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copied!<p>This is an extremely cool idea, but unfortunately its true potential can't be realized in Scala's type system. What you really want here is <a href="http://en.wikipedia.org/wiki/Dependent_types" rel="nofollow noreferrer">dependent types</a>, which allow you to impose a proof obligation on the caller of your method to verify that the argument is in range, such that <em>the method can't even be invoked</em> with an invalid argument. </p> <p>But without dependent types and the ability to verify specifications at compile-time, I think this has questionable value, even leaving aside performance considerations. Consider, how is it any better than using the <a href="http://daily-scala.blogspot.com/2010/03/assert-require-assume.html" rel="nofollow noreferrer"><code>require</code></a> function to state the initial conditions required by your method, like so: </p> <pre><code>def foo(i:Int) = { require (i >= 0) i * 9 + 4 } </code></pre> <p>In both cases, a negative value will cause an exception to be thrown at runtime, either in the <code>require</code> function or when constructing your <code>NonNegativeDouble</code>. Both techniques state the contract of the method clearly, but I would argue that there is a large overhead in building all these specialized types whose only purpose is to encapsulate a particular expression to be asserted at runtime. For instance, what if you wanted to enforce a slightly different precondition; say, that <code>i > 45</code>? Will you build an <code>IntGreaterThan45</code> type just for that method?</p> <p>The only argument I can see for building e.g. a <code>NonNegativeFoo</code> type is if you have many methods which consume and return positive numbers only. Even then, I think the payoff is dubious.</p> <p>Incidentally, this is similar to the question <a href="https://stackoverflow.com/questions/3181766/how-far-to-go-with-a-strongly-typed-language">How far to go with a strongly typed language?</a>, to which I gave a similar answer.</p>

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