On 2004-11-02 at 00:11+0100 Benjamin Franksen wrote:
On Monday 01 November 2004 22:34, I wrote:
data Shape = Circle Double | Square Double
is a convenience that declares three new names (Shape, Circle and Square), but five entities.
There's Shape: a type, Circle, Square:: Double -> Shape: constructor functions, and Circle, Square:: Shape -> (Double -> t) -> t -> t: destructor functions.
Interesting point of view! But... could you explain the types of the destructors? And how one would use them to deconstruct a Shape? Because, hmmm, isn't it rather *one* destructor with type
destructShape :: Shape -> (Double -> t) -> (Double -> t) -> t
There could be, but that wouldn't suit my argument ;-)
where the second and third arguments explain what to do with a Circle resp. a Square? So that
case s of Circle r -> f r Square l -> g l
is another way to write
destructShape s g f
They amount to the same thing: if Circle.destruct:: Shape -> (Double -> t) -> t -> t and similarly Square.destruct, we'd just have to write the case as Circle.destruct s f (Square.destruct s g (error "impossible")) ie the .destructs take a Shape, a function to apply if it matches and a value to return if it doesn't. Apart from matching up with the names there's not much to choose between one destructor and many, except possibly when one considers something like: case e of Square s -> ... _ -> ... particularly if the type has more than two constructors. Jón -- Jón Fairbairn Jon.Fairbairn@cl.cam.ac.uk