On Mon, 2007-12-17 at 13:51 +0000, Bayley, Alistair wrote:
From: haskell-cafe-bounces@haskell.org [mailto:haskell-cafe-bounces@haskell.org] On Behalf Of Nicholls, Mark
To recap...
"type" introduces a synonym for another type, no new type is created....it's for readabilities sake.
"Newtype" introduces an isomorphic copy of an existing type...but doesn't copy it's type class membership...the types are disjoint/distinct but isomorphic (thus only 1 constructor param).
"data" introduces a new type, and defines a composition of existing types to create a new one based on "->" and "(".
"class" introduces a constraint that any types declaring themselves to be a member of this class...that functions must exist to satisfy the constraint.
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?
More less than more. There is a context that can distinguish a newtype from a data type. This is explained on this wiki page that addresses exactly this question. http://www.haskell.org/haskellwiki/Newtype