alias. Let's go a little further and 
put the convertToHex() function between the template and the 
struct:

```d
template MyCon(T, string str = "0")
{
     MyCon convert = str;
     struct MyCon
     {
         T value; //alias value this;
         
         this(R)(R data)
         {
             import std.conv : to;
             value = data.to!T;
         }
         mixin DownRange;
     }

     enum _input = str;
     string convertToHex()
     {
         import std.string : format;
         return _input.format!"%-(%02X%)";
     }
     
}

import std.stdio;
void main()
{
     mixin MyCon!(double, "3.141") piNumber;
     piNumber.convertToHex().write(": ");
     piNumber.convert.writeln;
   
     auto iNumber = MyCon!double("0.123");
     piNumber.convertToHex().write(": ");
     iNumber.value.writeln;
   
     // mixin MyCon!(int, "123") testError;
     mixin myStatement!(int, "Hello");
     assert(_value == 42); // no error
     
     mixin myStatement!(double, "D!");
     // assert(_value == 42);
}

template myStatement(T, string value = "")
{
     auto doIt()
     {
         T.stringof.writeln(": ", value);
         return 42;
     }

     auto _value = doIt();
}

template DownRange()
{
     auto empty() => value <= 0;
     auto front() => value;
     auto popFront() => --value;
}

struct S
{
     int value;
     mixin DownRange;
}
```
I also added another template called DownRange (InputRange 
functions) inside a template. Since the structures of this mixin 
are static codes, unfortunately everything looks like const and 
cannot be changed. Please play around in the 
[playground](https://run.dlang.io/). I tried to show many things 
in the code above.


Here is the output of the code:

```d
/*
332E313431: [3.141, 2.141, 1.141, 0.141]
332E313431: 0.123
int: Hello
double: D!
*/
```

Please note iNumber, not piNumber.

SDB at 79