Re: "in" operator gives a pointer result from a test against an Associative Array?
On Friday, 10 May 2024 at 16:33:53 UTC, Nick Treleaven wrote: Arrays evaluate to true in boolean conditions if their `.ptr` field is non-null. This is bug-prone and I hope we can remove this in the next edition. ... A string literal's `.ptr` field is always non-null, because it is null-terminated. Thank you! Andy
Re: "in" operator gives a pointer result from a test against an Associative Array?
On Friday, 10 May 2024 at 15:23:39 UTC, Andy Valencia wrote: On Friday, 10 May 2024 at 03:07:43 UTC, Steven Schveighoffer wrote: Yes, we say that a type has "truthiness" if it can be used in a condition (`while`, `if`, `assert`, etc). So if I may ask for one more small clarification... WRT "truthiness", I've observed that empty arrays are treated as false, non-empty as true. Arrays evaluate to true in boolean conditions if their `.ptr` field is non-null. This is bug-prone and I hope we can remove this in the next edition. However, although I thought a string was basically an immutable array of characters, "" is treated as true, not false? A string literal's `.ptr` field is always non-null, because it is null-terminated.
Re: "in" operator gives a pointer result from a test against an Associative Array?
On Friday, 10 May 2024 at 03:07:43 UTC, Steven Schveighoffer wrote: Yes, we say that a type has "truthiness" if it can be used in a condition (`while`, `if`, `assert`, etc). So if I may ask for one more small clarification... WRT "truthiness", I've observed that empty arrays are treated as false, non-empty as true. However, although I thought a string was basically an immutable array of characters, "" is treated as true, not false? Thanks again, Andy
Re: D doesn't have weak references. So how can I make a associative array of objects without preventing their destruction?
On Friday, 10 May 2024 at 13:27:40 UTC, Dukc wrote: Steven Schveighoffer kirjoitti 10.5.2024 klo 16.01: On Friday, 10 May 2024 at 11:05:28 UTC, Dukc wrote: This also gets inferred as `pure` - meaning that if you use it twice for the same `WeakRef`, the compiler may reuse the result of the first dereference for the second call, without checking whether the referred value has changed! This would be weak pure since the reference is mutable. This cannot be memoized. The difference is the type. With a pointer parameter (both a bare one and one in a struct), the compiler can cache the result only when the pointed data is similar. However, here we have an integer parameter. It can be cached if it itself is similar to the one in the other function call. The compiler doesn't have to know, nor can know, when a `size_t` is a pointer in disguise. This why I would just use ref counting if I were the topic author, trying to be smart will often comes back when one doesn't expect. And as stated by previous author if the goal is to have self-clearable weak reference it will need some infrastructure anyway, so tbh this will greatly outweight any possible benefits of having weak refs.
Re: D doesn't have weak references. So how can I make a associative array of objects without preventing their destruction?
Steven Schveighoffer kirjoitti 10.5.2024 klo 16.01: On Friday, 10 May 2024 at 11:05:28 UTC, Dukc wrote: This also gets inferred as `pure` - meaning that if you use it twice for the same `WeakRef`, the compiler may reuse the result of the first dereference for the second call, without checking whether the referred value has changed! This would be weak pure since the reference is mutable. This cannot be memoized. The difference is the type. With a pointer parameter (both a bare one and one in a struct), the compiler can cache the result only when the pointed data is similar. However, here we have an integer parameter. It can be cached if it itself is similar to the one in the other function call. The compiler doesn't have to know, nor can know, when a `size_t` is a pointer in disguise.
Re: D doesn't have weak references. So how can I make a associative array of objects without preventing their destruction?
On Friday, 10 May 2024 at 11:05:28 UTC, Dukc wrote: This also gets inferred as `pure` - meaning that if you use it twice for the same `WeakRef`, the compiler may reuse the result of the first dereference for the second call, without checking whether the referred value has changed! This would be weak pure since the reference is mutable. This cannot be memoized. -Steve
Re: D doesn't have weak references. So how can I make a associative array of objects without preventing their destruction?
evilrat kirjoitti 9.5.2024 klo 18.19:
```d
struct WeakRef(T) {
private size_t _handle; // same size as a pointer
this(T* ptr) {
_handle = cast(size_t) ptr;
}
T* getRef() {
return cast(T*) _handle;
}
// do the rest ...
}
```
[1] https://code.dlang.org/packages/automem
There is a hidden danger with using this struct. Since `getRef` is a
template, it will be inferred as `pure`. Now, consider a function using it:
```D
auto derefer(WeakrefT)(WeakrefT wr) => *wr.getRef;
```
This also gets inferred as `pure` - meaning that if you use it twice for
the same `WeakRef`, the compiler may reuse the result of the first
dereference for the second call, without checking whether the referred
value has changed!
You probably should add some never-executed dummy operation to `getRef`
that prevents it from becoming `pure` if you go with this design.
Re: moving from classical lex/yacc to pegged parser
Dmitry Ponyatov kirjoitti 9.5.2024 klo 11.30: > And I also can't figure out how to inherit `ParseTree` with all my script language objects to get AST right from pegged parser. Should I use some superloop with lot of matches to process parsed `pt` tree into something I need myself, to drop all unneeded parsing meta info and get clean semantic AST? Pegged can help you with that filtering part, at least to some extent. Remember you can use : and ; prefix operators in the grammar spec, and Pegged will drop the needless nodes for you. Or do the reverse, use ^ prefix operator (or write a new rule) to make a node out of Pegged builtins.
