Error: a type annotation may be needed

c#, f#

Solution

`let Name item = item.name` has no type annotation. The parameter `item` is unconstrained other than by a member constraint which is not supported implicitly (see here for making it explicit -- don't do that...)

It looks like you got confused because the parameter name `item` is the same as the type alias `item`. If you want to explicitly make that so do this:

let Name (itemParameter:item) = itemParameter.name

I haven't checked if the parameter can have the same name as the alias, but it's probably a bad idea since it might be confused with the type parameter (we just saw this!).

Edit: Ok, I checked. You can have the parameter name be the same as the annotated type of the parameter, but it results in a horribly confusing type signature and implementation:

> let Name (item:item) = item.name;;

val Name : item:item -> string // EW!

If you're feeling particularly evil, you could even do something like this:

> let item (item:item) : item = item;;

val item : item:item -> item //Huh?

As an aside:

let rec sentence s item  = function
    | s when s="Action" -> ""
    | s when s="Client" -> ""
    | s when s="Classifier" -> ""    
    | s when s="Container" -> ""    
    | s when s="ControlFlow" -> ""
    | s when s="Gaurd" -> ""
    | s when s="Name" -> Name item
    | s when s="ObjectFlow" -> ""    
    | s when s="Source" -> ""    
    | _ -> ""

is probably better expressed as:

let rec sentence s item  = 
    match s with
    | "Action" -> ""
    | "Client" -> ""
    | "Classifier" -> ""    
    | "Container" -> ""    
    | "ControlFlow" -> ""
    | "Gaurd" -> "" //Guard maybe?
    | "Name" -> Name item
    | "ObjectFlow" -> ""    
    | "Source" -> ""    
    | _ -> ""

Here is a working example using a simple definition of `Item` (shown using the repl):

> type Item() =
    member val name = "" with get,set
type item = Item

let Name (itemParameter:item) = itemParameter.name

let test = item();;
test.name <- "test"
Name test;;

val it : string = "test"

Problem

Original source