17 Dec
2007
17 Dec
'07
1:17 p.m.
A newtype can only have one constructor, with one argument, and is essentially a wrapper for that argument type.
In the general case, you want to use "data" instead of "newtype":
data Rectangle = R Int Int
I'm sure there's a trivial explanation for this, but here's something that I've always kind of wondered about: Given a single constructor type like "data X = X A B C" can't that be transformed into "newtype X = X (A, B, C)"? There must be some difference, because if there weren't we could transform all single constructor types that way, and dispense with newtype entirely.