Using DMD. v2.098-beta-2

Not sure if right terminology. But I just wrote a nested function that uses a variable outside its body. The capture (right term?) is obvious where the invocation is. However, I have to move the declaration of the variable to above the nested function for it to compile.

Here is the code that wont compile:

void execute()
	{
	bool isKey(ALLEGRO_KEY key)
		{
		return (event.keyboard.keycode == key);
		}
	ALLEGRO_EVENT event;
	// later
	isKey(ALLEGRO_KEY_W);

// main.d(491): Error: undefined identifier `event`

This however, will compile:

void execute()
	{
	ALLEGRO_EVENT event; // <--Only change
	bool isKey(ALLEGRO_KEY key)
		{
		return (event.keyboard.keycode == key);
		}
	// later
	isKey(ALLEGRO_KEY_W);

It appears the nested function's variable capture depends on forward declaration (the right term?). Whereas, I was under the impression most/all of D worked on a multiple pass compilation so the order of declarations shouldn't matter.

Is this a D spec, or a compiler parsing error/oversight?

I guess if I had two different variables called event, this could become confusing code to read except that, mentally these should still link up, right?

Hypothetical:

void execute()
{
bool isKey(ALLEGRO_KEY key)
	{
	return (event.keyboard.keycode == key);
	}

   {
   ALLEGRO_EVENT event;
   isKey(ALLEGRO_KEY_W);
   } //lets say this does some memory housekeeping/deleting so that's why we use scope
   {
   ALLEGRO_EVENT event; //new 'event', same name, new memory
   isKey(ALLEGRO_KEY_W);
   }
}

in this case, 'event' under-the-hood could be renamed, say, "event2" and have the same expected compile time symbol linking occur. The second isKey call is obviously connected to the second 'event' variable.

I imagine this is a really odd edge case but it's piqued my interest.

On Thursday, 14 April 2022 at 08:55:25 UTC, Chris Katko wrote:

Declarations in function scope are processed in order. This includes nested functions:

https://dlang.org/spec/function.html#nested (see #8).

On Thursday, 14 April 2022 at 08:55:25 UTC, Chris Katko wrote:

Consider this:

void main() {
    void foo() { initRuntimeState(i); }
    foo();
    if(!modifyRutimeState()) return;
    int i = getRandomValue();
    i = returnSomethingBasedOnRuntimeState(i);
}

Which value of i should foo use? What if modifyRuntimeState and returnSomethingBasedOnRuntimeState are dependent on initRuntimeState?

At module scope, and in class/struct declarations, only compile-time values can be used to initialize variables and assignment is illegal, so the compiler can jump around initializing things in any order it wants. It has a well-defined and limited set of rules it can work with.

That just isn't the case in local scopes, where a variable's lifetime begins at the point of declaration, the scope can exit at any time, and the order of evaluation can have side effects that change the run-time state.

On Thursday, 14 April 2022 at 08:55:25 UTC, Chris Katko wrote:

This is cross-module what you're talking about, eg outside of main(). Something with most programming languages, it's not about nested function. Do it with lambda if you want, and you'll get the same result:

/* auto is_Key = (ALLEGRO_KEY key) =>
                 (event.keyboard.keycode == key); // Not compiling! */
ALLEGRO_EVENT event;

You have one more option, and that is to use OOP. You can even briefly encapsulate it into a struct.

SDB@79