Re: Nested function requires forward declaration?
On Thursday, 14 April 2022 at 08:55:25 UTC, Chris Katko wrote: [...] 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? [...] 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: ```d /* 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
Re: Nested function requires forward declaration?
On Thursday, 14 April 2022 at 08:55:25 UTC, Chris Katko wrote:
I imagine this is a really odd edge case but it's piqued my
interest.
Consider this:
```d
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.
Re: Nested function requires forward declaration?
On Thursday, 14 April 2022 at 08:55:25 UTC, Chris Katko wrote: 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. Declarations in function scope are processed in order. This includes nested functions: https://dlang.org/spec/function.html#nested (see #8).
Nested function requires forward declaration?
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:
D
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:
D
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:
D
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.
