Haskell type vs. newtype with respect to type safety

I think this is mostly a matter of the situation.

Consider pathnames. The standard prelude has "type FilePath = String" because, as a matter of convenience, you want to have access to all the string and list operations. If you had "newtype FilePath = FilePath String" then you would need filePathLength, filePathMap and so on, or else you would forever be using conversion functions.

On the other hand, consider SQL queries. SQL injection is a common security hole, so it makes sense to have something like

newtype Query = Query String

and then add extra functions that will convert a string into a query (or query fragment) by escaping quote characters, or fill in blanks in a template in the same way. That way you can't accidentally convert a user parameter into a query without going through the quote escaping function.

For simple X = Y declarations, type is documentation; newtype is type checking; this is why newtype is compared to data.

I fairly frequently use newtype 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 data declaration; there's no particular reason to chose one over the other. Note that with GHC's GeneralizedNewtypeDeriving extension, for either you can automatically derive classes such as Num, allowing your temperatures or yen to be added and subtracted just as you can with the Ints 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!

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 data, 39 uses of newtype, and 96 uses of type.

But the ratio, as far as "simple" types are concerned, is a bit closer than that demonstrates, because 32 of those 96 uses of type are actually aliases for function types, such as

type PlotDataGen t = PlotSeries t -> [String]

You'll note two extra complexities here: first, it's actually a function type, not just a simple X = Y alias, and second that it's parameterized: PlotDataGen is a type constructor that I apply to another type to create a new type, such as PlotDataGen (Int,Double). When you start doing this kind of thing, type is no longer just documentation, but is actually a function, though at the type level rather than the data level.

newtype is occasionally used where type 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 newtypes and 60% are types. Several of the newtype definitions used to be types, and were definitely converted for the exact reasons you mentioned.

So in short, yes, this is a frequent idiom.

I think it is quite common to use newtype for type distinctions. In many cases this is because you want to give different type class instances, or to hide implementations, but simply wanting to protect against accidental conversions is also an obvious reason to do it.