mtp/docs/TYPE-MAP.md

Type Map

This file documents the Type Map & Registry configuration used by the MTP protocol.

TypeMap & Compile-Time Type Safety

A TypeMap maps Communication-Types and Data-Types to their wire IDs. Each protocol version has its own TypeMap because the same type name may use different wire IDs in different versions.

Type names are defined in a YAML config and turned into Rust enums at compile time by a build.rs in the type-map crate. This means invalid type names are caught by the compiler instead of failing at runtime.

Defining Type Maps

Place your own type-maps.yaml in your project root. Browser apps should use the Vite plugin so the app-specific WASM package is generated into Vite's cache during dev/build:

import { defineConfig } from "vite";
import { mtp } from "mtp/vite";

export default defineConfig({
  plugins: [mtp({ typeMaps: "./type-maps.yaml" })],
});

Rust and manual WASM builds can set MTP_TYPE_MAPS directly (see Customizing Type Maps in Downstream Projects).

Using Generated Enums

After editing the config and rebuilding, CommunicationType and DataType enums are generated automatically. Use them in code:

use mtp::type_map::{CommunicationType, DataType, TypeMap};

let tm = TypeMap::v2_0();
let id = tm.data_id_enum(DataType::SomeType).unwrap();

The enums are a union across all versions; every type name from every version is a variant. The version-specific TypeMap maps each variant to the correct wire ID for that version. Types not defined in a version return None:

Encoding/decoding uses a TypeMap to resolve type names to wire IDs:

use mtp::codec::{encode, decode, DataValue};
use mtp::type_map::TypeMap;

let tm = TypeMap::v2_0();
let value = DataValue::Str("hello".into());

let bytes = encode(&value, &tm).unwrap();
let decoded = decode(&bytes, &tm).unwrap();

let tm_v2 = TypeMap::v2_0();
assert!(tm_v2.data_id_enum(DataType::SomeType).is_some());    // defined in v2.0
assert!(tm_v2.data_id_enum(DataType::ExampleType).is_none()); // NOT in v2.0

let tm_v1 = TypeMap::v1_0();
assert!(tm_v1.data_id_enum(DataType::ExampleType).is_some());  // defined in v1.0

Forward/Backward Compatibility Between Versions

Because enums are a union of all types across versions, a variant might exist that has no wire mapping in the negotiated version:

v2.0 client sends DataType::SomeType    → host encodes with v2.0 TypeMap → wire ID 32
v2.0 host receives DataType::ExampleType (from v1.0 client) → not in v2.0 TypeMap → None → Error

This is by design: the host maps unknown types to Error, and the client should only send types that exist in its compiled-in version.

Registry

The registry feature of the Codec crate adds VersionedCodec for version-aware encoding:

Requires the host feature (which enables mtp-codec's registry feature):

[dependencies]
mtp = { path = "..", features = ["host"] }

use mtp::codec::registry::{Registry, VersionedCodec};

let registry = Registry::builtin();
let codec = VersionedCodec::new(registry);

// Encode with a specific version
let bytes = codec.encode(&value, Version(2, 0)).unwrap();

// Decode with a specific version
let decoded = codec.decode(&bytes, Version(2, 0)).unwrap();

Customizing Type Maps in Downstream Projects

External projects must provide their own type map configuration. Browser projects should install mtp and configure mtp/vite; they do not need to publish, fork, or copy a generated WASM package.

import { defineConfig } from "vite";
import { mtp } from "mtp/vite";

export default defineConfig({
  plugins: [mtp({ typeMaps: "./type-maps.yaml" })],
});

For Rust builds, or when invoking wasm-pack manually, set the MTP_TYPE_MAPS environment variable. If the variable points to an invalid file, the build fails. If MTP_TYPE_MAPS is not set, the build script emits a warning and generates reserved protocol types only; application-specific communication and data types will not be available.

1. Create a type-maps.yaml in your project root
2. Set the MTP_TYPE_MAPS environment variable in .cargo/config.toml:

# .cargo/config.toml
[env]
MTP_TYPE_MAPS = { value = "type-maps.yaml", relative = true }