Perhaps you should look at the Clean languages, which is similar to Haskell, but has a feature called "uniqueness typing". Using the type system, you can figure out the information that you are asking for and as far as I know, their implementation is optimzed in the manner you want. In fact, their whole IO system is built around that idea. Have a look at http://www.cs.kun.nl/~clean/ -Peter ML> Now, Haskell has a garbage collector, so Haskell must know how many ML> pointers there are to all objects in the heap (or?). Then, if a finite map ML> for example only has one single pointer to it, then it ought to be all ML> right to modify the finite map (or whatever datastructure we are ML> considering), I mean really modify the map without making any copies, just ML> like in imperative languages. Perhaps there might be pointers to nodes ML> inside the tree and I guess that could complicate the matter somewhat. But ML> for Haskell arrays it ought to be possible to really modify the array if ML> it is used by only one pointer ? ML> Are such optimizations possible, and if they are, are they already ML> implemented in for example GHC ? Or am I wrong somewhere ?