Re: [Haskell-cafe] how to make this work recursive ?

Don't worry. Keep these points in mind and re-read my previous mail. Regarding data vs. type constructors. 1) Leaf :: MessageTree (Leaf is a MessageTree) 2) MessageTree is a type constructor (takes types and gives new types) 3) Node is a data constructor (takes values and gives a new MessageTree) Regarding insertion. 1) The "insert" function takes a message and tree, and returns a tree with that message inserted into it 2) We absolutely have to provide it a tree 3) If we insert an invalid message in the tree, it does not get added. Then we get ..... the same tree back. If you still cannot see a problem in your code, ask yourself the following: 1) What type does the output of insert (MessageTree Leaf) have, and what type should it have 2) What happens if we insert an invalid string in a non-empty tree (according to your code) Don't forget to re-read the previous mail. Hope this drives the point home. On 28 February 2015 at 22:59, Roelof Wobben wrote:

Sorry,

But I do not understand what you mean.

What I try to do is this

If you have a LogMessage which contains " This is not the right format " then the LogMessage is not inserted in the tree. Every other message are inserted in the tree.

So that is why I make a MessageTree with only one Leaf.

Roelof

Sumit Sahrawat, Maths & Computing, IIT (BHU) schreef op 28-2-2015 om 16:08:

You're missing an argument to insert, assuming that it is indeed there, consider this:

insert msg tree = tree' where tree' = undefined -- Take a LogMessage (msg) and a MessageTree (tree) -- and return a new MessageTree (tree')

Therefore, the result of insert in your case, i.e. "MessageTree Leaf" should be of type MessageTree. And thus, we get

MessageTree Leaf :: MessageTree -- Takes a (Leaf :: LogMessage) and produces a MessageTree

which implies,

MessageTree :: LogMessage -> MessageTree -- A Data constructor

Obviously no such data constructor exists. A data constructor of this type would exist only if you wrote something like:

data MessageTree = MessageTree LogMessage | ...

which would not make sense.

On 28 February 2015 at 19:09, Roelof Wobben wrote:

...

I tried this :

insert :: LogMessage -> MessageTree -> MessageTree insert s = case words s of (_:_: "This is not the right format") -> MessageTree Leaf _ -> MessageTree Node LogMessage Leaf

But I see this errormessages which I do not understand :

src/LogAnalysis.hs@38:49-38:60 Not in scope: data constructor MessageTree src/LogAnalysis.hs@39:49-39:60 Not in scope: data constructor MessageTree

Mike Meyer schreef op 28-2-2015 om 12:17:

On Sat, Feb 28, 2015 at 5:07 AM, Roelof Wobben wrote:

...

That part I understand .

it's more how I do it here :

Lets say I have this :

data MessageType = Info | Warning | Error Int deriving (Show, Eq)

type TimeStamp = Int

data LogMessage = LogMessage MessageType TimeStamp String | Unknown String deriving (Show, Eq)

data MessageTree = Leaf | Node MessageTree LogMessage MessageTree deriving (Show, Eq)

Now I have to put all the LogMessages in a binary tree. with this signature : insert :: LogMessage -> MessageTree -> MessageTree

I think I need recursion here but I still not see how I can make the clauses. Normally you can say somethig like this [] -> 0 or 1 -> but here it seems to be all seperate logMessages

Whether you need recursion or not depends on where you're to insert the message. For instance, you could turn them into a list with:

insert message tree = Node tree message Leaf

Try drawing a few pictures of what that produces, and you'll see why it's probably not what you want.

You should be able to tell by looking at the MessageTree type that you have two cases: one where you're passed something matching a Node, and one where you're passed something that's just a Leaf. The latter can't be recursive, so it's going to terminate any recursion. Following the advice given to you for the Hanoi problem, try writing it first.

Then all you have to do is figure out how to handle the case where you're given a Node instead of a Leaf. Figuring out where in the MessageTree you want to insert the Logmessage will probably be required to do that.

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-- Regards

Sumit Sahrawat

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-- Regards Sumit Sahrawat