Computation expression used to create Saturn controllers - abstraction representing REST-ish endpoint for serving HTML views or returning data. It supports:

• a set of predefined actions that are automatically mapped to the endpoints following standard conventions
• embedding sub-controllers for modeling one-to-many relationships
• versioning
• adding plugs for a particular action which in principle provides the same mechanism as attributes in ASP.NET MVC applications
• defining a common error handler for all actions
• defining a not-found action

The result of the computation expression is a standard Giraffe HttpHandler, which means that it's easily composable with other parts of the ecosytem.

Example:

let commentController userId = subcontroller {
    index (fun ctx -> (sprintf "Comment Index handler for user %i" userId ) |> Controller.text ctx)
    add (fun ctx -> (sprintf "Comment Add handler for user %i" userId ) |> Controller.text ctx)
    show (fun (ctx, id) -> (sprintf "Show comment %s handler for user %i" id userId ) |> Controller.text ctx)
    edit (fun (ctx, id) -> (sprintf "Edit comment %s handler for user %i" id userId )  |> Controller.text ctx)
}

let userControllerVersion1 = controller {
    version 1
    subController "/comments" commentController

    index (fun ctx -> "Index handler version 1" |> Controller.text ctx)
    add (fun ctx -> "Add handler version 1" |> Controller.text ctx)
    show (fun (ctx, id) -> (sprintf "Show handler version 1 - %i" id) |> Controller.text ctx)
    edit (fun (ctx, id) -> (sprintf "Edit handler version 1 - %i" id) |> Controller.text ctx)
}

let userController = controller {
    subController "/comments" commentController

    plug [All] (setHttpHeader "user-controller-common" "123")
    plug [Index; Show] (setHttpHeader "user-controller-specialized" "123")

    index (fun ctx -> "Index handler no version" |> Controller.text ctx)
    add (fun ctx -> "Add handler no version" |> Controller.text ctx)
    show (fun (ctx, id) -> (sprintf "Show handler no version - %i" id) |> Controller.text ctx)
    edit (fun (ctx, id) -> (sprintf "Edit handler no version - %i" id) |> Controller.text ctx)
}