[Haskell-cafe] computational overhead of the "data" declarations

[Daryoush Mehrtash]

I Haskell School of Expression  (p172), it says:

A newtype declaration is just like a data declaration, except that it can
> only be used to defined data types with single constructor.  The new data
> type is different from the analogous one created by a data declaration, in
> that there is no computational overhead in having the constructor.... You
> can think of a newtype as defining  a "new type" with exactly the same
> structure, behaviour, and performance as the underlying type.
>


What is (or where do you see) the computational overhead of the "data"
declrations?
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