• Platform.worker

What can you do with Elm?

• Html
• Http
• Ports
• Web Components

Different techniques for extending elm

• elm-pages StaticHttp API

• Elm radio episode 1 on elm pages

• elm-graphql

• Codegen

• Macros

• Elixir exunit

• Wrapper elm apps

• Can emulate effect managers

• Platform.worker

• Introspection

• elm-typescript-interop

• Ports and flags

• Web Components

• Code transformation

• Elm asset loader webpack

• Hacking JS to get FFI

• Depending on internal details could end up with broken code

elm-hot and elm-hot-webpack-loader

Pitfalls and considerations

Codegen

• Have a single clear source of truth for codegen
• Prevent bad states with airtight abstractions, rather than having lots of caveats
• Make sure public APIs for generated code look nice
• Use doc comments

Macros

• Elm code that doesn’t look like elm code
• Tooling doesn’t work then - see Babel ecosystem
• Violates Open close principle - you’re modifying the language, not extending it

Provide a platform with extensions in mind when you build tools so you don’t require users to hack

When you build a tool, think about the mental model for uses, let that guide you. Avoid leaky abstractions

Be opinionated about the core things, and unopionated about what’s not essential to the tool

• [elm-spa](https://github.com/ryannhg/elm-spa)