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copied!<p>You're looking for <a href="http://disciple.ouroborus.net/" rel="nofollow noreferrer">Disciple</a>.</p> <p>So there are two kinds of laziness to distinguish in Haskell. There's lazy I/O, which is an abomination and is solved by iteratee libraries (Shameless plug: including my <a href="http://hackage.haskell.org/package/pipes-2.0.0" rel="nofollow noreferrer">pipes</a> library). Then there's laziness in pure computations, which is still open to debate, but I'll try to summarize the key advantages of laziness since you are already familiar with the disadvantage:</p> <p><strong>Laziness is more efficient</strong></p> <p>A simple example is:</p> <pre><code>any = foldr (||) False </code></pre> <p><code>any</code> finds if any value in a list is <code>True</code>. This only evaluates the elements up to the first <code>True</code>, so it doesn't matter if the list is very long.</p> <p>Laziness only computes as much as it has to, meaning that if you chain together two lazy computations, it can actually improve the time complexity of the resulting computation. <a href="https://stackoverflow.com/a/284180/1026598">This Stack Overflow comment</a> gives another good example of this.</p> <p>This is actually the same reason why iteratee libraries are very resource-efficient. They only do as much work as they have to generate results, and this leads to very efficient memory and disk usage with very easy-to-use semantics.</p> <p><strong>Laziness is inherently more composable</strong></p> <p>This is well known by people who have programmed in both strict and functional languages, but I actually inadvertently demonstrated a limited proof of this while working on the <code>pipes</code> library, where the lazy version is the only version that permits a <code>Category</code> instance. Pipes actually work in any monad, including the pure <code>Identity</code> monad, so my proofs translate to pure code as well.</p> <p>This is the true reason why I believe that laziness in general is really the future of programming, however I still think that it's an open question of whether or not Haskell implemented laziness "right".</p>

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