Try this: data Super a = A a | B [a] | C [Super a] -- Markus Läll On Sat, Nov 6, 2010 at 6:14 AM, aditya siram wrote:

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 wrote:

...

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

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