Bolero

Routing

Bind the page's URL to the Elmish model.

Prerequisites

In order to use routing, you need to make sure that the page's base URL is set correctly. In most cases, you simply need to add the following inside the <head> of your page (Pages/_Host.cshtml for hosted apps, wwwroot/index.html for client-only apps):

<base href="/">

If the root of your application is a subpath, then you need to set that subpath as the base, including the trailing slash. For example, if you have an application served at https://someone.github.io/myrepo/, then you need the following:

<base href="/myrepo/">

Inferred router

The easiest way to create a router is by using an inferred router. In this mode of operation, you create an endpoint type which has a 1-to-1 correspondence with your supported URLs, and store it in the Elmish model.

Here are the steps to set up an inferred router:

  1. Create the endpoint type. Typically, it will be an F# union type where each case corresponds to a path prefixed by the case's name and the case's arguments are the consecutive fragments of the path. For example:

    type Page =
    | Home                                  // -> /Home
    | BlogArticle of id: int                // -> /BlogArticle/42
    | BlogList of user: string * page: int  // -> /BlogList/tarmil/1

    See Format for all the supported types and how to customize the corresponding path.

  2. Add a field in the Elmish model that stores the endpoint:

    type Model =
    {
        page: Page
        // other fields...
    }

  3. Add an Elmish message that sets the endpoint:

    type Message =
    | PageChanged of Page
    // other messages...

let update message model =
    match message with
    | PageChanged page -> { model with page = page }
    // other messages...

  4. Create the router with Router.infer:

    let router = Router.infer PageChanged (fun m -> m.page)

  5. Attach the router to the Elmish program:

    Program.mkSimple initModel update view
|> Program.withRouter router

You can specify the endpoint to show when the user visits a URL that can't be parsed with Router.withNotFound. Note that this endpoint is used when initially loading a page with an unrecognized URL; however it is not used when the user clicks a link in the Bolero page to an unrecognized URL, as that would prevent using external links.

let router =
    Router.infer PageChanged (fun m -> m.page)
    |> Router.withNotFound Home

Note: the message PageChanged is dispatched automatically when the URL has been modified (eg by clicking a link). It is not recommended to dispatch it directly yourself, as it will result in the same message being dispatched twice: first by you, and then by the router.

Instead, if you want to trigger a page change programmatically, you can add another message for this purpose:

type Message =
    | PageChanged of Page
    | ChangePage of Page

let update message model =
    match message with
    | PageChanged page -> { model with page = page }
    | ChangePage page -> { model with page = page }

Alternatively, to trigger a page change programmatically from the update function, simply return a model with the desired target page:

type Message =
    | PageChanged of Page
    | LoginSucceeded of UserData

let update message model =
    match message with
    | PageChanged page -> { model with page = page }
    | LoginSucceeded userData ->
        { model with userData = userData; page = Home }

The router has a few helpful utilities:

  • router.Link takes an endpoint value and returns the corresponding URL.

    a { attr.href (router.Link Home); "Go to Home" }

  • router.HRef takes an endpoint and returns an href attribute pointing to the corresponding URL.

    a { router.HRef Home; "Go to Home" }

  • Both these methods take an optional hash parameter to add a hash component to the URL, in order to make the browser scroll to the element with the given id after displaying the corresponding page.

    a {
    router.HRef(Home, "heading-1")
    "Go to Home and scroll to #heading-1"
}

Format

Router.infer supports the following types:

  • Standard library types:

    • string
    • bool
    • integer: int8 (aka sbyte), uint8 (aka byte), int16, uint16, int32 (aka int), uint32, int64, uint64
    • decimal
    • float: single, double (aka float)

  • F# union types. Each case corresponds to a path prefixed by the case's name. The case's arguments are the consecutive fragments of the path:

    type Page =
    | Home                                  // -> /Home
    | BlogArticle of id: int                // -> /BlogArticle/42
    | BlogList of user: string * page: int  // -> /BlogList/tarmil/1

    To customize the path, you can use the EndPoint attribute, with parameters indicated by {curly_braces}.

    type Page =
    | [<EndPoint "/">]
      Home                                  // -> /
    | [<EndPoint "/article/{id}">]
      BlogArticle of id: int                // -> /article/42
    | [<EndPoint "/list/{user}/{page}">]
      BlogList of user: string * page: int  // -> /list/tarmil/1

    An {*asterisk} indicates that this parameter catches the rest of the path. It must be the last parameter in the path, and correspond to a value of type string, list or array.

    type Page =
    | [<EndPoint "/list/{user}/tagged/{*tags}">]
      ListTagged of user: string * tags: list<string>

ListTagged("tarmil", ["bolero"; "webassembly"])
// -> /list/tarmil/tagged/bolero/webassembly

    Several cases can share a common prefix in their paths, as long as any parameters in this common prefix have the same type.

    type Page =
    | [<EndPoint "/user/{user}/favorites">]
      Favorites of user: string
    | [<EndPoint "/user/{user}/comments">]
      Comments of user: string

    Query parameters can also be specified:

    type Page =
    | [<EndPoint "/article?id={articleId}">]
      BlogArticle of articleId: int

    They can of course be mixed together with path parameters:

    type Page =
    | [<EndPoint "/list/{user}?page={pageNumber}&count={pageSize}"]
      BlogList of user: string * pageNumber: int * pageSize: int

    The syntax ?{param} is short for ?param={param}:

    type Page =
    | [<EndPoint "/list/{user}?{page}&{count}"]
      BlogList of user: string * page: int * count: int

    Query parameters can have type string, bool, integer or float. In these cases, they are mandatory. Alternatively, they can be option or voption of these types, making them optional.

    type Page =
    | [<EndPoint "/list/{user}?{page}&{count}"]
      BlogList of user: string * page: int option * count: int option

    Together, the path and query must contain a {parameter} for each of its case's values. Alternatively, the endpoint can consist of a single non-parameter path fragment; in this case, the values are appended in order as separate fragments.

    type Page =
    | [<EndPoint "/list">]                  // Equivalent to /list/{user}/{page}
      BlogList of user: string * page: int  // -> /list/tarmil/1

  • Tuples. The values of the tuple are the consecutive fragments of the path.

    type Page = int * string    // -> /42/abc

  • F# record types. The fields of the record are the consecutive fragments of the path.

    type Page =
    {
        x: int
        y: string
    }
// -> /42/abc

  • Lists and arrays. The first fragment of the path is the number of items, and the items themselves are the consecutive fragments.

    type Page = list<string>   // -> /3/abc/def/ghi

  • Any combination of the above, including recursive types.

    type Page =
    | [<EndPoint "/article">]
      Article of ArticleId              // -> /article/123/announcing-bolero
    | [<EndPoint "/list/{*tags}">]
      List of tags: list<string>        // -> /list/bolero/blazor
    | [<EndPoint "/login/{page}">]
      LoginAndRedirectTo of page: Page  // -> /login/list/bolero/blazor

and ArticleId =
    {
        uid: int
        slug: string
    }

Custom router

To have more control over the exact shape of your URLs, you can create a custom router like follows.

let customRouter : Router<Page, Model, Message> =
    {
        // getEndPoint : Model -> Page
        getEndPoint = fun m -> m.page
        // setRoute : string -> option<Message>
        setRoute = fun path ->
            match path.Trim('/').Split('/') with
            | [||] -> Some Home
            | [|"article"; id|] -> Some (BlogArticle (int id))
            | [|"list"; user; page|] -> Some (BlogList (user, int page))
            | _ -> None
            |> Option.map PageChanged
        // getRoute : Page -> string
        getRoute = function
            | Home -> "/"
            | BlogArticle(id) -> sprintf "/article/%i" id
            | BlogList(user, page) -> sprintf "/list/%s/%i" user page
    }

Note: if the URL is changed in such a way that setRoute returns None, then the model is not updated.

You can also create a router that maps directly to the model without an intermediary endpoint type. However this router type doesn't have some utilities such as Link and HRef.

let customRouter2 : Router<Model, Message> =
    {
        // setRoute : string -> option<Message>
        setRoute = fun path ->
            match path.Trim('/').Split('/') with
            | [||] -> Some Home
            | [|"article"; id|] -> Some (BlogArticle (int id))
            | [|"list"; user; page|] -> Some (BlogList (user, int page))
            | _ -> None
            |> Option.map PageChanged
        // getRoute : Model -> string
        getRoute = fun model ->
            match model.page with
            | Home -> "/"
            | BlogArticle(id) -> sprintf "/article/%i" id
            | BlogList(user, page) -> sprintf "/list/%s/%i" user page
    }

To provide the default route for such a router that doesn't have an endpoint type, instead of Router.withNotFound, you can use Router.withNotFoundMsg and pass the Elmish message to send when the user visits an unrecognized URL.

Page Models

It is common to have a part of the application's model that is specific to a page. For example, a login page has the username and password that the user is typing. A "list of items" page has the items, which may have been downloaded from a remote function. And so on.

Let's take for example an application like the following:


type PersonModel = { name: string; age: int }

type LoginModel = { username: string; password: string }

type Page =
    | [<EndPoint "/people">] People  // Displays a list of Person
    | [<EndPoint "/login">] Login    // Shows a login screen with Login data

There are essentially two ways to handle page models:

  1. Simply store the page model as a field of the application model. This is a reasonable way to go if you want to persist the state across page changes, and simply reuse the already loaded model when switching back to this page. For example, it may be a nice way to store the people in our application.

    type Model =
    {
        page: Page
        people: PersonModel list
    }

  2. Store the page model in the Page union, so that it only exists when that is the current page. This is a better solution for our Login page: for security reasons, we don't want to keep the credentials in memory beyond the login page!

    For this purpose, Bolero has the type PageModel<'T>. When a page has an argument of type PageModel<'T>, it is not included in the page's URL and simply kept as internal state.

    type Page =
    | [<EndPoint "/people">] People
    | [<EndPoint "/login">] Login of PageModel<LoginModel>

    PageModel<'T> is simply a record with a field Model : 'T:

    type Message =
    | PageChanged of Page
    | SetUsername of string
    | SetPassword of string

let updateLogin (update: LoginModel -> LoginModel) (model: Model) : Model =
    match model.page with
    | Login login -> { model with page = Login { Model = update login.Model } }
    | _ -> model

let update (message: Message) (model: Model) =
    match message with
    | PageChanged p -> { model with page = p }
    | SetUsername username -> model |> updateLogin (fun l -> { l with username = username })
    | SetPassword password -> model |> updateLogin (fun l -> { l with password = password })

    There is one more requirement to be able to use PageModel: you must define the default value for the page model. Indeed, when the user clicks a link /login, Bolero needs to know what LoginModel value it needs to pass to Login! In order to define this default page model, instead of creating the router with Router.infer, you must use Router.inferWithModel. It takes an additional function of type Page -> unit. In this function, you must call Router.definePageModel with the default value of every page model in the router:

    let defaultModel = function
    | People -> ()
    | Login model -> Router.definePageModel model { username = ""; password = "" }

let router = Router.inferWithModel PageChanged (fun m -> m.page) defaultModel

    And finally, when calling the functions router.Link or router.HRef to create a link to a page, you need to have a dummy page model value to pass to Login. You can use Router.noModel for this purpose:

    let s = router.Link (Login Router.noModel)
// s = "/login"
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