FFI

Some things cannot be written in Mew. Reading a file, writing to a terminal and asking the operating system for the time all happen somewhere else, and the foreign function interface is how a program reaches them.

An external function is a declaration with no body. The library named by its ffi attribute supplies the body, and the declaration ends in ; where a block would otherwise go.

[ffi("mylib")]
pub static external fn measure(first: i8) -> void;

The standard library is built on this and gets no special treatment: println and atoi reach a native library called mewstd through exactly these declarations, with no privilege the language does not give your own code.

It has to be static

An external function has no value to be called on, so it has to be static.

[ffi("mylib")]
pub external fn measure(first: i8) -> void;

Naming the function on the other side

ffi says which library. host says which function inside it, for when the name on the other side is not the name you want to write in Mew.

[ffi("mewstd")]
[host("mew_write_line")]
pub static external fn write_line(text: string) -> void;

Without host, the function is looked up under its own name.

What can cross

Only some types can go across the boundary, and the set is not the same in both directions.

A string or an array may go in but cannot come back, so a native function that produces text writes into a buffer it was handed rather than returning one.

char crosses in neither direction, and neither does any, a type you declare, a union or an interface. Anything else is an error at the declaration rather than a surprise at run time.

pub type Point {
    pub field x: i32;
}
  
[ffi("mylib")]
pub static external fn take(point: Point) -> void;

To hand a declared type across, take it apart and pass what it holds.

Nothing checks the other side

A declaration is a promise about a library the compiler cannot see. If the signature does not match what is really there, nothing says so until the program runs, and what happens then is the operating system's business rather than Mew's.

If the function ends the program rather than returning, say so with [noreturn]. That is exactly the case the attribute exists for, and why the compiler takes it on trust rather than checking a body it does not have.