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copied!<p>Some good things have been said about why you shouldn't shy away from Haskell's laziness, but I feel that the original question remains unanswered.</p> <blockquote> <p>Function application in Haskell is non-strict; that is, a function argument is evaluated only when required.</p> </blockquote> <p>~ <a href="http://www.haskell.org/onlinereport/haskell2010/haskellch6.html#x13-1260006.2" rel="nofollow">Haskell Report 2010 > Predefined Types and Classes # Strict Evaluation</a></p> <p>This is a little misleading, however. Implementations <em>can</em> evaluate function arguments before they are required, but only to a limited extent. You have to preserve non-strict semantics: so if the expression for an argument results an infinite loop, and that argument is not used, then a function call with that argument must not infinite loop.</p> <p>So you <em>are</em> allowed to implement Haskell in a way which is not fully "Lazy", but it nevertheless cannot be "Strict" either. This seems like a contradiction at first blush, but it is not. A few related topics you might want to check out:</p> <ul> <li><a href="http://csg.csail.mit.edu/pubs/haskell.html" rel="nofollow">Eager Haskell</a>, an implementation of the Haskell programming language which by default uses eager evaluation. I believe this is what you were thinking of (though it's not a branch of GHC).</li> <li><a href="http://en.wikipedia.org/wiki/Speculative_execution" rel="nofollow">Speculative Execution</a> and Speculative Parallelism (see e.g. the <a href="http://hackage.haskell.org/package/speculation" rel="nofollow">speculation</a> package).</li> <li><a href="http://research.microsoft.com/en-us/um/people/simonpj/papers/optimistic/" rel="nofollow">Optimistic Evaluation</a>, a paper by SPJ about speeding up GHC with strictness optimizations.</li> </ul>

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