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copied!<p><strong><code>seq</code> and <code>($!)</code> <a href="http://users.aber.ac.uk/afc/stricthaskell.html#seq" rel="nofollow noreferrer">only evaluate</a> enough to check that something is not bottom.</strong></p> <p>The following program will only print "there".</p> <pre><code>main = print "hi " `seq` print "there" </code></pre> <p>For those unfamiliar with Haskell, Haskell is non-strict in general, meaning that an argument to a function is only evaluated if it is needed.</p> <p>For example, the following prints "ignored" and terminates with success.</p> <pre><code>main = foo (error "explode!") where foo _ = print "ignored" </code></pre> <p><code>seq</code> is known to change that behavior by evaluating to bottom if its first argument is bottom.</p> <p>For example:</p> <pre><code>main = error "first" `seq` print "impossible to print" </code></pre> <p>... or equivalently, without infix ...</p> <pre><code>main = seq (error "first") (print "impossible to print") </code></pre> <p>... will blow up with an error on "first". It will never print "impossible to print".</p> <p>So it might be a little surprising that even though <code>seq</code> is strict, it won't evaluate something the way eager languages evaluate. In particular, it won't try to force all the positive integers in the following program. Instead, it will check that <code>[1..]</code> isn't bottom (which can be found immediately), print "done", and exit.</p> <pre><code>main = [1..] `seq` print "done" </code></pre>

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