On 17 Dec 2007, at 7:39 AM, David Menendez wrote:
On Dec 17, 2007 8:51 AM, Bayley, Alistair
<[EMAIL PROTECTED]> 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.
So there is a program (or, rather, type) you can write with newtype
that can't be written with data:
newtype T = T T
jcc
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