Blog post about linking C code

blog/2024-09-25-js-link-c.md

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+---
+slug: 2024-09-25-ghc-update
+title: "Linking C code with GHC's JavaScript backend"
+authors: [sylvain]
+tags: [ghc,javascript]
+---
+
+Since GHC 9.10 it is possible to build and to link C code with Haskell code
+compiled with the JavaScript backend. In this post we show to do it, using the
+well-known SQLite database as an example.
+
+<!-- truncate -->
+
+Using JavaScript code produced from C code from JavaScript code produced
+from Haskell code is [documented in GHC's users
+guide](https://downloads.haskell.org/ghc/latest/docs/users_guide/javascript.html#linking-with-c-sources),
+however it's probably simpler to understand with examples.
+We'll start with a simple Hello World example and then directly leap to an
+example using the SQLite database.
+
+TODO: motivation. Why would we want to do that? Avoid reimplementing in Haskell.
+Performance (need example + benchmark). Maybe a graphics C lib?
+
+## Hello World
+
+TODO
+
+## SQLite
+
+The SQLite database is released as a single C file. To use it from Haskell, we
+can use the [direct-sqlite
+package](https://hackage.haskell.org/package/direct-sqlite) which embeds this C
+file and provides Haskell bindings for it.
+
+Since GHC 9.10, if the JavaScript backend on GHC is used, by default the C files
+are compiled to WebAssembly (using the [Emscripten](https://emscripten.org/)
+compiler) and embedded into the produced JavaScript file. (You can disable this
+behavior by passing the `-ddisable-js-c-sources` flag to GHC.)
+
+The generated code can't be directly called into from Haskell. We need to write
+some JavaScript code to call into the WebAssembly code, passing arguments into
+the WebAssembly heap as needed. This is no as difficult as it should, especially
+thanks to JavaScript helper functions provided by GHC's JavaScript RTS.
+
+## Building the project
+
+Note that we need to use at least GHC 9.10.2 for this to work as it contains a
+fix for [#25288](https://gitlab.haskell.org/ghc/ghc/-/issues/25288).
+
+FIXME: checkout branch in fork instead? https://github.com/hsyl20/direct-sqlite/tree/hsyl20/js
+
+```bash
+> git clone https://github.com/IreneKnapp/direct-sqlite -- FIXME: checkout MR
+> cd direct-sqlite
+> cabal build . -w /path/to/javascript-unknown-ghcjs-ghc --with-ghc-pkg=/path/to/javascript-unknown-ghcjs-ghc-pkg
+> cabal test . -w /path/to/javascript-unknown-ghcjs-ghc --with-ghc-pkg=/path/to/javascript-unknown-ghcjs-ghc-pkg
+Running 1 test suites...
+Test suite test: RUNNING...
+Cases: 25  Tried: 25  Errors: 0  Failures: 0
+Cases: 1  Tried: 1  Errors: 0  Failures: 0
+Cases: 26  Tried: 26  Errors: 0  Failures: 0
+Cases: 1  Tried: 1  Errors: 0  Failures: 0
+Test suite test: PASS
+```
+