Add more details to the main README.md

README.md

@@ -2,12 +2,76 @@
 
 ---
 
-Integrating components written in different languages (like Haskell/Rust/C).
+This repository is a small Rust/Haskell interop demo.
 
-Current demo work lives in `haskell/` and `rust/`.
+The current project demonstrates communication in both directions:
 
-- `haskell/` contains a small Cabal project with a Haskell executable that calls into Rust and a foreign library that Rust can call back into.
+- Haskell calling into Rust through a C ABI exposed by the Rust crate
-- `rust/` contains the Rust C ABI exports plus a CLI path for Rust calling the Haskell foreign library.
+- Rust calling back into Haskell through a Cabal `foreign-library`
+
+The code is intentionally small. The goal is to show the main integration mechanics and the main failure points without adding code generation or a large build stack.
+
+## What The Demo Contains
+
+- `rust/` - Rust exports a C ABI for Haskell and provides a CLI path that loads and calls the Haskell shared library
+- `haskell/` - Haskell contains a small Cabal project with:
+  - an executable that calls Rust
+  - a shared foreign library that Rust calls
+  - small pure tests for the shared Haskell-side logic
+
+The boundary is deliberately C-shaped:
+
+- integers
+- a shared struct layout
+- owned C strings with explicit free functions on each side
+
+## Why The Build Uses Both Static And Shared Libraries
+
+This demo uses different library styles for the two directions because that keeps each path simpler:
+
+- Haskell -> Rust uses the Rust crate as a static library
+- Rust -> Haskell uses a Haskell shared foreign library that Rust loads at runtime
+
+That is not the only possible design. It is just a practical one for a two-way demo.
+
+## Build And Run
+
+Build the Haskell project and the Rust library it links against:
+
+```sh
+make haskell-build
+```
+
+Run the Haskell -> Rust demo:
+
+```sh
+make haskell-run
+```
+
+Run the Rust -> Haskell demo:
+
+```sh
+make rust-calls-haskell
+```
+
+You can also run the underlying commands directly:
+
+```sh
+cargo test
+```
+
+```sh
+cd haskell
+CABAL_DIR=$PWD/../.cabal XDG_STATE_HOME=$PWD/../.cabal/state XDG_CACHE_HOME=$PWD/../.cabal/cache XDG_CONFIG_HOME=$PWD/../.cabal/config cabal test --project-file=cabal.project
+```
+
+## What This Project Demonstrates
+
+- the ABI boundary must stay simple and explicit
+- Rust and Haskell do not share ownership rules automatically
+- struct layout must match on both sides
+- Rust calling Haskell is the harder direction because it must initialize the GHC runtime correctly
+- build tooling is part of the integration problem, not just an implementation detail
 
 ### License