[Haskell-cafe] Structural sharing in haskell data structures?

[Andrew Wagner]

So I've been reading a lot about a (relatively) new language called Clojure.
One of its goals is to make concurrency easier via a built-in home-grown
STM. Anyway, one of the ways it tries to do this is to have completely
immutable data structures. Every time I read a tutorial about this in
Clojure, it says "...yes, it sounds awful to think that your whole data
structure gets copied every time you want to make a change to it, but it's
sane because of a technique called structural sharing". Yet every time I
hear immutability talked about in Haskell, what I hear is "...yes, it sounds
awful to think that your whole data structure gets copied every time you
want to make a change to it, but it's sane because of laziness...unless you
need the whole data structure...". So I'm just curious, does GHC use
structural sharing or something similar? Do other implementations? Does it
matter?
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