Just wanted to point out you can get accessor/deconstructor functions using record notation: data Shape = Circle { radius :: Double } | Square { length :: Double } Keean. Benjamin Franksen wrote:
On Monday 01 November 2004 22:34, you wrote:
On 2004-11-01 at 23:01+0100 Benjamin Franksen wrote:
On Monday 01 November 2004 21:51, Jon Fairbairn wrote:
Put the data declaration in a module, export the type, but
not the constructor you want to hide:
module Shape (Shape(Square), circle) where
Since we were talking about 'what can one do with a constructor as compared with a function', it should be mentioned that this also has a disadvantage: you loose pattern matching outside the module. I.e. no longer can say
... case x of Circle radius -> ... Square edge -> ...
BTW, would it be possible (in principle) to allow constructor export 'read-only', so matching against a constructor is allowed but not constructing a value?
I don't see why not. To add something to the debate about what Circle /is/, I'd like to mention that the declaration
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
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
Ben _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe