servant-to-elm-example

This example is a full-stack web application, built in a typesafe functional way.

What's cool here is that servant-to-elm does the job of generating types and decoders/encoders from Haskell types and Servant definition to Elm, which not only catches regressions in the compile-time but also provides ready (and highly configurable) Elm functions to fetch necessary data from the server.

Prepare

• Install Stack and Elm
• Run code generation

  cd backend && stack run codegen

• Install frontend dependencies

  cd frontend && npm i

Set up editor (optional)

• If you are using VSCode and you want to open both frontend and backend as a single workspace, use File > Open Workspace and choose servant-to-elm-example.code-workspace. In this scenario, VSCode will work correctly with both languages simultaneously. Also, the editor may recommend extensions, and installing them is a wise choice.

Run

• Start the server on port 8000:

  cd backend && stack run server

• Run frontend in dev mode

  cd frontend && npm start

• Open http://localhost:8080

Domain model: Library

• A book can have exactly 1 author
• An author can have zero to many books
• Book title must be unique per author
• The author's name must be unique

This example does demonstrate:

• How to fetch data and handle errors and "loading" state
• How to post a new entity related to another, either new or existing one (submit a book with a new or existing author)

This example does not demonstrate:

• User input validation parsing best practices
• Client-side routing best practices
• Debouncing user input
• Client-side validation before submitting a form
• Subscriptions via WebSockets (which could be implemented, but they are related more to Servant or Elm, rather than to code-generation)

Other choices

• New entries that are in relation are submitted together and inserted transactionally.
• Search is case-insensitive

Technical notes

• This app was not tested on Windows systems yet. While is not a showcase of cross-platform compatibility, you can still open an issue or PR if something does not work as expected.

Adding new types

• in backend/src/DomainModel.hs:
  • Define the type
  • Derive necessary instances, also specify Elm module name and Elm type (it's better to keep type name the same, but it's convenient to use one module for several coupled types - see Book, NewBook, and NewBookAuthor types and corresponding generated Elm module)
  • Add the type to typeDefinitions list - jsonDefinitions @YourNewType
• Use your type in backend/src/Server.hs or wherever it's intended to
• Run code-generation again

Design choices and alternatives

• An obsolete generated Elm code can be removed before writing the new ones. Subdirectories that must be deleted are listed in backend/src/Codegen.hs. In this example, only the frontend/src/Api directory is removed before writing new files.
• One of the alternatives to this approach is GraphQL, and there is a Haskell/Elm full-stack GraphQL example app. This example app was shamelessly inspired by that example.