No, they are actually being mutated. ST is basically IO with a
universal state thread (IO uses RealWorld) to prevent you from letting
any of the mutable structures out (or any in) of the block. The whole
point of ST is to have real mutable references/arrays that have a
referentially transparent if you look at them from outside.

Dan

On Tue, Dec 15, 2009 at 9:23 AM, Alberto G. Corona <agocorona@gmail.com> wrote:
> AFAIK, ST Arrays are pure data structures. They are not really mutable. They
> are destroyed and recreated on every update. The mutation is just simulated
> thanks to the hidden state in the state monad. Sure, the garbage collector
> must have a hard work in recycling all the "backbones" of the discarded
> arrays (not the elements).
>
> 2009/12/14 Brad Larsen <brad.larsen@gmail.com>
>>
>> Is anyone working on fixing ticket #650
>> <http://hackage.haskell.org/trac/ghc/ticket/650>?  In short, STArray
>> and the garbage collector don't play well together, resulting in array
>> updates being non-constant time operations.  This bug makes it very
>> difficult/impossible to write efficient array algorithms that depend
>> upon mutation in Haskell.
>>
>> On another note, does this (or perhaps better phrased, will this) bug
>> also affect Data Parallel Haskell?
>>
>> I would really like to see highly efficient, mutable, boxed arrays in
>> Haskell!  Unfortunately, I don't have the know-how to fix Ticket 650.
>>
>> Sincerely,
>> Brad
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