help-rust/README.md

## setup

In module `common`, I have:

```rust
pub enum Value {
  Bool(bool),
  Str(String),
  Int(isize),
  Date(DateTime<chrono::Utc>)
}

pub type Record<'a> = HashMap<&'a str, Value>;
```

(in a future version, `Value` will probably become recursive, with a variant `Vec<Value>` for example)

In module `modules`, I have:

```rust
pub trait Module {
  fn run(&self, record: &mut Record) -> Result<bool, ()>;
}
```

In module `filters`, I have:

```rust
pub struct Equals { … }

impl Equals {
  pub fn from_args(mut args: ModuleArgs) -> Equals {
    Equals { … }
  }
}

impl Module for Equals {
  fn run(&self, record: &mut Record) -> Result<bool, ()> { … }
}
```

## goal, “naive-style”

What I really want (for a start) is that, in module `modules`:

```rust
struct Available {
  name: String,
  cons: Box<fn(ModuleArgs) -> dyn Module>
}

const available: &[Available] = &[
  Available { name: String::from("equals"), cons: Box::new(move |a| filters::Equals::from_args(a)) }
];
```

with `available` being an array of available modules, each having a name and a constructor.

However, I rather fail to see the reason for:

* `Box`: In my case, it will be set forever, and be unmutable; and fn is a pointer anyway, so fixed-size…
* `&[…]` 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.

## problem

The above code gives me these errors:

* first:

  ```
  error[E0277]: the size for values of type `dyn modules::Module` cannot be known at compilation time
    --> src/modules.rs:10:69
    |
  10 |   Available { name: String::from("equals"), cons: Box::new(move |a| filters::Equals::from_args(a)) }
    |                                                                     ^ doesn't have a size known at compile-time
    |
    = help: the trait `std::marker::Sized` is not implemented for `dyn modules::Module`
  ```

* second:

  ```
  error[E0308]: mismatched types
    --> src/modules.rs:10:69
    |
  10 |   Available { name: String::from("equals"), cons: Box::new(move |a| filters::Equals::from_args(a)) }
    |                                                                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected trait modules::Module, found struct `filters::Equals`
    |
    = note: expected type `dyn modules::Module`
                found type `filters::Equals`
  ```