Modules ... "import" vs. "compilation" ... what is the real process here?

[Ali Çehreli]

On 9/27/21 10:38 AM, james.p.leblanc wrote:

 > I have trouble understanding "module imports" vs. "module compilations".

In addition to what Mike Parker said, templates do complicate matters 
here: Templates are instantiated (i.e. compiled for a specific set of 
template arguments) by modules that actually use those templates.

So, if no non-template definition is used from a module, that module 
need not appear separately on the build line.

For example, 'module a' defines just a function template:

```
module a;

auto doubled(T)(T value) {
   return value * 2;
}
```

The main module imports 'a' and uses and instantiation of a template 
from that module:

```
module main;

import a;

void main() {
   assert(42.doubled == 84);
}
```

The build line may be confusing because in this case a.d need not appear 
on the build line:

$ dmd main.d

Everything works because the compilation of instance doubled!int is 
compiled when compiling main.d and the linker happily finds all symbols 
that are called in the program.

Ali