Hello everyone. I'm trying to write a code which works on binary trees. When I write code like this with a tree with empty nodes : data Tree = Empty | Node Integer Tree Tree function Node a (Node b Empty Empty) (Node c Empty Empty) it works fine. But when I try to create a more generic code like this which could work with trees who don't have empty nodes in grandchild level : function Node a (Node b Tree Tree) (Node c Tree Tree ) I get this error : Undefined data constructor "Tree" Can you help me with creating more generic code please? Thanks
I think this is because "Tree" is a data type and not a data constructor.
According to your definition of Tree, Empty and Node are the constructors.
Node a (Node b (Node c Empty Empty) (Node d Empty Empty)) (Node e (Node f
Empty Empty) (Node g Empty Empty))
is a valid tree if a, b, c, d, e, f, g are integers.
Hope this helps.
--
Aditya.
On 10 March 2012 15:58, bahadýr altan
Hello everyone. I'm trying to write a code which works on binary trees. When I write code like this with a tree with empty nodes :
data Tree = Empty | Node Integer Tree Tree
function Node a (Node b Empty Empty) (Node c Empty Empty)
it works fine. But when I try to create a more generic code like this which could work with trees who don't have empty nodes in grandchild level :
function Node a (Node b Tree Tree) (Node c Tree Tree )
I get this error : Undefined data constructor "Tree"
Can you help me with creating more generic code please? Thanks
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On Sat, Mar 10, 2012 at 10:28:43AM +0000, bahadýr altan wrote:
Hello everyone. I'm trying to write a code which works on binary trees. When I write code like this with a tree with empty nodes :
data Tree = Empty | Node Integer Tree Tree
function Node a (Node b Empty Empty) (Node c Empty Empty)
it works fine. But when I try to create a more generic code like this which could work with trees who don't have empty nodes in grandchild level :
function Node a (Node b Tree Tree) (Node c Tree Tree )
I get this error : Undefined data constructor "Tree"
Can you help me with creating more generic code please? Thanks
Hello, You defined a Tree as an Empty or a Node. So to build it, you can use only Empty or Node, which is what you do in your first function. In the second one however, you use Tree, which is the type, not the constructor. If you use them it will look like : function Node a (Node b t1 t2) (Node c t3 t4) where t1 = Empty t2 = Node d Empty Empty ... Adding the function signature before your function implementatio help sort things out for the parameters you need: t1, to t4, to a to c, etc Hope that helps. Adrien -- Adrien Haxaire www.adrienhaxaire.org | @adrienhaxaire
But when I try to create a more generic code like this which could work with trees who don't have empty nodes in grandchild level :
function Node a (Node b Tree Tree) (Node c Tree Tree )
The problem is that 'Tree' is a type, not a constructor. Someone correct me, if I'm mistaken (this is my first post to the mailing-list, yieah :-)), but what seems to cause the problem is that the pattern matcher needs constructors, so it can determine, whether a pattern can produce an input data. There are several solutions. If you don't need the further subtrees, leave them fully unspecified via the underscore: function (Node a (Node b _ _) (Node c _ _)) = ... or you could give them variable names like this: function (Node a (Node b bl br) (Node c cl cr)) = ... where bl, br, cl, and cr are variables of the type Tree. However, what you might want to accomplish is a recursive function over the recursive type to get a fully generic code. This usually looks something like this: -- a generic recursive function cFunction :: Tree -> a cFunction Empty = ... cFunction (Node i l r) = f i (cFunction l) (cFunction r) where f :: Integer -> a -> a -> a f int recLeft recRight = ... Hope that helped.
Someone correct me, if I'm mistaken but what seems to cause the problem is that the pattern matcher needs constructors, so it can determine, whether a pattern can produce an input data. Yes, I would agree with you on that. It seems correct to me.
Angelos
Date: Sat, 10 Mar 2012 12:45:07 +0100 From: Micha-Schober@web.de To: beginners@haskell.org Subject: Re: [Haskell-beginners] Empty or Tree?
But when I try to create a more generic code like this which could work with trees who don't have empty nodes in grandchild level :
function Node a (Node b Tree Tree) (Node c Tree Tree )
The problem is that 'Tree' is a type, not a constructor. Someone correct me, if I'm mistaken (this is my first post to the mailing-list, yieah :-)), but what seems to cause the problem is that the pattern matcher needs constructors, so it can determine, whether a pattern can produce an input data.
There are several solutions. If you don't need the further subtrees, leave them fully unspecified via the underscore:
function (Node a (Node b _ _) (Node c _ _)) = ...
or you could give them variable names like this:
function (Node a (Node b bl br) (Node c cl cr)) = ...
where bl, br, cl, and cr are variables of the type Tree. However, what you might want to accomplish is a recursive function over the recursive type to get a fully generic code. This usually looks something like this:
-- a generic recursive function cFunction :: Tree -> a cFunction Empty = ... cFunction (Node i l r) = f i (cFunction l) (cFunction r) where f :: Integer -> a -> a -> a f int recLeft recRight = ...
Hope that helped.
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participants (5)
-
Aditya Manthramurthy -
Adrien Haxaire -
Angelos Sphyris -
bahadýr altan -
Michael Schober