[Haskell-beginners] Empty or Tree?

[Angelos Sphyris]

>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 at web.de
> To: beginners at 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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