fsharp help from stackoverflow 20160924

gistfile1.txt

this compiles and works:

    [<HttpGet("{id}", Name = "GetFsToDo")>]
    member __.Get(id) = 
        async {
            let! data = Async.AwaitTask(__.Queries.Find(id))
            if isNull data 
                then return  __.NotFound() :> IActionResult
                else
                    return new ObjectResult(data) :> IActionResult  } 
            |> Async.StartAsTask

when I changed it like this as suggested in comments:

    [<HttpGet("{id}", Name = "GetFsToDo")>]
    member __.Get(id) : IActionResult = 
        async {
            let! data = Async.AwaitTask(__.Queries.Find(id))
            if isNull data 
                then return  __.NotFound() :> _
                else
                    return new ObjectResult(data) :> _  } 
            |> Async.StartAsTask
            
or like this:

    [<HttpGet("{id}", Name = "GetFsToDo")>]
    member __.Get(id) : Task<IActionResult> = 
       async {
            let! dataOrDefault = __.Queries.Find(id) |> Async.AwaitTask
            match Option.ofObj dataOrDefault with 
            | None -> return __.NotFound() :> _
            | Some data -> return ObjectResult(data) :> _ 
       } |> Async.StartAsTask
            
I get

    error FS0013: The static coercion from type
    error FS0013: The static coercion from type
    error FS0001: This expression was expected to have type
    
if I change it like this:

    [<HttpGet("{id}", Name = "GetFsToDo")>]
    member __.Get(id) : IActionResult = 
        async {
            let! data = Async.AwaitTask(__.Queries.Find(id))
            if isNull data 
                then return  __.NotFound() 
                else
                    return new ObjectResult(data)  } 
            |> Async.StartAsTask
            
  I get:
  
  error FS0001: All branches of an 'if' expression must return the same type. This expression was expected to have type 'NotFoundResult' but here has type 'ObjectResult'.
  error FS0001: This expression was expected to have type
  
  it does work with the option like this:
  
      [<HttpGet("{id}", Name = "GetFsToDo")>]
    member __.Get(id) = 
       async {
            let! dataOrDefault = __.Queries.Find(id) |> Async.AwaitTask
            match Option.ofObj dataOrDefault with 
            | None -> return __.NotFound() :> IActionResult
            | Some data -> return ObjectResult(data) :> IActionResult 
       } |> Async.StartAsTask
  
  
    
        

You def need a cast somewhere, and you need to specify the type - the :> _ just lets you not redundantly state it, and F# will never just pick a common base (C# wouldn't either in a var-initialisation)

    [<HttpGet("{id}", Name = "GetFsToDo")>]
    member __.Get(id) : Task<IActionResult> = 
       async {
            let! dataOrDefault = __.Queries.Find(id) |> Async.AwaitTask
            match Option.ofObj dataOrDefault with 
            | None -> return __.NotFound() :> _
            | Some data -> return ObjectResult(data) :> _ 
       } |> Async.StartAsTask

thanks, but when I try that I still get the same compiler error, I've updated the gist to show what I tried which I think is exactly as you have shown.

I'm also unclear on the guidance about __ vs this and what circumstances to use one or the other, googling about that it seems there is no standard convention for choosing but use of "this" at least would be more expressive of what it is and more familiar for C# devs to read the code.