On 17 Dec 2007, [EMAIL PROTECTED] wrote: > Ooo > > "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"
You want 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). Jed
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