Box::new + &[…] notation
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49
README.md
49
README.md
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@ -1,5 +1,20 @@
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## setup
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In module `common`, I have:
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```rust
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pub enum Value {
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Bool(bool),
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Str(String),
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Int(isize),
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Date(DateTime<chrono::Utc>)
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}
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pub type Record<'a> = HashMap<&'a str, Value>;
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```
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(in a future version, `Value` will probably become recursive, with a variant `Vec<Value>` for example)
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In module `modules`, I have:
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```rust
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@ -34,35 +49,43 @@ struct Available {
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cons: Box<fn(ModuleArgs) -> dyn Module>
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}
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const available: [Available] = [
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Available { name: "equals", cons: Box.new(filters::Equals::from_args) }
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const available: &[Available] = &[
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Available { name: String::from("equals"), cons: Box::new(move |a| filters::Equals::from_args(a)) }
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];
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```
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with `available` being an array of available modules, each having a name and a constructor.
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However, I rather fail to see the reason for:
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* `Box`: In my case, it will be set forever, and be unmutable; and fn is a pointer anyway, so fixed-size…
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* `&[…]` notation: In this case, `available` is a constant, and I suppose its size is known at compile-time, so there shouldn’t be the need for a reference.
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## problem
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The above code gives me these errors (which I suspect are hiding the next error about `"equals"` probably being `&str` instead of `String`…):
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The above code gives me these errors:
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* first:
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```
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error[E0423]: expected value, found struct `Box`
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--> src/modules.rs:10:37
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10 | Available { name: "equals", cons: Box.new(filters::Equals::from_args) }
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| ^^^ constructor is not visible here due to private fields
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error[E0277]: the size for values of type `dyn modules::Module` cannot be known at compilation time
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--> src/modules.rs:10:69
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10 | Available { name: String::from("equals"), cons: Box::new(move |a| filters::Equals::from_args(a)) }
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| ^ doesn't have a size known at compile-time
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= help: the trait `std::marker::Sized` is not implemented for `dyn modules::Module`
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```
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* second:
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```
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error[E0277]: the size for values of type `[modules::Available]` cannot be known at compilation time
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--> src/modules.rs:9:18
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error[E0308]: mismatched types
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--> src/modules.rs:10:69
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9 | const available: [Available] = [
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| ^^^^^^^^^^^ doesn't have a size known at compile-time
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10 | Available { name: String::from("equals"), cons: Box::new(move |a| filters::Equals::from_args(a)) }
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| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected trait modules::Module, found struct `filters::Equals`
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= help: the trait `std::marker::Sized` is not implemented for `[modules::Available]`
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= note: expected type `dyn modules::Module`
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found type `filters::Equals`
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```
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