rhai/doc/src/engine/metadata/gen_fn_sig.md

Generate Function Signatures
===========================

{{#include ../../links.md}}


`Engine::gen_fn_signatures`
--------------------------

As part of a _reflections_ API, `Engine::gen_fn_signatures` returns a list of function _signatures_
(as `Vec<String>`), each corresponding to a particular function available to that [`Engine`] instance.

> `fn_name ( param_1: type_1, param_2: type_2, ... , param_n : type_n ) -> return_type`

### Sources

Functions from the following sources are included, in order:

1) Native Rust functions registered into the global namespace via the `Engine::register_XXX` API
2) _Public_ (i.e. non-[`private`]) functions (native Rust or Rhai scripted) in global sub-modules registered via
   [`Engine::register_module`]({{rootUrl}}/rust/modules/create.md)
3) Native Rust functions in registered [packages] (optional)


Functions Metadata
------------------

Beware, however, that not all function signatures contain parameters and return value information.

### `Engine::register_XXX`

For instance, functions registered via `Engine::register_XXX` contain no information on
the names of parameter and their actual types because Rust simply does not make such metadata
available natively. The return type is also undetermined.

A function registered under the name `foo` with three parameters and unknown return type:

> `foo(_, _, _)`

An operator function - again, unknown parameters and return type.
Notice that function names do not need to be valid identifiers.

> `+(_, _)`

A [property setter][getters/setters] - again, unknown parameters and return type.
Notice that function names do not need to be valid identifiers.
In this case, the first parameter should be `&mut T` of the custom type and the return value is `()`:

> `set$prop(_, _, _)`

### Script-Defined Functions

Script-defined [function] signatures contain parameter names. Since all parameters, as well as
the return value, are [`Dynamic`] the types are simply not shown.

A script-defined function always takes dynamic arguments, and the return type is also dynamic:

> `foo(x, y, z) -> Dynamic`

probably defined as:

```rust
fn foo(x, y, z) {
    ...
}
```

is the same as:

> `foo(x: Dynamic, y: Dynamic, z: Dynamic) -> Result<Dynamic, Box<EvalAltResult>>`

### Plugin Functions

Functions defined in [plugin modules] are the best.  They contain all the metadata
describing the functions.

For example, a plugin function `merge`:

> `merge(list: &mut MyStruct<i64>, num: usize, name: &str) -> Option<bool>`

Notice that function names do not need to be valid identifiers.

For example, an operator defined as a [fallible function] in a [plugin module] via
`#[rhai_fn(name="+=", return_raw)]` returns `Result<bool, Box<EvalAltResult>>`:

> `+=(list: &mut MyStruct<i64>, num: usize, name: &str) -> Result<bool, Box<EvalAltResult>>`

For example, a [property getter][getters/setters] defined in a [plugin module]:

> `get$prop(obj: &mut MyStruct<i64>) -> String`