Re: [Haskell-beginners] trees on Haskell : Do I understand it right ?

26 Feb 2015

      Hi Roelof,

I think you misunderstood it.

There are two things here: types and values (value-constructors). They
exist in different world, not touching each other.

In Haskell, you define a type as:

data  =   

You can create values as:

let varName =  <Value> <Value> <Value>

You need to put <Value> of some type, not type name itself in place of
those <Value>s.

So, with datatype you provided, you have two data-constructors:

Leaf

and

Node <val> <val> <val>

You can create a leaf:

let leaf = Leav

or a node:

let node = Node Leaf "msg" Leaf

You can see that Node is a data-constructor that takes 3 values, not
type-names as it's parameters.

Hope this helps.

On Thu, Feb 26, 2015 at 3:21 PM, Roelof Wobben  wrote:
...
Hello,
Suppose we have this definition of a tree :
data MessageTree = Leaf
                 | Node MessageTree LogMessage MessageTree
  deriving (Show, Eq)
let Message  = LogMessage "E 1 this is a test error"
let Message = LogMessage "e 2 this is the second test error "
As I understand it right I can make the first entry like this :
first_entry = Node Messagetree  Message Messagetree
And the second one like this second_entry = Node Message Messagetree
Message2 Messagetree ??
Roelof
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Re: [Haskell-beginners] trees on Haskell : Do I understand it right ?

Konstantine Rybnikov