Foreign Function Interface (FFI)

Nexus interoperates with WebAssembly modules, allowing extension with functions written in Rust, C, or other languages that compile to WASM.

Importing WASM Modules

Load a WASM module with import external:

import external math.wasm

The module’s exports become available for binding.

External Bindings

Bind a WASM export to a Nexus name:

pub external add_ints = "add" : (a: i64, b: i64) -> i64
external internal_helper = "helper" : (x: i64) -> unit

• pub makes the binding visible to other modules
• The string literal after = is the WASM export name
• The type after : must be an arrow type

Generic External Bindings

Polymorphic externals require explicit type parameters:

pub external length = "array_length" : <T>(arr: &[| T |]) -> i64

Using an undeclared type variable (e.g., T without <T>) is a type error. This prevents typos from silently becoming type variables.

Type Mapping

Nexus Type	WASM Type	Notes
i64	i64	Direct
float / f64	f64	Direct
i32	i32	Via i64 casting
f32	f32	Via float casting
bool	i32	0 = false, 1 = true
string	i64	Packed as (offset, length) pair
unit	(none)	No WASM parameter generated
Records	i32	Heap pointer

Example

import external utils.wasm

external process_data = "process" : (val: float) -> float

let main = fn () -> unit require { PermConsole } do
    inject stdio.system_handler do
        let result = process_data(val: 42.0)
        Console.println(val: string.from_float(val: result))
    end
    return ()
end