[JoelMcCracken]

Great point. This also reminds me of discussion I saw somewhere else about programming languages with execution capabilities support for security, the author saying that no languages supported it, and a commenter saying that this was basically the same as an effects system.

I'm personally a professional Haskell programmer and quite like it, but I think we are circling a core notion: There are many problems with having programming languages where any code can do literally anything at all, and being able to restrict it is extremely powerful.

Constraints liberate, liberties constrain. You can always loosen strictures, but once loosened, they are extremely hard to reintroduce.