Nicholls, Mark wrote:
data Shape = Circle Int | Rectangle Int Int | Square Int
Isn't this now "closed"...i.e. the statement is effectively defining that shape is this and only ever this....i.e. can I in another module add new "types" of Shape?
Yes, but in most cases, this is actually a good thing. For instance, you can now define equality of two shapes: equal :: Shape -> Shape -> Bool equal (Circle x) (Circle y) = x == y equal (Rectangle x1 x2) (Rectangle y1 y2) = x1 == x2 && y1 == y2 equal (Square x) (Square y) = x == y In general, the "open" approach is limited to functions of the form Shape -> ... -> Shape / Int / something else with no Shape occurring in the other ... arguments.
I'm trying to teach myself Haskell....I've spent a few hours going through a few tutorials....and I sort of get the basics... [...] After many years of OOP though my brain is wired up to construct software in that 'pattern'....a problem for me at the moment is I cannot see how to construct programs in an OO style in Haskell....I know this is probably not the way to approach it...but I feel I need to master the syntax before the paradigm.
This approach is probably harder than it could be, you'll have a much easier time learning it from a paper-textbook like http://www.cs.nott.ac.uk/~gmh/book.html http://web.comlab.ox.ac.uk/oucl/publications/books/functional/ http://haskell.org/soe/ After all, it's like learning programming anew. Regards, apfelmus