Is it ever possible to detect sharing in Haskell?

Question

In Scheme, the primitive eq? tests whether its arguments are the same object. For example, in the following list

(define lst
  (let (x (list \'a         


        

Answer 1

There's lots of approaches.

1. Generate unique IDs and stick everything in a finite map (e.g. IntMap).
2. The refined version of the last choice is to make an explicit graph, e.g. using fgl.
3. Use stable names.
4. Use IORefs (see also), which have both Eq and Ord instances regardless of the contained type.
5. There are libraries for observable sharing.
6. As mentioned above, there is reallyUnsafePtrEquality# but you should understand what's really unsafe about it before you use it!

See also this answer about avoiding equality checks altogether.