[Haskell-cafe] Newbie Question on type constructors
Jon.Fairbairn at cl.cam.ac.uk
Mon Nov 1 15:51:11 EST 2004
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,
> but I don't know how to prevent users from calling "Circle"
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 at cl.cam.ac.uk
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