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 book http://en.wikibooks.org/wiki/Haskell/Pattern_matching for more information.

On Sun, Sep 5, 2010 at 2:18 PM, Rohit Garg <rpg.314@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/
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Regards
Lakshmi Narasimhan T V