[Haskell-cafe] OOP'er with (hopefully) trivial questions.....
jed at 59A2.org
Mon Dec 17 07:04:08 EST 2007
On 17 Dec 2007, Nicholls.Mark at mtvne.com wrote:
> "The constructor of a newtype must have exactly one field but `R' has
> two In the newtype declaration for `Rectangle'"
> It doesn't like
> "newtype Rectangle = R Int Int"
data Rectangle = R Int Int
A newtype declaration will be completely erased at compile time. That
is, when you have a declaration like
newtype Circle = C Int
the compiled code will not be able to distinguish between a Circle and
an Int. You do, however, get all the benefits of a separate entity in
the type system. When your type only has one constructor, newtype is
preferred over data, but they are semantically equivalent. There are
extensions which provide impressive newtype-deriving-foo (getting the
compiler to write fairly non-trivial instance declarations for you).
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