Elmish: Does the Program.withErrorHandler handle the error case of Cmd.ofAsync?

Question

I am trying to capture and log errors at every possible location in my Elmish (Fable) application.

If I have added a logging handler using Program.withErrorHandler, do I also need to add logging code to the Error case of each cmd.ofAsync invocation as well?

StackOverflow says to show some code, so here is my state update function:

| ZipCodeChanged (ZipCode s) ->
    let newZip, cmd = 
        if s |> ZipCode.looksLikeAZipCode then
            let cmd = Cmd.ofAsync api.LookupZipCode (ZipCode s) LookupZipCodeResponse LookupZipCodeError 
            InvalidInput (ZipCode s, [ValidationError "Validating ZIP Code ..."]), cmd
        else
            InvalidInput (ZipCode s, [ValidationError "Please inputs a 5 digit numeric ZIP Code."]), Cmd.none

    { model with ZipCode = newZip }, cmd
| LookupZipCodeResponse (posOpt) ->
    (... eliminated for brevity ...)
| LookupZipCodeError e ->
    { model with ZipCode = InvalidInput (ZipCode "", [ValidationError (sprintf "Error validating ZIPCode: %s" e.Message)]) }, Cmd.none

This code treats the ZipCode as "InvalidInput" with a user-friendly validation message the entire time until a successful LookupZipCodeResponse is received.

But if that call to the backend API fails, do I need to add my own logging code in LookupZipCodeError, if I have already provided logging the function that I provided to Program.withErrorHandler?

Answer 1

You do have to add logging code to LookupZipCodeError: Program.withErrorHandler handles uncaught errors in update, and Cmd.ofAsync actually catches error cases of the call and wraps them in a LookupZipCodeError msg.