On Dec 17, 2007 8:51 AM, Bayley, Alistair <Alistair_Bayley@invescoperpetual.co.uk> wrote:

As an aside, I was wondering exactly what the differences are between
newtype and data i.e. between

> newtype A a = A a

and

> data A a = A a

According to:
  http://www.haskell.org/onlinereport/decls.html#sect4.2.3
newtype is, umm, stricter than data i.e. newtype A undefined =
undefined, but data A undefined = A undefined. Other than that, newtype
just seems to be an optimization hint. Is that a more-or-less correct
interpretation?

Pretty much. Newtype is nice to have, but I don't think there's any program you can write that couldn't be rewritten to use data (with a possible loss of efficiency).

The difference that surprised me is the difference between

    newtype A a = A a

and

    data A a = A !a

If we define a function like this,

    seqA (A a) = ()

Under the first definition of A,

    seqA undefined = ()

Under the second,

   seqA undefined = undefined

The difference is that pattern-matching a newtype doesn't do any evaluation.