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README.md

Haskell WASM REPL Template

A template project to create REPLs with Haskell that run in the browser using WASM.

Running

The project is orchestrated using Nix. After you enter the Nix shell by running nix-shell, you can the following commands:

  • clean: Cleans the Cabal build directory.
  • compile: Build the project using Cabal.
  • run: Builds and run the project using Cabal.
  • build: Builds the project using Nix. Builds dynamically linked executables.
  • build-static: Builds the project using Nix. Builds statically linked executables. Requires either to be run on Linux, or to have a Nix Linux builder configured.
  • build-docker: Runs the previous step and creates a Docker image with the statically linked executables using Nix. Requires Docker to be set up on the build machine. The image will also contain the contents of the data directory.
  • build-wasm: Runs the previous step and converts the Docker image into a WASM binary out.wasm, and copies it into the web directory.
  • serve: Serves the contents of the web directory over HTTP locally.

The build-static, build-docker, and build-wasm commands take a long time to finish when run for the first time, maybe even hours. The built artifacts are cached so subsequent builds finish in minutes.

Deploying

You can copy the contents of the web directory to your server and serve them as a static website. For the browser to load the WASM file, the server needs to send the following headers:

Cross-Origin-Opener-Policy: same-origin
Cross-Origin-Embedder-Policy: require-corp

It is recommended to configure your server to serve pre-compressed assets, and to pre-compress the out.wasm file using Brotli or Gzip. This reduces the browser load time of the website by more than half.

It is also recommended to configure cache-control headers for the assets in your server so that browsers cache them. If you change the out.wasm file frequently, it is recommended to figure out a cache-busting policy for it.

Developing

This project uses the following tools/components:

  • Nix is used to build statically linked executables and Docker images.
  • Niv is used to pin Nix dependencies.
  • container2wasm is used to convert Docker images to WASM binaries. The WASI example from container2wasm forms the basis of the web directory.
  • Xterm.js is used to create the web terminal in browsers.
  • Xterm-pty is used to run the WASM binary on Xterm.
  • Xterm-fit-addon is used to make the web terminal fullscreen.

Using

For unknown reasons, the WASM REPL does not work on Safari. Please use it in Firefox or Chrome.