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Error monitoring

In Fabulous, everything happens in a centralized message loop that is responsible for calling your init, update and view functions. Fabulous allows you to plug into this loop to run custom logic such as logging and error handling.
There’s a few built-in functions available already:
Call custom tracing function everytime Fabulous needs to update the app. Signature: trace: 'msg -> 'model -> unit
Call custom error handling logic. Returns a boolean to indicate if the exception has been handled or not. Signature: handler: exn -> bool
Fabulous has a lot of internal logs to help debugging what is happening. By default, the logs won't be printed anywhere but it can be configured. Signature: logger: Logger
F#To use them, you will need to add them when declaring the runner. You can add multiple trace functions, one after another.
module App =
let runner =
Program.stateful init update view
|> Program.withConsoleTrace
|> Program.withErrorHandler (fun (message, exn) -> writeToDisk exn)
In this example, logs will be written to the console before the error handler can write the exceptions to the disk.

Writing a custom trace function

Writing a custom trace function is simple. You need to make a function that accepts a Program<'args, 'model, 'msg, 'view> and that outputs another Program<'args, 'model, 'msg, 'view>.
This Program defines the handlers that are responsibles for calling init, update and views as well as handle errors. You can define your own handlers instead to do additional logic. Make sure to call the previous Program handlers in your owns, otherwise you will completely bypass Fabulous.
Here’s a simple example that prints a message each time something happens
let withSimpleTrace (program: Program<'args, 'model, 'msg, _>) =
let traceInit args =
Console.WriteLine "Init"
program.init args
let traceUpdate msg model =
Console.WriteLine "Update"
program.update msg model
let traceView model dispatch =
Console.WriteLine "View"
program.update msg model
let traceOnError (message, exn) =
Console.WriteLine "Error"
program.onError (message, exn)
{ program with
init = traceInit
update = traceUpdate
view = traceView
onError = traceOnError }
module App =
let runner =
Program.stateful init update view
|> Program.withSimpleTrace
You’re not required to implement all handlers, if you only need to override update then just override that one.
let withSimpleTrace (program: Program<'args, 'model, 'msg, _>) =
let traceUpdate msg model =
Console.WriteLine "Update"
program.update msg model
{ program with update = traceUpdate }


Here’s a good example of implementing our own trace function.
Visual Studio App Center is a DevOps portal tailored for mobile application development. It handles everything from build, tests, distribution, analytics and crashes reporting, for Android, iOS and UWP.
We can define our own trace functions to send analytics and crashes to AppCenter.
AppCenter provides us with a really simple way to report to it. We only need to install the following packages:
Once the packages added, we can access 3 methods:
  • Start: Initialize AppCenter by providing our app secrets
  • Analytics.TrackEvent: Track a custom event with associated data
  • Crashes.TrackError: Track an exception.
AppCenter will provide a dashboard of those events and exceptions along with stack traces
Here we override update to call Analytics.TrackEvent, and onError to call Crashes.TrackError.
module AppCenter =
type AppCenterUpdateTracer<'msg, 'model> =
'msg -> 'model -> (string * (string * string) list) option
/// Trace all the updates to AppCenter
let withAppCenterTrace (shouldTraceUpdate: AppCenterUpdateTracer<_, _>) (program: Program<_, F#_, _, _>) =
let traceUpdate msg model =
match shouldTraceUpdate msg model with
| Some (key, value) -> Analytics.TrackEvent (key, dict value)
| None -> ()
program.update msg model
let traceError (message, exn) =
Crashes.TrackError(exn, dict [ ("Message", message) ])
{ program with
update = traceUpdate
onError = traceError }
We could trace everything, but you should consider to trace the minimum to protect your users' privacy. To do that, we have added a AppCenterUpdateTracer function that will filter the messages that interest us and what data we should extract from it.
module Tracing =
let hasValue = (not << String.IsNullOrEmpty) >> string
let rules msg _ =
match msg with
| App.Msg.GoToAbout ->
Some ("Navigation", [ ("Page", "About") ])
| App.Msg.NavigationPopped ->
Some ("Back Navigation", [])
| App.Msg.UpdateWhenContactAdded c ->
Some ("Contact added", [
("Has Email", hasValue c.Email)
("Has Phone", hasValue c.Phone)
("Has Address", hasValue c.Address)
| _ -> None
In our App class, we need to call AppCenter.Start("appsecrets", typeof<Analytics>, typeof<Crashes>) to initialize it.
module App =
do AppCenter.Start("ios=(...);android=(...)", typeof<Analytics>, typeof<Crashes>)
let runner =
Program.stateful init update view
|> AppCenter.withAppCenterTrace Tracing.rules
Last modified 27d ago