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    <p>First of all, <code>Vector</code> is an immutable data structure. Any time you "modify" the <code>Vector</code>, you're creating a whole new copy of it, meaning each modification takes O(n) time (where n is the length of the vector).</p> <p>Second of all, values in Haskell are immutable. Any time you modify a <code>TVar</code>, you are replacing the old value with a new value.</p> <p>I think what you want is a purely functional data structure supporting efficient update. Two examples:</p> <ul> <li><p><a href="http://hackage.haskell.org/packages/archive/containers/latest/doc/html/Data-Map.html" rel="nofollow">Data.Map</a>: Key-value dictionary. This is like <code>std::map</code> in C++.</p></li> <li><p><a href="http://hackage.haskell.org/packages/archive/containers/latest/doc/html/Data-Sequence.html" rel="nofollow">Data.Sequence</a>: Like a mutable array, but better.</p></li> </ul> <p>Any time you "modify" one of these data structures, you're actually constructing a new value that internally points to parts of the old value.</p> <p>A couple guidelines:</p> <ul> <li><p>If you're only modifying a single value, <a href="http://hackage.haskell.org/packages/archive/base/latest/doc/html/Data-IORef.html#v%3aatomicModifyIORef" rel="nofollow"><code>atomicModifyIORef</code></a> may be sufficient. If more sophisticated synchronization is needed besides atomic update, <code>STM</code> would be a better choice.</p></li> <li><p>Watch out for laziness when working with mutable variables. Any time you modify the shared state, be sure to force it. This can be done using <a href="http://hackage.haskell.org/packages/archive/base/latest/doc/html/Prelude.html#v%3aseq" rel="nofollow"><code>seq</code></a>. A more convenient way is to use <a href="http://hackage.haskell.org/trac/haskell-prime/wiki/BangPatterns" rel="nofollow">bang patterns</a>. Example:</p> <pre><code>!x &lt;- atomically $ do x &lt;- readTVar shared_state writeTVar shared_state (changeSomething x) return x </code></pre> <p>This forces <code>x</code> to be evaluated after the transaction completes. If a variable (<code>IORef</code>, <code>STRef</code>, <code>TVar</code>, etc) is modified a bunch of times but never forced, thunks will pile up in memory. Evaluating the resulting thunk may even produce a stack overflow.</p></li> </ul> <p>If your program needs to be massively parallel (that is, have multiple cores/CPUs access the variable simultaneously), updating a single value like this may be less efficient, as computations may be duplicated among processors. However, for a small number of cores, duplication is pretty rare.</p>
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    1. POSoftware transactional memory with a big, shared piece of data
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    1. USJoey Adams
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    1. COThe questionar probably was referring to mutable vectors. [Data.Vector.Mutable](http://hackage.haskell.org/packages/archive/vector/0.6/doc/html/Data-Vector-Mutable.html)
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      1. USPhilip JF
 

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