# Tutorial: Dependency Interfaces — Bind a Module by Contract This tutorial builds `calc_via_interface`, a **core module that depends on an *interface*, not a concrete module**. Instead of naming `calc_module` (from [Part 1](tutorial-wrapping-c-library.md)) as a dependency and getting a fixed `modules().calc_module` wrapper, it declares a small **`calculator` interface** — a list of methods and one event — and **binds that interface to a module name chosen at runtime**. Any module whose API is a *superset* of the interface can satisfy it; `calc_module` is one such provider. You drive the whole thing from `logoscore` on the command line. **What you'll build:** A `calc_via_interface` core module that: - declares a **`calculator` interface** in its own language (a pure-C++ header with a `logos_events:` block) — `interfaces/calculator.h` - lists it under `metadata.json`'s `interface_dependencies` — and names **no concrete module** in `dependencies` - **binds** the interface to a runtime-chosen module with `modules().bind_calculator("calc_module")`, then calls it through the usual type-safe wrappers — **synchronously** (`add`, `multiply`, `libVersion`), **asynchronously** (`fibonacciAsync` + callback), and via a typed **event** subscription (`onVersionReady`) - proves the **no-validation** contract: binding to a module that does not satisfy the interface fails as an ordinary remote-call error — no crash No Qt, no `LogosAPI`, no plugin boilerplate — one plain C++ class, plus a one-file interface contract. **What you'll learn:** - The difference between a concrete dependency (`dependencies`) and a dependency interface (`interface_dependencies`) - How to declare an interface in pure C++ (methods + a `logos_events:` block) — or equivalently in `.lidl` - How the generator turns an interface into a **bound** wrapper whose target module is a constructor argument, exposed as `modules().bind_(moduleName)` - How to call a bound interface **synchronously** and **asynchronously**, and how to subscribe to its events — all type-safely - Why binding is decoupled from loading, and what the "superset" / no-validation rule means in practice - How to share one interface across repos via a flake input (the same wiring `dependencies` use) ## Prerequisites - Completed [Part 1](tutorial-wrapping-c-library.md) — you have a working `calc_module` whose shared library is built (`libcalc.so`/`.dylib` in `logos-calc-module/lib/`). This tutorial only needs `calc_module` as a *runtime* provider; it is never named at build time. - Nix with flakes enabled - Basic familiarity with C++ --- ## Step 1: Scaffold the Module Project Create a new directory and initialise it from the minimal module template: `mkdir logos-calc-via-interface-module && cd logos-calc-via-interface-module` ### 1.1 Create the project from the template ```bash nix flake init -t github:logos-co/logos-module-builder/0.2.0 ``` This scaffolds a `flake.nix`, `metadata.json`, `CMakeLists.txt`, and a `src/` directory pre-wired for `logos-module-builder`. As in Part 1 we use the **pure-C++ (`interface: universal`) pattern**, so we replace the template's example `src/` files with our own plain `*_impl.h` / `*_impl.cpp`. ### 1.2 Remove the template's example sources Delete the example Qt plugin the template ships — this tutorial supplies its own pure-C++ `src/` files: ```bash rm -f src/minimal_interface.h src/minimal_plugin.h src/minimal_plugin.cpp ``` --- ## Step 2: Declare the Interface An **interface** is a method/event contract decoupled from any concrete module. You write it in the *same language as your module* — for a universal module that's a plain C++ header. It looks like an impl class, but you only declare signatures: the generator reads them to build a typed client. Put it in an `interfaces/` directory: ### 2.1 `interfaces/calculator.h` — the contract The `calculator` interface names four methods and one event. It is deliberately a **subset** of what `calc_module` exposes (which also has `factorial`, `libVersionNotify`, …) — that is the *superset rule*: a provider may expose more than the interface requires. ```cpp #pragma once // A DEPENDENCY INTERFACE: a method/event contract that names no // module. Any module whose API is a superset of this can satisfy // it; the consumer binds it to a concrete module name at runtime. // // Written in the module's own language (pure C++). The generator // reads the public methods + the `logos_events:` block and emits a // BOUND wrapper class `Calculator` whose target module name is a // runtime constructor argument — not baked in. // // Types are std (int64_t / std::string) because the consuming // module is `interface: "universal"`; the bound wrapper inherits // that api-style. #include #include // Defines the `logos_events` token (expands to `public`) so this // header is valid C++ on its own, not only as generator input. #include class ICalculator { public: int64_t add(int64_t a, int64_t b); int64_t multiply(int64_t a, int64_t b); int64_t fibonacci(int64_t n); std::string libVersion(); logos_events: // Emitted by the provider; the consumer subscribes through the // bound wrapper's generated onVersionReady(...) accessor. void versionReady(const std::string& version); }; ``` A few things to notice: - The class name (`ICalculator`) and method signatures are all the generator needs — no `LogosAPI`, no Qt, no reference to any concrete module. The one include (`logos_module_context.h`) just defines the `logos_events` token so the header is valid C++ on its own. - `logos_events:` (like Qt's `signals:`) marks event declarations. The generator turns each into a typed `on(callback)` subscriber on the bound wrapper. - You could write the exact same contract as a `.lidl` file instead — `interfaces/calculator.lidl` with `method add(a: int, b: int) -> int` … `event versionReady(version: tstr)`. The `.h` form is shown here because it matches a universal module's own language. --- ## Step 3: Configure the Module Three config files declare the module, point it at the interface, and tell CMake how to build it. The key contrast with [Composing Modules](tutorial-composing-modules.md): there is **no concrete module** in `dependencies`. ### 3.1 `metadata.json` — declare the interface dependency `interface_dependencies` lists the contracts this module binds at runtime. Each entry gives the interface `name` and the `file` that defines it (and, for a `.h` file, the `impl_class` whose signatures define the contract). `dependencies` stays **empty** — we never name `calc_module` at build time. ```json { "name": "calc_via_interface", "version": "1.0.0", "type": "core", "category": "general", "description": "Binds a calculator interface to a module chosen at runtime", "main": "calc_via_interface_plugin", "interface": "universal", "dependencies": [], "interface_dependencies": [ { "name": "calculator", "file": "interfaces/calculator.h", "impl_class": "ICalculator" } ], "nix": { "packages": { "build": [], "runtime": [] }, "external_libraries": [], "cmake": { "find_packages": [], "extra_sources": [], "extra_include_dirs": [], "extra_link_libraries": [] } } } ``` | Field | What it does | | ------------------------ | ----------------------------------------------------------------------------------------------------------------------------- | | `interface` | `"universal"` — pure C++ impl, the builder generates the Qt plugin glue | | `dependencies` | `[]` — **no concrete module** is named at build time | | `interface_dependencies` | `[{ name, file, impl_class }]` — the builder generates the bound wrapper `Calculator` + the `modules().bind_calculator(...)` factory | For a `.h` interface the `impl_class` field is required (the class whose signatures define the contract). For a `.lidl` interface, omit it. To pull an interface from **another repo**, add an `"input"` field naming a flake input — covered in the final step. ### 3.2 `CMakeLists.txt` — list your sources You list only your plain C++ files. The generated interface wrapper and plugin glue are compiled automatically. ```cmake cmake_minimum_required(VERSION 3.14) project(CalcViaInterfacePlugin LANGUAGES CXX) if(DEFINED ENV{LOGOS_MODULE_BUILDER_ROOT}) include($ENV{LOGOS_MODULE_BUILDER_ROOT}/cmake/LogosModule.cmake) elseif(EXISTS "${CMAKE_CURRENT_SOURCE_DIR}/cmake/LogosModule.cmake") include(cmake/LogosModule.cmake) else() message(FATAL_ERROR "LogosModule.cmake not found") endif() logos_module( NAME calc_via_interface SOURCES src/calc_via_interface_impl.h src/calc_via_interface_impl.cpp ) ``` `NAME` must match `name` in `metadata.json` (`calc_via_interface`). No module dependency to wire here — the interface file is local to this repo. ### 3.3 `flake.nix` — no module inputs needed Because there is no concrete dependency, the only input is the builder itself. (Contrast with Composing Modules, which had to add a `calc_module.url` input.) ```nix { description = "Core module that binds a calculator interface at runtime"; inputs = { logos-module-builder.url = "github:logos-co/logos-module-builder/0.2.0"; }; outputs = inputs@{ logos-module-builder, ... }: logos-module-builder.lib.mkLogosModule { src = ./.; configFile = ./metadata.json; flakeInputs = inputs; }; } ``` `flakeInputs = inputs` hands the builder everything it needs. It reads `interface_dependencies` from `metadata.json`, resolves the local `interfaces/calculator.h`, and runs `logos-cpp-generator` to emit the bound `Calculator` wrapper into `generated_code/`. --- ## Step 4: Write the Module Class The module is one plain C++ class inheriting `LogosModuleContext` — that base gives it `modules()`, through which the generated `bind_calculator(name)` factory is reachable. Each method takes the **provider module name** as its first argument, so we can bind to different modules at runtime from `logoscore`. ### 4.1 `src/calc_via_interface_impl.h` — the class Every `public` method becomes callable over IPC. They fall into three groups: synchronous binds, an asynchronous bind, and an event subscription. ```cpp #pragma once #include #include #include // LogosModuleContext base → modules() // Binds the `calculator` interface (interfaces/calculator.h) to a // module name chosen at runtime and calls it through the generated, // type-safe bound wrapper. It names no concrete module of its own — // the provider is whatever string you pass in. class CalcViaInterfaceImpl : public LogosModuleContext { public: CalcViaInterfaceImpl() = default; ~CalcViaInterfaceImpl() = default; // ── Synchronous binds ────────────────────────────────────── // Bind `calculator` to `provider`, then call it. The module // name appears only at bind time, never on the call. int64_t sumVia(const std::string& provider, int64_t a, int64_t b); int64_t productVia(const std::string& provider, int64_t a, int64_t b); std::string versionVia(const std::string& provider); // ── Asynchronous bind ────────────────────────────────────── // Fire calculator.fibonacci(n) asynchronously against `provider` // and return immediately ("queued"). Read the reply later with // lastFib(). std::string startFibVia(const std::string& provider, int64_t n); int64_t lastFib() const; // ── Event subscription ───────────────────────────────────── // Subscribe to the interface's `versionReady` event on // `provider` via the generated onVersionReady(...) accessor. std::string watchVersion(const std::string& provider); std::string lastVersion() const; private: int64_t m_lastFib = -1; std::string m_lastVersion; }; ``` The provider name is a plain `std::string` parameter — that is the whole "bind at runtime" idea. The same handle code works for any module that satisfies `calculator`. ### 4.2 `src/calc_via_interface_impl.cpp` — the implementation The `.cpp` includes the generated `logos_sdk.h` (which defines `LogosModules` and the `bind_calculator` factory), so the bind/call sites live here rather than in the header the generator parses. ```cpp #include "calc_via_interface_impl.h" // Generated at build time by logos-cpp-generator. Because // metadata.json lists `interface_dependencies`, LogosModules gains a // `bind_calculator(moduleName)` factory returning the bound // `Calculator` wrapper. Included only in the .cpp so the impl header // the generator parses stays free of generated types. #include "logos_sdk.h" // ── Synchronous binds ─────────────────────────────────────────────── int64_t CalcViaInterfaceImpl::sumVia(const std::string& provider, int64_t a, int64_t b) { // Bind once, then call normally — no module name on the call. // Every generated method also takes an optional trailing // logos::CallError* — the explicit way to tell a failed remote // call apart from a legitimate result (without it, a failed // call returns the type's default and only logs a warning). auto calc = modules().bind_calculator(provider); logos::CallError err; const int64_t sum = calc.add(a, b, &err); if (!err.ok()) return -1; // e.g. the bound module isn't loaded return sum; } int64_t CalcViaInterfaceImpl::productVia(const std::string& provider, int64_t a, int64_t b) { return modules().bind_calculator(provider).multiply(a, b); } std::string CalcViaInterfaceImpl::versionVia(const std::string& provider) { return modules().bind_calculator(provider).libVersion(); } // ── Asynchronous bind ──────────────────────────────────────────────── std::string CalcViaInterfaceImpl::startFibVia(const std::string& provider, int64_t n) { // The generated async overload is `Async(args..., // callback, timeout)`. It returns immediately; the reply lands in // the callback on this module's event loop. The bound handle is a // temporary, but the call is registered on the LogosAPI-owned // client and the callback captures `this`, so it outlives it. modules().bind_calculator(provider).fibonacciAsync(n, [this](int64_t value) { m_lastFib = value; }); return "queued"; } int64_t CalcViaInterfaceImpl::lastFib() const { return m_lastFib; } // ── Event subscription ─────────────────────────────────────────────── std::string CalcViaInterfaceImpl::watchVersion(const std::string& provider) { // onVersionReady(...) is generated from the interface's // `logos_events:` block; the callback's arg type matches the event. bool ok = modules().bind_calculator(provider).onVersionReady( [this](const std::string& version) { m_lastVersion = version; }); return ok ? "ok" : "failed"; } std::string CalcViaInterfaceImpl::lastVersion() const { return m_lastVersion; } ``` Everything flows through `modules().bind_calculator(provider)` — the factory the builder generated from `interface_dependencies`. There is no `modules().calc_module`, because `calc_module` is never a build-time dependency. The bound `Calculator` exposes the same typed sync/async/event API the name-baked wrappers do; the only difference is the target module is chosen when you bind. --- ## Step 5: Build the Module ### 5.1 Add a `.gitignore` and init the repo Nix flakes require a git repository, and only tracked files are visible — so `interfaces/calculator.h` must be committed for the generator to find it. Exclude build artifacts first: ```text # Nix build output result result-* # CMake build directory build/ ``` Initialise the repo and stage the files (including `interfaces/`): ```bash git init && git add -A ``` ### 5.2 Build For a universal module with an interface dependency, this is where `logos-cpp-generator` runs over both `src/calc_via_interface_impl.h` (plugin glue) and `interfaces/calculator.h` (the bound `Calculator` wrapper + `bind_calculator` factory), emitting everything under `generated_code/`: ```bash nix build ``` ### 5.3 Check the output ```bash ls -la result/lib/ ``` You should see your plugin (extension depends on platform): ``` calc_via_interface_plugin.so # Linux calc_via_interface_plugin.dylib # macOS ``` --- ## Step 6: Inspect the Module Use `lm` to confirm the public API made it into the binary — and, tellingly, that there is **no module dependency**. ### 6.1 Build `lm` ```bash nix build 'github:logos-co/logos-module/0.2.0#lm' --out-link ./lm ``` ### 6.2 View metadata — note the empty dependency list ```bash ./lm/bin/lm metadata result/lib/calc_via_interface_plugin.so # Linux ./lm/bin/lm metadata result/lib/calc_via_interface_plugin.dylib # macOS ``` ``` Plugin Metadata: ================ Name: calc_via_interface Version: 1.0.0 Description: Binds a calculator interface to a module chosen at runtime Type: core Dependencies: ``` `Dependencies:` is empty — the module is coupled to the `calculator` *contract*, not to any module. ### 6.3 List methods ```bash ./lm/bin/lm methods result/lib/calc_via_interface_plugin.so # Linux ./lm/bin/lm methods result/lib/calc_via_interface_plugin.dylib # macOS ``` Every `public` method is here, published in the **LIDL contract** vocabulary rather than in C++ or Qt names: `int64_t` shows up as `int` and `std::string` as `tstr`. `lm` is reporting what the module says about itself, and what a module publishes is its contract. --- ## Step 7: Run it with `logoscore` Now the payoff: run `calc_via_interface` and bind its `calculator` interface to the real `calc_module` from Part 1. We use the `logoscore` **daemon** (`-D`) so module processes stay alive between `call` commands — needed for the async reply and the event subscription to survive from one call to the next. (Same daemon flow as [Part 1](tutorial-wrapping-c-library.md#step-6-test-with-logoscore) and [Composing Modules](tutorial-composing-modules.md#run-it-with-logoscore).) ### 7.1 Build the runtime and package both modules Build `logoscore` and the package manager, then install **both** modules into a `modules/` directory. `calc_via_interface` comes from this project; `calc_module` from your Part 1 checkout — it is the *provider* we bind to, even though this module never declared it: ```bash nix build 'github:logos-co/logos-logoscore-cli/0.2.0' --out-link ./logos ``` ```bash nix build 'github:logos-co/logos-package-manager/0.2.0#cli' --out-link ./pm ``` ```bash mkdir -p modules ``` ### 7.2 Install calc_via_interface ```bash nix build '.#lgx' --out-link result-iface-lgx ./pm/bin/lgpm --modules-dir ./modules install --file result-iface-lgx/*.lgx ``` ### 7.3 Install calc_module (the runtime provider) Make sure `calc_module`'s shared library is built (from [Part 1](tutorial-wrapping-c-library.md#15-build-the-shared-library)), then package and install it: ```bash # Build libcalc if needed (Part 1, Step 1.5): cd ../logos-calc-module/lib gcc -shared -fPIC -o libcalc.so libcalc.c # Linux # gcc -shared -fPIC -o libcalc.dylib libcalc.c # macOS cd - ``` ```bash nix build 'path:../logos-calc-module#lgx' --out-link result-calc-lgx ./pm/bin/lgpm --modules-dir ./modules install --file result-calc-lgx/*.lgx ``` `modules/` now holds `calc_via_interface/` and `calc_module/`. Neither knows about the other at build time — they meet only at runtime, through the interface. ### 7.4 Start the daemon and load both modules ```bash ./logos/bin/logoscore -D -m ./modules & ``` ```bash sleep 4 ``` Load the provider and the consumer. The consumer declares no dependency, so we load `calc_module` explicitly: ```bash ./logos/bin/logoscore load-module calc_module ``` ```bash ./logos/bin/logoscore load-module calc_via_interface ``` ### 7.5 Bind and call synchronously `sumVia` / `productVia` / `versionVia` each bind `calculator` to the module name you pass, then call through the bound wrapper. Bind to `calc_module`: ```bash ./logos/bin/logoscore call calc_via_interface sumVia calc_module 3 5 ``` ```bash ./logos/bin/logoscore call calc_via_interface productVia calc_module 3 5 ``` ```bash ./logos/bin/logoscore call calc_via_interface versionVia calc_module ``` `sumVia(calc_module, 3, 5) = 8`, `productVia(calc_module, 3, 5) = 15`, and `versionVia(calc_module) = "1.0.0"` — all through `modules().bind_calculator("calc_module")`, with `calc_module` chosen at call time. ### 7.6 Bind and call asynchronously `startFibVia` fires `calculator.fibonacci(n)` asynchronously against the bound module and returns `"queued"`. The reply arrives on the daemon's event loop; `lastFib()` reads it. With `n = 20`, `fib(20) = 6765`: ```bash ./logos/bin/logoscore call calc_via_interface startFibVia calc_module 20 ``` ```bash sleep 1 ``` ```bash ./logos/bin/logoscore call calc_via_interface lastFib ``` The bound wrapper's generated `fibonacciAsync(..., callback)` delivered `6765` to the callback after `startFibVia` had already returned — the typed **async** path, over a runtime-bound interface. ### 7.7 Subscribe to a bound interface event `watchVersion` subscribes to the interface's `versionReady` event on the bound module. `calc_module.libVersionNotify()` makes `calc_module` emit it, and `lastVersion()` reads what the typed callback captured: ```bash ./logos/bin/logoscore call calc_via_interface watchVersion calc_module ``` ```bash ./logos/bin/logoscore call calc_module libVersionNotify ``` ```bash sleep 1 ``` ```bash ./logos/bin/logoscore call calc_via_interface lastVersion ``` `watchVersion` registered the callback via the generated `onVersionReady(...)`; the event fired in between; `lastVersion()` returned `1.0.0` — a typed event subscription on a runtime-bound interface. ### 7.8 Bind to a non-satisfying module (the no-validation rule) Binding does **not** validate that the target satisfies the interface — there is no build-time coupling to check against. A bad bind isn't caught at bind time; it surfaces when you **call** through it — no crash, and the daemon keeps serving. `sumVia` checks the wrapper's `logos::CallError` out-parameter (see its implementation above) and returns `-1` when the inner call fails; on a transport that fails slowly the outer call may instead time out (`RPC_FAILED` / `"status":"error"`). Either way `calc_module` keeps answering (we keep going with `|| true` so the tour continues): ```bash ./logos/bin/logoscore call calc_via_interface sumVia no_such_module 3 5 2>&1 || true ``` The bound `no_such_module` couldn't be resolved, so the inner `add` call failed — exactly like any other call to an absent module. No crash, no conformance check. Because `sumVia` passes a `logos::CallError*`, it *sees* the failure (`err.code == "object_unavailable"`) and maps it to its own error convention; a call without the out-parameter would get the type's default value plus a warning in the module log. Swapping providers is just changing the string: `sumVia calc_module 3 5` returns `8`; `sumVia no_such_module 3 5` fails. **Any** module that really exposes `add`/`multiply`/`fibonacci`/`libVersion`/`versionReady` satisfies `calculator` and slots in unchanged. ```bash ./logos/bin/logoscore stop ``` That completes the tour: a single interface, bound at runtime to a concrete module, driven type-safely for sync calls, async calls, and events — with no build-time dependency on the provider. --- ## Step 8: Share an Interface Across Repos So far `interfaces/calculator.h` lived in this repo. To let *several* modules depend on the **same** contract, move it to its own repo (or a provider repo that publishes the interface it implements) and pull it in as a flake input — exactly how concrete `dependencies` are wired. Add an `"input"` field to the `interface_dependencies` entry, naming a flake input, with `file` relative to that input's root: ```json "interface_dependencies": [ { "name": "calculator", "input": "calc_interfaces", "file": "interfaces/calculator.h", "impl_class": "ICalculator" } ] ``` and declare the matching input in `flake.nix` (the input attribute name must equal the `input` value): ```nix inputs = { logos-module-builder.url = "github:logos-co/logos-module-builder/0.2.0"; calc_interfaces.url = "github:your-org/logos-calc-interfaces"; }; ``` The builder resolves `calc_interfaces` to a store path, hands the generator the resolved file, and emits the same bound `Calculator` wrapper — only the *source* of the contract moved. Nothing in `src/` changes. (In a workspace, run `ws sync-graph` after editing flake inputs.) That's the full picture. An interface is a contract you can keep local or share across repos; a module binds it to whatever provider it's given at runtime; and the generated, type-safe wrappers make the bound calls feel exactly like calling a concrete dependency — minus the coupling. --- ## Recap | Concept | In the code | Seen via `logoscore` | | -------------------------------- | -------------------------------------------------------- | --------------------------------------------------- | | Interface declaration | `interfaces/calculator.h` (methods + `logos_events:`) | — | | Declared, not depended-on | `interface_dependencies` set, `dependencies: []` | `lm metadata` shows empty `Dependencies:` | | Bind at runtime | `modules().bind_calculator(provider)` | provider is a `call` argument | | Typed **sync** call | `sumVia` / `productVia` / `versionVia` | `8`, `15`, `1.0.0` | | Typed **async** call | `startFibVia` → `fibonacciAsync(..., cb)` | `queued`, then `6765` | | Typed **event** subscription | `watchVersion` → `onVersionReady(cb)` | captured payload `1.0.0` | | No-validation / superset rule | bind to any module name | `calc_module` → `8`; `no_such_module` → RPC error | | Share across repos | `interface_dependencies[].input` + flake input | — | The interface coupled `calc_via_interface` to a *contract*, never to `calc_module`. Any module exposing that contract can be bound in its place — at runtime, by name. **Next:** see [Composing Modules](tutorial-composing-modules.md) for the concrete-dependency counterpart (`modules().calc_module`), or give this module a UI with [Part 2 (QML-only)](tutorial-qml-ui-app.md) / [Part 3 (C++ backend)](tutorial-cpp-ui-app.md).