Question

Consider the following:

type T () =
  member x.y = 4

let a =
  let fn () (k: T) = ()
  fn ()

let b =
  let fn () (k: System.IO.Directory) = ()
  fn ()

a fails while b is ok. The error message is:

The value ‘a’ has been inferred to have generic type val a : (‘_a -> unit) when ‘_a :> T Either make the arguments to ‘a’ explicit or, if you do not intend for it to be generic, add a type annotation

Why and how to fix that?

Answer 1

The error message itself tells you exactly what you need to do – add a type annotation:

let a : T -> unit = 
  let fn () (k: T) = () 
  fn () 

The reason that you see the error in the first place is that the compiler tries to generalize the definition of a (see this part of the spec), which results in the odd signature that you see in the error message.

The reason that you don’t need to do this for b is that System.IO.Directory is sealed, so there is no need to generalize.