Can D do some sort of automatic template to "dynamic template" wrapping

[bitwise via Digitalmars-d]

On Friday, 18 September 2015 at 20:55:06 UTC, Adam wrote:
> Fact: Templates are static!
>
> But templates are still normal functioning code and can 
> actually work dynamically. We can see this by including a D 
> compiler inside an app and have the app re-compile the 
> templates for specific dynamic types. It doesn't need templates 
> to do of course, but it shows that "Templates are static" is 
> not quite as simple as it sounds.
>
> But could D actually create some sort of internal expression of 
> D templates as dynamic objects that can be used in run-time?
>
> If have a template T that only uses 3 types, then one can 
> simple do something like(pseudo):
>
> t(object o)
> {
> // use dynamic reflection or whatever to get the type of the 
> object o
> if (o.type == 'int')
>     return T!(int)(o.value, o.unit);
> if (o.type == 'float')
>     return T!(float)(o.value, o.unit);
> return T!(unknown)(o.value, o.unit);
> }
>
> etc...
>
> t then is a "dynamic representation" of the static 
> functionality of T.
>
> But my guess is that the compiler could essentially do this for 
> us and allow one to use templates in a dynamic way. Most of the 
> time it may not be of much use but, say, if one is interacting 
> with console and T converts a string to a number. T could 
> convert a number with unit such as "3mm" in to some base 
> representation such as 0.003(m). float's are handled 
> differently than ints(rounding depending on some criteria).
>
> Maybe we use the T internally and its lightning fast. But for 
> the console case, we  have write a dynamic version of the 
> template or create something similar to the code above.
>
> But the compiler already knows, at least, some of the times 
> that will be used and it should be able to instantiate the 
> template for each of those types.
>
> e.g., if you use T!(float) in the code, it will know to create 
> the "dynamic wrapper" to include that type in the list. If you 
> don't every use T!(float) explicitly then you can simply create 
> a useless statement like {auto x = T!(float)("3.04mm");} which 
> will be optimized but the compiler will then include float as a 
> type used in the dynamic version of T. I may not be making much 
> sense here, but it's the best I'll do.
>
>
>
> The compiler then can do something like assemble the dynamic 
> version of the template into a new function which can be used 
> at compile time on any object.
> Using a simple special character could inform the compiler to 
> use the dynamic template version.
>
> object u = userInput();
> &T(u); // (instead of using t(u)).
>
>
> Here the compiler does all the work for us. If u has type float 
> then this gets effectively dispatched to 
> T!(float)(cast(float)u). If it's an int, then 
> T!(int)(cast(int)u).
>
> What it mainly does for us is simplify having to do the cast 
> our self along with the string of if checking for all the types 
> or having to code that works dynamically
>
> Or is it impossible for the D compiler to accomplish such a 
> task automatically for us?

I think that what you're asking can only be accomplished in an 
interpreted or JIT compiled language. With a compiled language, 
you have to know all the possible types you will support up front.

If you're getting the input from a console though, you must be 
parsing it right? The parser cannot complete it's task without 
knowing what it's parsing, and the parser should pass along or 
store any relevant information for future use.

For compiled languages, there is the idea of "Type Erasure". 
Polymorphism is one way to get this done. You will have to create 
your own base object which specifies all possible functionality.

Example:

import std.stdio;
import std.conv;
import std.random;

abstract class BaseObj {
     abstract void print();
}

class FloatObj : BaseObj {
     float value;
     this(float val) { value = val; }
     override void print() { writeln("float: " ~ value.to!string); 
}
}

class IntObj : BaseObj {
     int value;
     this(int val) { value = val; }
     override void print() { writeln("int: " ~ value.to!string); }
}

BaseObj userInput()
{
     if(uniform(0, 2) % 2 == 0)
         return new FloatObj(uniform(0.0f, 100.0f));
     else
         return new IntObj(uniform(0, 100));
}

void main(string[] args)
{
     BaseObj[] objs;

     foreach(i; 0..10)
     {
         BaseObj obj = userInput();
         obj.print();
     }
}