The mess with variable ids.

Variable identification tags can contain four different types of information. In Haskell we would write it as such:

data Id = Empty            -- Unused binding. Eg: '\ _ -> ...'.
        | Etherial Int     -- Internal variable. Only used when type-checking.
        | Anonymous Int    -- Anonymous variable created by the compiler.
        | Named Name       -- Named variable created by the user.

However, in LHC this data structure was unrolled and packed into an Int. The encoding went as following:

  Empty        = zero
  Etherial     = negative numbers
  Anonymous    = even, positive numbers
  Named        = odd, positive numbers, used as keys in a global hash table.

This encoding gives us very fast operations on Sets and Maps but it also punishes mistakes with a vengeance. The increased performance is definitely not worth it and we've been working on untangling the Ids from day-1.

As of today, I'm glad to say that we've finally restored the beautiful ADT and we can now hack without fear of segfaulting.