• Ary Borenszweig (84/84) Apr 25 2008 First I'll post the new thing I learned today about D, then I'll explain...
• Janice Caron (13/15) Apr 25 2008 Yes.
• Bruno Medeiros (8/12) Apr 29 2008 Yes, but only in the case of mixins. Regular template instantiation

Ary Borenszweig <ary esperanto.org.ar> writes:

First I'll post the new thing I learned today about D, then I'll explain 
why I made this experiment.

I have a module named "other":

---
module other;

class Something {
	char[] name = "other";
}

template Foo() {
	Something something;
	static this() {
		something = new Something();
	}
}
---

Then I have a module named "main":

---
module main;

import std.stdio;
import other;

mixin Foo!();

void main() {
	writefln("%s", something.name);
}
---

The output of compiling and running this is: other

Now... I modify "main" like this:

---
module main;

import std.stdio;
import other;

// Here is the change, I added this
class Something {
	char[] name = "main";
}

mixin Foo!();

void main() {
	writefln("%s", something.name);
}
---

The output of compiling and running this is: main

Not only that. If I change it to this:

---
module main;

import std.stdio;
import other;

class Something {
	// Here is the change this time
	char[] nameX = "main";
}

mixin Foo!();

void main() {
	writefln("%s", something.name);
}
---

I can't compile main anymore:
main.d(15): Error: no property 'name' for type 'main.Something'

So, basically, I can change a template's instantiation by overriding the 
symbols it uses. Is that expected behaviour?

I did the experiment because you currently don't get autocompletion or 
any cool stuff in Descent inside templates (as well as inside templated 
classes and functions), and won't probably in the next release. Why? 
Because these features rely heavily in DMD's semantic, but semantic 
isn't done for templates, not even checking that an identifier resolves 
correctly, until they are instantiated. So in the above example:

---
module other;

class Something {
	char[] name = "other";
}

template Foo() {
	Something something;
	static this() {
		something. // I'd like to suggest you the field "name",
                            // but who knows if it even exists!
	}
}
---

I know, no one will do that, but because template logic is evaluated at 
the location of the instantiation, I'd like to ask: could templates have 
at least some kind of semantic analysis done? I'd like the compiler to 
show an error if "Something" is not defined in the module I defined the 
template. What I do with template parameters doesn't care, but at least 
some kind of logic...

"Janice Caron" <caron800 googlemail.com> writes:

On 25/04/2008, Ary Borenszweig <ary esperanto.org.ar> wrote:
  So, basically, I can change a template's instantiation by overriding the
 symbols it uses. Is that expected behaviour?

Yes.

But there is a way of unhiding it. Just give the mixin instantiation a name.

    module main;

    import std.stdio;
    import other;

    class Something {
       char[] nameX = "main";
    }

    mixin Foo!() foo;

   void main() {
       writefln("%s", foo.something.name);
    }

Bruno Medeiros <brunodomedeiros+spam com.gmail> writes:

Ary Borenszweig wrote:
 
 So, basically, I can change a template's instantiation by overriding the 
 symbols it uses. Is that expected behaviour?
 

Yes, but only in the case of mixins. Regular template instantiation 
behaves as you'd expect.
If I recall correctly, there have been those who have argued against 
this behavior of mixins.



-- 
Bruno Medeiros - Software Developer, MSc. in CS/E graduate
http://www.prowiki.org/wiki4d/wiki.cgi?BrunoMedeiros#D