Type Mapping

Every formalang type that crosses the WIT boundary maps to a WIT shape. The full table is below; for the rules governing what may cross, see Boundary Policy.

Internal types (closures, ranges, the bump-allocator's free pointer) live entirely inside the core module and never appear in the WIT file.

Primitives

Containers

Aggregates

Examples

A function with primitives only:

#![allow(unused)]
fn main() {
pub fn id(x: I32) -> I32 { x }
}

emits:

package formawasm:generated;

world component {
  export id: func(x: s32) -> s32;
}

A pub struct:

#![allow(unused)]
fn main() {
pub struct Point { x: I32, y: I32 }
}

emits:

package formawasm:generated;

interface types {
  record point {
    x: s32,
    y: s32,
  }
}

world component {
  use types.{point};
}

An enum with mixed arms:

#![allow(unused)]
fn main() {
pub enum Action {
    reset
    add(value: I32)
    replace(x: I32, y: I32)
}
}

emits:

package formawasm:generated;

interface types {
  variant action {
    reset,
    add(s32),
    replace(tuple<s32, s32>),
  }
}

world component {
  use types.{action};
}

Identifier conversion

formalang identifiers use snake_case or camelCase; WIT identifiers are required to be kebab-case. The backend converts every identifier crossing the boundary:

• call_host → call-host
• Point → point
• MyEnum → my-enum

This applies to function names, type names, record field names, and variant arm names. Inside a component, the original identifier is preserved (and visible in the wasm name custom section for debugger tooling); only the boundary view is kebab-cased.

Internal-only types

These never appear in WIT and are documented here only so you know they exist:

If you mark a function pub whose signature mentions any of these as a top-level type, pre-flight rejects it. (Some — like anonymous structs nested inside a pub record's fields — flow through transparently because the layout planner handles them.)