Hello,
You are more or less right; except thatNow, Haskell has a garbage collector, so Haskell must know how many pointers there are to all objects in the heap (or?). Then, if a finite map for example only has one single pointer to it, then it ought to be all right to modify the finite map (or whatever datastructure we are considering), I mean really modify the map without making any copies, just like in imperative languages. Perhaps there might be pointers to nodes inside the tree and I guess that could complicate the matter somewhat. But for Haskell arrays it ought to be possible to really modify the array if it is used by only one pointer ?Are such optimizations possible, and if they are, are they already implemented in for example GHC ? Or am I wrong somewhere ?
-- the Haskell implementations I have seen do not keep number of pointers to
objects (gc in principle works by traversing the graph made by pointers,
marking reachable nodes and finally removing the unreachable ones; just
keeping the number of references is not sufficient due to cyclic structures)
-- due to lazy evaluation, the situation where there would be just one
pointer to array is rare
Zdenek
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