title: Can a thunk be duplicated to improve memory performance?
body: <p>One of my struggles with lazy evaluation in Haskell is the difficulty of reasoning about memory usage.  I think the ability to duplicate a thunk would make this much easier for me.  Here's an example.</p>
  
  <p>Let's create a really big list:</p>
  
  <pre><code>let xs = [1..10000000]
  </code></pre>
  
  <p>Now, let's create a bad function:</p>
  
  <pre><code>bad = do
      print $ foldl1' (+) xs
      print $ length xs
  </code></pre>
  
  <p>With no optimizations, this eats up a few dozen MB of ram.  The garbage collector can't deallocate xs during the fold because it will be needed for calculating the length later.</p>
  
  <p>Is it possible to reimplement this function something like this:</p>
  
  <pre><code>good = do
      (xs1,xs2) &lt;- copyThunk xs
      print $ foldl1' (+) xs1
      print $ length xs2
  </code></pre>
  
  <p>Now, xs1 and xs2 would represent the same value, but also be independent of each other in memory so the garbage collector can deallocate during the fold preventing memory wasting.  (I think this would slightly increase the computational cost though?)</p>
  
  <p>Obviously in this trivial example, refactoring the code could easily solve this problem, but It seems like it's not always obvious how to refactor.  Or sometimes refactoring would greatly reduce code clarity.</p>