[Haskell-cafe] Type trickery

[Andrew Coppin]

The well-known ST monad uses an ingenious hack to make it impossible for 
distinct ST computations to interact with each other.

Is there a way to do something similar so that I can create "cursors" 
that reference a (mutable) container, and then write a function that 
takes two cursor arguments which are statically guaranteed to refer to 
the same container?