//[snippet:Definition]
open System

type Hole = Hole

[<CompilerMessage("Incomplete hole", 130)>]
let (?) (_ : Hole) (id : string) : 'T = 
    sprintf "Incomplete hole '%s : %O'" id typeof<'T>
    |> NotImplementedException
    |> raise

//[/snippet]

//[snippet:Example]
let abs n =
    if n >= 0 then n
    else Hole ?TODO_Negation

// compile time warning:
// Test.fs(28,15): warning FS0130: Incomplete hole
// val abs : n:int -> int

abs 2 // successful
abs -1 // System.NotImplementedException: Incomplete hole 'TODO_Negation : System.Int32' 

//[/snippet]