Not really with this... The open case (as in OO) seems to be more like the Haskell class construct, i.e. if new types declare themselves to be members of a class then they must satisfy certain constaints....I can then specify "equals" with the class and leave the onus on the implementor to implement it....the data construct seems more analogous to a OO class definition...which is closed in the same way. The approach is deliberate...but I accept may be harder than it needs to be...I'm interested in Haskell because of the alleged power/formality of it's type system against the relatively weakness of OO ones...the irony at the moment is that they do not really seem to correspond directly....and OO type system seems to (loosely) correlate to Haskell type class system, and an OO class system (loosely) to Haskels type system, though in OOP's they are unpleasantly tangled. -----Original Message----- From: haskell-cafe-bounces@haskell.org [mailto:haskell-cafe-bounces@haskell.org] On Behalf Of apfelmus Sent: 17 December 2007 12:34 To: haskell-cafe@haskell.org Subject: [Haskell-cafe] Re: OOP'er with (hopefully) trivial questions..... 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?
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
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. 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 _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe