Holes in GHC

[Simon Peyton-Jones]

|  This is where you would want to use a hole. Just like undefined, it
|  has type `a`, so it can be used anywhere (and when compiling, we
|  intend to turn it into an exception too), but the difference with
|  undefined is that after the typechecking has succeeded, you get a list
|  of your holes, with the type that was inferred for them, as a sort of
|  todo-list.

I'm sorry to be slow, but I still don't understand what you intend.  I wonder whether you could give a series of examples?  Is this something to do with GHCi?  Or some hypothetical IDE? Or do you expect to compile Foo.hs with some holes in it, and get some output relating to the holes?   Be as concrete as you possibly can.  Precisely how do you expect people to interact with the system you envisage? What do they type in?  What output do they see on the screen?

Simon