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