Why is there “data” and “newtype” in Haskell? [duplicate]

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Both newtype and the single-constructor data introduce a single value constructor, but the value constructor introduced by newtype is strict and the value constructor introduced by data is lazy. So if you have

data D = D Int
newtype N = N Int

Then N undefined is equivalent to undefined and causes an error when evaluated. But D undefined is not equivalent to undefined, and it can be evaluated as long as you don’t try to peek inside.

Couldn’t the compiler handle this for itself.

No, not really—this is a case where as the programmer you get to decide whether the constructor is strict or lazy. To understand when and how to make constructors strict or lazy, you have to have a much better understanding of lazy evaluation than I do. I stick to the idea in the Report, namely that newtype is there for you to rename an existing type, like having several different incompatible kinds of measurements:

newtype Feet = Feet Double
newtype Cm   = Cm   Double

both behave exactly like Double at run time, but the compiler promises not to let you confuse them.

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