Very interesting, I did not know that! I thought newtype was an optimization of data, and that "newtype" was bad terminology. But if newtype is just a wrapper around a type, then the name is choosen well. I'm a bit confused why then one needs a data-constructor-like tag to construct a newtype value then? Is this to avoid having to add a type signature (for type inference)? I find this a bit weird since newtype Foo = Foo Int bar = Foo 123 does not safe a lot of keystrokes ;) compared to -- Incorrect Haskell follows newtype Foo = Int bar = 123::Foo -----Original Message----- From: haskell-cafe-bounces@haskell.org [mailto:haskell-cafe-bounces@haskell.org] On Behalf Of Wolfgang Jeltsch Sent: Monday, December 17, 2007 5:39 PM To: haskell-cafe@haskell.org Subject: Re: [Haskell-cafe] OOP'er with (hopefully) trivial questions..... Am Montag, 17. Dezember 2007 13:04 schrieb Jed Brown:
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When your type only has one constructor, newtype is preferred over data, but they are semantically equivalent.
They are *not* semantically equivalent, as has already been said more or less. data adds an extra level of indirection. With data A a = MkA a, _|_ (i.e., undefinedness) is different from MkA _|_. If I don’t know anything about a value of A a then this is not the same as knowing that the value is at least an application of MkA. newtype just creates wrapper types and it’s very unfortunate that it uses syntax similar to data because it’s very different. With newtype A a = MkA a, you just create a wrapper type A a for each type a. Applying the constructor just means casting from a to A a, and pattern matching just means casting from A a to a. Type-casting _|_ yields botton, that’s why MkA _|_ is _|_ and pattern matching _|_ against A x doesn’t fail but assigns _|_ to x.
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Best wishes, Wolfgang _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe