Help needed to learn templates

[Ali Çehreli]

On 3/19/22 06:38, Vinod K Chandran wrote:
 > On Saturday, 19 March 2022 at 11:47:53 UTC, Stanislav Blinov wrote:
 >>
 >> No.
 >>
 > First of all Thanks for the reply. The answer "No" is a wonder to me.

I wrote a chapter about the is expression but it's still mysterious to 
me. :)

   ddili.org/ders/d.en/is_expr.html

I may be saying some things wrong there but that's my mental model.

 > Because, from my point of view, `U` is coming from nowhere.

Agreed. It is similar to U's coming from nowhere below:

void foo(U)(U[] array) {
}

So, in my mental model, that use of the is expression is the same but 
written in the reverse order from foo above:

     static if (is(T t == U[], U))

It means "if T matches U[] and U is a type". "a type" because it is just 
U in the is expression list.

I believe at least some of the traits have been added since that doc 
document was written. I would write it in a much simpler way using 
template constraints today:

template rank(T) {
   import std.traits : isArray;
   import std.range : ElementType;

   static if (isArray!T) {
     enum size_t rank = 1 + rank!(ElementType!T);

   } else {
     enum size_t rank = 0;
   }
}

Or one can separate the logic in two template definitions:

import std.traits : isArray;

template rank(T)
if (isArray!T)
{
   import std.range : ElementType;
   enum size_t rank = 1 + rank!(ElementType!T);
}

template rank(T)
if (!isArray!T)
{
   enum size_t rank = 0;
}

However, note how the template constraints had to be repeated as 
isArray!T and !isArray!T in that case.

 > My
 > understanding is, we can use any parameter of a template inside the
 > template. So in this case `U` is not in the parameter list. It is
 > suddenly appearing in that `static if`.

In my mental model, the is expression uses at least a part of the 
template system.

 >> you can simply write
 >> `is(T == U[], U)`.
 >>
 > So the `T` is not the type.

T is the type because it is introduced as simply T in the parameter 
list. If it were 'int T', then it would be an int. So in that sense, it 
is a type-kind template parameter.

 > It's the parameter. Right ? So a template
 > doesn't need a type. Only the parameter, right ?

The way I read it is: "T is a type that matches U[] where U is a type as 
well."

 > (I think I am too dumb
 > to ask this. Please forgive me.)

Not at all! The is expression is the weirdest part of D.

Ali