Does the following code help?
type Point = (Int,Int)
type PolyLines = [Point]
data Shape = Point Point | PolyLines PolyLines | Shape [Shape]
-- here you might want to write:
-- data Shape = Point | PolyLines | [Shape]
-- but Haskell requires that "type constructors" like "data Shape ..."
-- use "data constructors" like "Point ...", "PolyLines ..." to group
-- different values.
-- Type synonyms, however, don't require a data constructor but
-- are restricted to one value
-- Example usage :
shapes = Shape [(Point (1,2)), PolyLines [(2,3),(4,5)],
Shape[Shape[Point (6,7)]]]
-deech
On Fri, Nov 5, 2010 at 10:37 PM, Nikolay Artamonov
Hi all! How can we model in Haskell three hypothetic data structures A, B, C which obey following conditions:
1. A is anything 2. B is list of A's 3. C is list of A's or B's or another C's
So, for example, it would be possible to write something like: C [A, B [A, A], A, C [A, B [A]]].
I wrote this code:
data Super = A | B [A] | C[Super]
Not surprisingly, Haskell refused to accept subexpression `B[A]`, because "Not in scope: type constructor or class `A'". How to solve this problem?
More "real" example of A, B and C:
data Shape = Point Int Int | Polyline [Point] | Shape [Shape]
Interestingly, if I replace Polyline [Point] to Polyline [Shape] then definition will be accepted. But what if I want *polylines* consists of *points*, not any *shapes*?
Thank you!
-- Nikolay Artamonov _______________________________________________ Beginners mailing list Beginners@haskell.org http://www.haskell.org/mailman/listinfo/beginners