[Haskell-beginners] Pattern matching on binary trees

[lakshminaras2002 at gmail.com]

Hi Rohit,
You can probably ignore the first line of my reply, if it feels a bit
confusing :)

On Sun, Sep 5, 2010 at 3:16 PM, lakshminaras2002 at gmail.com <
lakshminaras2002 at gmail.com> wrote:

> The idea behind pattern matching is that you can use any of the data
> constructor for the type to dismantle the value coming in.
>
> For depthTree, the type of value expected is Tree a. The Node constructor
> is used to dismantle the incoming value into three parts and bind it to
> variables x u v. Note here that depthTree accepts only one argument as input
> and the type of that value is (Tree a)
>
> When you do not want to dismantle the incoming value, then you would not
> use any data constructor. Your function would then become,
>     depthTree :: Tree a -> Int
>     depthTree x = ..
>
> You cannot use depthTree (x u v) because the compiler would then try to
> look for a data constructor x . And in Haskell, data constructor names begin
> with a uppercase letter.
>
> You can refer to this page on Haskell wiki bookhttp://en.wikibooks.org/wiki/Haskell/Pattern_matching for
> more information.
>
>
> On Sun, Sep 5, 2010 at 2:18 PM, Rohit Garg <rpg.314 at gmail.com> wrote:
>
>> Hi,
>>
>> I am working on exercises in RWH, chapter 3 and I have a a question
>> about the pattern matching in the depth-of-tree question.
>>
>> The tree datatype is defined as
>>
>> data Tree a = Node a (Tree a) (Tree a)
>>            | Empty
>>              deriving (Show)
>>
>> And to pattern match on it, I need to write
>>
>> depthTree (Node x u v) = 1 + max (depthTree u) (depthTree v)
>>
>> My question is why do I need to prefix x with Node? Why can't it
>> pattern match on depthTree (x u v)? More generally, where do I need to
>> put data constructors in pattern matches?
>>
>> --
>> Rohit Garg
>>
>> http://rpg-314.blogspot.com/
>> _______________________________________________
>> Beginners mailing list
>> Beginners at haskell.org
>> http://www.haskell.org/mailman/listinfo/beginners
>>
>
>
>
> --
> Regards
> Lakshmi Narasimhan T V
>



-- 
Regards
Lakshmi Narasimhan T V
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