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 Jon.Fairbairn@cl.cam.ac.uk