On 2004-11-01 at 12:30PST "Brian Beckman" wrote: > Most interesting discussion -- in reading it, I realized that I had a > 'hidden agenda' in asking my question (hidden even from myself), and > that is: can I put interesting functionality, like precondition checks & > data validation, in data constructors?
No, though one could make a case that you should be able to. > I suspect not, and that's why I tend to write something > like the following: > > data Shape = Circle Float > | Square Float > deriving (Eq, Show) > > circle :: Float -> Shape > circle x = if (x <= 0) then error "Bad radius!" else Circle x > > That's fine, Yup. > but I don't know how to prevent users from calling "Circle" > directly Put the data declaration in a module, export the type, but not the constructor you want to hide: > module Shape (Shape(Square), circle) where -- Jón Fairbairn [EMAIL PROTECTED] _______________________________________________ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
