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copied!<p>For simple <code>X = Y</code> declarations, <code>type</code> is documentation; <code>newtype</code> is type checking; this is why <code>newtype</code> is compared to <code>data</code>.</p> <p>I fairly frequently use <code>newtype</code> for just the purpose you describe: ensuring that something which is stored (and often manipulated) in the same way as another type is not confused with something else. In that way it works just as a slightly more efficient <code>data</code> declaration; there's no particular reason to chose one over the other. Note that with GHC's <code>GeneralizedNewtypeDeriving</code> extension, for either you can automatically derive classes such as <code>Num</code>, allowing your temperatures or yen to be added and subtracted just as you can with the <code>Int</code>s or whatever lies beneath them. One wants to be a little bit careful with this, however; typically one doesn't multiply a temperature by another temperature!</p> <p>For an idea of how often these things are used, In one reasonably large project I'm working on right now, I have about 122 uses of <code>data</code>, 39 uses of <code>newtype</code>, and 96 uses of <code>type</code>.</p> <p>But the ratio, as far as "simple" types are concerned, is a bit closer than that demonstrates, because 32 of those 96 uses of <code>type</code> are actually aliases for function types, such as</p> <pre><code>type PlotDataGen t = PlotSeries t -> [String] </code></pre> <p>You'll note two extra complexities here: first, it's actually a function type, not just a simple <code>X = Y</code> alias, and second that it's parameterized: <code>PlotDataGen</code> is a type constructor that I apply to another type to create a new type, such as <code>PlotDataGen (Int,Double)</code>. When you start doing this kind of thing, <code>type</code> is no longer just documentation, but is actually a function, though at the type level rather than the data level.</p> <p><code>newtype</code> is occasionally used where <code>type</code> can't be, such as where a recursive type definition is necessary, but I find this to be reasonably rare. So it looks like, on this particular project at least, about 40% of my "primitive" type definitions are <code>newtype</code>s and 60% are <code>type</code>s. Several of the <code>newtype</code> definitions used to be types, and were definitely converted for the exact reasons you mentioned.</p> <p>So in short, yes, this is a frequent idiom.</p>

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