Re: Modules ... "import" vs. "compilation" ... what is the real process here?
On 9/28/21 1:59 AM, james.p.leblanc wrote:
On Tuesday, 28 September 2021 at 05:26:29 UTC, Ali Çehreli wrote:
On 9/27/21 10:38 AM, james.p.leblanc wrote:
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.
Ali, this is great! ...I had been tempted to also ask about how templates
figure into this, but realized that including this in my question would be
over complicating the question, so it remained unasked.
But, now I have this part answered as well. I very much appreciate the
mind-reading tricks going on here on the forum!
Be aware that the compiler might not include the code for the template
in the instantiating module if it detects that the instantiation could
already have been generated in an imported module (not one passed on the
command line).
For example, if Ali's module `a` contained an alias:
```d
module a;
auto doubled(T)(T value) {
return value * 2;
}
alias doubleInt = doubled!int;
```
Now the compiler might say "hey, a.d already has instantiated that one,
and it's not being built by me, so I'll assume it has already generated
the code" and doesn't do it.
-Steve
Re: Modules ... "import" vs. "compilation" ... what is the real process here?
On Tuesday, 28 September 2021 at 05:26:29 UTC, Ali Çehreli wrote: On 9/27/21 10:38 AM, james.p.leblanc wrote: 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. Ali Ali, this is great! ...I had been tempted to also ask about how templates figure into this, but realized that including this in my question would be over complicating the question, so it remained unasked. But, now I have this part answered as well. I very much appreciate the mind-reading tricks going on here on the forum! Thank You, and Best Regards, James
Re: Modules ... "import" vs. "compilation" ... what is the real process here?
On Tuesday, 28 September 2021 at 02:05:43 UTC, Mike Parker wrote: On Monday, 27 September 2021 at 17:38:29 UTC, james.p.leblanc mpilations". Does that help? **Yes!...** === ... this helped immensely! This explanation gave me a much better understanding of how the whole process works. Mike, thanks very much for the time and thought you put into this. Best Regards, James
Re: Modules ... "import" vs. "compilation" ... what is the real process here?
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
Re: Modules ... "import" vs. "compilation" ... what is the real process here?
On Monday, 27 September 2021 at 17:38:29 UTC, james.p.leblanc wrote: Dear D-ers, I have trouble understanding "module imports" vs. "module compilations". A module is implemented in a source file. Though we often use the term "module" to refer to both, it may help to think in terms of importing modules and compiling source files. Given the source files `A.d` and `B.d`, which implement the modules A and B respectively, and given that module A uses symbols from module B, then we can say the following: 1. When the compiler is compiling `A.d`, it must be aware of which symbols from module B are accessible from module A. This is what imports are for and has no relation to the compilation status of `B.d`. 2. The compiler will produce a `A.o/obj` object file that it will then pass to the linker, including references to the symbols in module B. At that point, the linker will also need an object file from a compiled `B.d` in order to fix up the symbol references and produce the final executable. To achieve #1, the compiler needs to read either the source file `B.d` or a D interface file, `B.di`, in order to know which symbols are available to module A. There are a couple of ways this can happen: ``` dmd A.d B.d ``` Here, when the compiler encounters `import B` in `A.d`, it will recognize that `B.d` has been passed on the command line. If `B.d` has no module statement, then the file name `B` is used as the module name. If it has a module statement, it the file can be named anything when it's passed on the command line like this. It could be `foo.d`, but as long as it has a `module B` at the top, then `A.d` can `import B`. ``` dmd -c A.d ``` Here, when the compiler encounters `import B` in `A.d`, it will see that no `module B` declaration has been encountered in any other files on the command line, so it will search for `B.di` and, if it's not found, `B.d` on the import path (to which we can append directories with `-I`). I've included `-c` here, which will just compile `A.d` and not attempt to link it, because without it the linker will spew errors for every missing symbol from module B. This is how D supports separate compilation. Assuming object files with the `.obj` extension on Windows, you could do this: ``` dmd -c B.d dmd A.d B.obj ``` Now, the compiler uses the source of `B.d` to assess symbol accessibility as before, and it will pass both `A.obj` and `B.obj` to the linker to produce the executable. Or you could compile `B.d` into `B.lib` and pass that on the command line as well. Finally, there have been discussions about allowing new ways of "compiling a module" by including its name on the command line. For example this from 2017: https://forum.dlang.org/post/tcrdpvqvwxffnewzo...@forum.dlang.org This is what resulted in the `-i` compiler switch. Modifying the example above: ``` dmd -i A.d ``` Now, when the compiler encounters `import B` in `A.d`, if there is no `B.di` and it finds `B.d`, it will compile `B.d` alongside `A.d`, just as if the command line had been `dmd A.d B.d`. Does that help?
Modules ... "import" vs. "compilation" ... what is the real process here?
Dear D-ers, I have trouble understanding "module imports" vs. "module compilations". For example, in the dlang.org/tour, we have: **"The import statement makes all public functions and types from the given module available."** And from the dlang.org/spec we have: **"Modules are always compiled at global scope and are unaffected by surrounding attributes or other modifiers."** Finally, there have been discussions about allowing new ways of "compiling a module" by including its name on the command line. For example this from 2017: https://forum.dlang.org/post/tcrdpvqvwxffnewzo...@forum.dlang.org The more I look into this, the more that I realize that I do not understand this as well as I had hoped. So, when we specify the module name on the command line, then it gets compiled along with the other files on the command line (seems reasonable). But, if it is NOT a command line argument, then when does it get compiled?? (I believe that at first cut, only the public functions and types are "grabbed"). But, when and how are the subsequent and necessary module compilation (and linking) steps performed? Finally, we use a "-I" for telling dmd to look for imports in a directory, but why only "look for imports", and not "grab anything from there that is used, AND compile them"? (This question is prompted by some recent desires to use ldc and "fastmath"... which, if I understand correctly, will require me to include all of the related modules on the command line for immediate compilation). But, in a broader sense, I need to understand the related issues better. Thank for all responses, as well as continued patience with my questions. Best Regards, James
