hygiene problem

[liufengyun]

Hygiene is a common problem in macros.

We've some initial design ideas about hygiene here:

https://github.com/liufengyun/gestalt/pull/77#pullrequestreview-38177513

Generally, hygiene becomes a problem when a name either in user-program or meta-program changes its originally intended meaning. There are mainly three cases:

• introduction of new names (i.e. definitions) in the meta-program that cause names in user-program to have a different meaning than intended
• use of names in the meta-program when mixed in user-program has a different meaning than intended
• restructure of user-program that changes the intended meaning of names.

[liufengyun]

Note that https://github.com/liufengyun/gestalt/pull/81 provides a solution to the 2nd problem above, both for syntatic macros and semantic macros.

It seems there's no simple way to do better for the 1st and 3rd problem, especially for syntactic macros.

[liufengyun]

During semester project presentation, @sjrd mentioned the current approach to hygiene will cause inconvenience:

def macro: Unit = {
  q"""
      val x = 4
      println(x)
  """
}

The code above will not type check, unless gestalt.options.unsafe is imported, which is a little unfortunate.