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

It is not possible in Haskell, the pure language.

But in its implementation in GHC, there are loopholes, such as

• the use of reallyUnsafePtrEquality# or
• introspection libraries like ghc-heap-view.

In any case, using this in regular code would be very unidiomatic; at most I could imagine that building a highly specialized library for something (memoizatoin, hash tables, whatever) that then provides a sane, pure API, might be acceptable.