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

[Sumit Sahrawat, Maths & Computing, IIT (BHU)]

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 <r.wobben at home.nl> 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 at 38:49-38:60
> Not in scope: data constructor
> MessageTree
>   src/LogAnalysis.hs at 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 <r.wobben at home.nl> 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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