Question

Adding attributes to members works well because the syntax is exactly like it would be when defining the member in a type.

[<TypeAttribute>]
let Foo () =
  {
    new Baz with
      [<MethodAttribute>]
      member x.Booga () = ()
  }

Above fails to compile with AttributeUsage exception of “This attribute is not valid for use on this language element”.

This error doesn’t go away even if I add an Attribute hint:

[<type: TypeAttribute>]

If the attribute is moved next to the object expression, where it seems to make sense to go, the code doesn’t even parse.

let Foo () =
  [<TypeAttribute>]
  {
    new Baz with
      member x.Booga () = ()
  }

The parse error is “Unexpected symbol ‘[<‘ in binding.

let Foo () =
  {
    [<TypeAttribute>]
    new Baz with
      member x.Booga () = ()
  }

Moving the attribute here fails to parse with “Unexpected symbol ‘[<‘ in expression. Expected ‘}’ or other token”, “Unmatched ‘{‘” and “Unexpected keyword ‘member’ in definition…”. The worst result yet.

So at this point my option is to give up using an object expression in favor of a private type in the module.

[<TypeAttribute>]
type private myBaz () =
 interface Bax with
   member x.Booga () = ()

let Foo () = new myBaz ()

That works fine, but I’d prefer an alternative that allows object expressions to be used instead.

Is there an alternative?

Answer 1

Attributes cannot be used on object expressions, though it does seem like a reasonable suggestion for a future extension to the language (I’ll log a suggestion).

http://msdn.microsoft.com/en-us/library/dd233179(VS.100).aspx