Practical difference between template "alias" arguments/normal generic arguments in this case?

[Mike Parker via Digitalmars-d-learn]

On Wednesday, 12 April 2017 at 11:06:13 UTC, Juanjo Alvarez wrote:
> Hi!
>
> With "alias this" accepting runtime variables I'm struggling to

FYI, you are not talking about "alias this", but "alias template 
parameters", two very different concepts.

> understand the difference between a generic function with an 
> "alias this" parameter and another one with a "runtime" 
> parameter of template type.
>
> Example:
>
> // ---- example code ----
> import std.stdio: writeln;
>
> void writevalue1(alias param)() { writeln(param); }
>
> void writevalue2(T)(T param) { writeln(param); }
>
> void main() {
>   import std.random: uniform;
>   auto someNum = uniform(0, 1000); // runtime value
>   writevalue1(someNum);
>   someNum = uniform(0, 1000);
>   writevalue2(someNum);
> }
> // ---- example end -----
>
> Since both versions work with runtime values, what's are the 
> differences? When I should prefer one version over the other?

Neither template cares or knows anything about runtime values. 
When the compiler encounters them, it instantiates an instance of 
each by creating functions that work with runtime values. But it 
does so differently for each.

In your instantiation of writevalue1, you are passing the symbol 
"someNum" as an alias template parameter. This instantiation will 
only ever be used with "someNum". If you pass it a different 
symbol, you'll get a separate instantiation, even if it has the 
same type as "someNum".

In your instantiation of writevalue2, you are getting a function 
that takes a single argument of `typeof(someNum)`. This 
instantiation will be used for any value you pass to it that has 
the same type.

If you're only interested in the value of a variable, you almost 
certainly want to use template type parameters most of the time, 
especially if you are going to be calling the function with 
multiple variables. That way, variables of the same type all have 
one instantiation and you avoid bloat.

Use alias parameters when you actually care about the *symbol* 
and not the value.