Why is is the compiler telling me "Type misMatch for App message" when they are the same type - xamarin.forms

Question

So, I've been fighting with the compiler on a type error.
This code was working as of a couple days ago.
Type misMatch for App level message
App.fs snippets
module App =
type Msg =
| ConnectionPageMsg of ConnectionPage.Msg
| CodeGenPageMsg of CodeGenPage.Msg
//...
let update (msg : Msg) (model : Model) =
match msg with
| ConnectionPageMsg msg ->
let m, cmd = ConnectionPage.update msg model.ConnectionPageModel
{ model with ConnectionPageModel = m }, cmd
| CodeGenPageMsg msg ->
let m, cmd = CodeGenPage.update msg model.CodeGenPageModel
{ model with CodeGenPageModel = m }, cmd
//...
let runner =
Program.mkProgram init update view
|> Program.withConsoleTrace
|> XamarinFormsProgram.run app
I've added explicit aliases and the original error :
Type mismatch. Expecting a
'App.Msg -> App.Model -> App.Model * Cmd<App.Msg>'
but given a
'App.Msg -> App.Model -> App.Model * Cmd<Msg>'
The type 'App.Msg' does not match the type 'Msg'
Became these:
App.fs(50,50): Error FS0001: The type 'PocoGen.Page.ConnectionPage.Msg' does not match the type 'PocoGen.Page.CodeGenPage.Msg' (FS0001) (PocoGen)
App.fs(32,32): Error FS0001: Type mismatch.
Expecting a 'App.Msg -> App.Model -> App.Model * Cmd<App.Msg>'
but given a 'App.Msg -> App.Model -> App.Model * Cmd<Msg>'
The type 'App.Msg' does not match the type 'Msg' (FS0001) (PocoGen)
Other remarks
Right before these errors started appearing I was working on converting a blocking syncronous call to a async command in the ConnectionTestPage and removed the calling code for the cmd hoping that would fix it. (It did not)
ConnectionPage.fs Messages
type Msg =
| UpdateConnectionStringValue of string
| UpdateConnectionStringName of string
| TestConnection
| TestConnectionComplete of Model
| SaveConnectionString of ConnectionStringItem
| UpdateOutput of string
ConnectionPage.fs update
let update (msg : Msg) (m : Model) : Model * Cmd<Msg> =
match msg with
| UpdateConnectionStringValue conStringVal ->
{ m with
ConnectionString =
{ Id = m.ConnectionString.Id
Name = m.ConnectionString.Name
Value = conStringVal }
CurrentFormState =
match hasRequredSaveFields m.ConnectionString with
| false -> MissingConnStrValue
| _ -> Valid }, Cmd.none
| UpdateConnectionStringName conStringName ->
{ m with
ConnectionString =
{ Id = m.ConnectionString.Id
Name = conStringName
Value = m.ConnectionString.Value }
CurrentFormState =
match hasRequredSaveFields m.ConnectionString with
| false -> MissingConnStrValue
| _ -> Valid }, Cmd.none
| UpdateOutput output -> { m with Output = output }, Cmd.none
| TestConnection -> m, Cmd.none
| TestConnectionComplete testResult -> { m with Output = testResult.Output + "\r\n" }, Cmd.none
| SaveConnectionString(_) -> saveConnection m, Cmd.none
I've played with the Fsharp Version (because incidentally I did update to 4.7.2 a bit before getting this error)
The Full Repo:
https://github.com/musicm122/PocoGen_Fsharp/tree/master/PocoGen

Answer 1

The two branches of the match inside App.update have different types. The first branch has type App.Model * Cmd<ConnectionPage.Msg> and the second page has type App.Model * Cmd<CodeGenPage.Msg>.
You can't generally do that. This, for example, wouldn't compile:
let x =
match y with
| true -> 42
| false -> "foo"
What type is x here? Is it int or is it string? Doesn't compute. A match expression has to have all branches of the same type.
To convert Cmd<ConnectionPage.Msg> into a Cmd<App.Msg> (by wrapping the message in ConnectionPageMsg) you can use Cmd.map:
let update (msg : Msg) (model : Model) =
match msg with
| ConnectionPageMsg msg ->
let m, cmd = ConnectionPage.update msg model.ConnectionPageModel
{ model with ConnectionPageModel = m }, Cmd.map ConnectionPageMsg cmd
| CodeGenPageMsg msg ->
let m, cmd = CodeGenPage.update msg model.CodeGenPageModel
{ model with CodeGenPageModel = m }, Cmd.map CodeGenPageMsg cmd