kenota / lamdera-issue

Lamdera type safety issue

This repository shows how it is possible to create a lamdera app which compiles but sends incorrect messages from backend to frontend.

Model is defined as following:

type ToFrontend
    = ServerState SharedState


type SharedState
    = Lobby Int
    | NotLobby

Then we are trying to broadcast the state to clients using this code:

updateClients : Model -> Cmd BackendMsg
updateClients m =
    Lamdera.broadcast (Lobby (Set.size m.clients))

Which is incorrect, since it should be SharedState (Lobby (Set.size m.clients)), but the code compiles and we have undefined behaviour. The message from bakend is descoded as NotLobby shared state.

You can see the behaviour in action by checking out this repository, running lamdera live and opening http://localhost:8000 in the browser.

You can also see it on this screenshot:

Environment:

Linux minty 5.3.0-59-generic #53~18.04.1-Ubuntu SMP Thu Jun 4 14:58:26 UTC 2020 x86_64 x86_64 x86_64 GNU/Linux
➜  lamdera-issue git:(master) ✗ lamdera --version
0.0.1-alpha8-0.19.0
➜  lamdera-issue git:(master) ✗