• Juanjo Alvarez (23/23) Apr 12 2017 Hi!
• Mike Parker (23/43) Apr 12 2017 FYI, you are not talking about "alias this", but "alias template
• Nicholas Wilson (13/36) Apr 12 2017 (As noted by Mike the examples you present are actually template
• Juanjo Alvarez (8/8) Apr 12 2017 Thanks to both, I got it. Type templates for generic types, and

Juanjo Alvarez <gorthol protonmail.com> writes:

Hi!

With "alias this" accepting runtime variables I'm struggling to 
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?

If objdump is not lying to me, both calls jump to the same 
assembly and the only diffence is that the call to writevalue1 
does a "mov -0x8(%rdi),%edi" just before the callq instruction.

Mike Parker <aldacron gmail.com> writes:

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.

Nicholas Wilson <iamthewilsonator hotmail.com> writes:

On Wednesday, 12 April 2017 at 11:06:13 UTC, Juanjo Alvarez wrote:
 Hi!

 With "alias this" accepting runtime variables I'm struggling to 
 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?

 If objdump is not lying to me, both calls jump to the same 
 assembly and the only diffence is that the call to writevalue1 
 does a "mov -0x8(%rdi),%edi" just before the callq instruction.

(As noted by Mike the examples you present are actually template 
alias parameters, not alias this which affects name lookup.)

There are three kinds of template parameters types, values and 
aliases.
I assume you understand what the first two _do_ in terms of 
parameterisation.

Alias parameters are for symbols, and are used generally when the 
thing you want to template on not a value or a type (although 
alias parameters can accept anything).

These typically include lambdas e.g. the predicate to 
std.algorithm.filter or the transformation to std.algorithm.map, 
(global) variables to do introspection (for e.g. logging).

Juanjo Alvarez <gorthol protonmail.com> writes:

Thanks to both, I got it. Type templates for generic types, and 
alias for other things knowing that the instantiation is by 
symbol.

Yes, the "alias this" in my message  was a (double) brainfart, I 
actually wanted to write "alias template".

In this case both functions had the same assembler body which was 
part of what had me confused but I guess that's a compiler 
optimization.