elm-spa-template

There is a live demo of this template here.

This project is something of a playground for the wonderful Elm language and how to build SPAs in particular. Much of the structure is taken from the excellent elm-mdl and its live demo in particular.

I used the tabs infrastrucutre in the mdl-demo as a starting point. It uses an array of tab details to set up the many contained tabs. For the mdl demo this is very effective, but for a "real-world" application we need on occasion to provide extra information to the tab (for example who is currently logged in etc). I very much wanted the contained tabs to not store any state that was not directly their own, so have experimented with injecting "context information" into the calls to their views.

There is a major convenience in having the tabs boilerplate defined as concisely as possible. Here is one such boilerplate for the tabs:

tabList : List Tab
tabList =
    [ { info = { tabName = "Tables", tabUrl = "tables", requiredRole = Auth.User }, tabViewMap = tableTabViewMap }
    , { info = { tabName = "Puppies", tabUrl = "puppies", requiredRole = Auth.Admin }, tabViewMap = .tabPuppies >> Tabs.Puppies.view >> App.map PuppiesMsg }
    , { info = { tabName = "Encoders", tabUrl = "encoders", requiredRole = Auth.User }, tabViewMap = .tabEncoders >> Tabs.Encoders.view >> App.map EncodersMsg }
    , { info = logonTabInfo, tabViewMap = logonTabViewMap }
    ]

As we are strongly typed and lists must be homogeneous, the tabViewMap element must all be of the same type (in this case Model -> Html Msg) In this experiment, the Encoders and Puppies tabs don't need anything more than their own models, but taking the case of the Logon tab, it would be nice to be able to say why we ended up there if, for example, we have been redirected there while trying to access a different tab for which we do not have permission. That's obviously data that has no place in Logon's model (it should not need to know other tabs even exist).

In the case of Tables tab we inject the Auth state into its view call and it is straightforward enough. In the case of the Logon tab we pass whether we have been redirected and if so where from and what the required Auth role is. This is done as follows:

logonTabViewMap : Model -> Html Msg
logonTabViewMap model =
    let
        desiredTabInfo =
            -- We would really like to just pull the info from the static tabInfoArray
            -- The problem is that creates a circular dependency as this LogonTabViewMap is part
            -- of the tabList.
            -- Instead we copy the tabInfoArray (without the viewmap functions) into the model
            -- on startup and that breaks the self-reference.
            Array.get model.desiredTab model.tabInfoArray |> Maybe.withDefault logonTabInfo
    in
        let
            viewWithInjectedArgs =
                Tabs.Logon.view (model.selectedTab /= model.desiredTab) desiredTabInfo.tabName desiredTabInfo.requiredRole
        in
            .tabLogon model |> viewWithInjectedArgs |> App.map LogonMsg

See the comment in the code snippet: I don't much like having this static data in the model, so any suggestions on how to avoid this much appreciated!

(As an aside, if you do have the circualr reference it compiles just fine but crashes when you load the SPA with the generated Javascript referencing the array the line before it sets it - fair enough really!)

Todo

• Build to .js with an Index.html that pulls in all the CSS etc