On 26/01/2012, Thijs Alkemade
Let me try to describe the goal better. The intended users are people new to Haskell or people working with existing code they are not familiar with. When starting with Haskell, at least in my experience, it happens lot that you have an idea about what you need to write, but there are some parts in the expression you're working on you don't know yet.
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.
Megawin. Many times, lost in a tangle of code, this was just what I needed, and, alas, just what I lacked.
Regards, Thijs Alkemade
Also, I think it would be nice to have a mode in which GHC would just print the type of each hole in a module. Otherwise, one might have to load each needed module in GHCi, which could be awkward, especially if the holed term is in definition of others. Cheers, MFD