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

Richard A. O'Keefe ok at cs.otago.ac.nz
Sun Mar 1 23:49:46 UTC 2015

On 1/03/2015, at 2:39 am, 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 

This makes no sense.
You have declared that insert takes a LogMessage argument
and a MessageTree argument and returns a MessageTree result.


You want to have

insert logmsg msgtree = 

and the case analysis should be on the message tree, not the log message.

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

The compiler is telling you, as clearly as it knows how,
that you DO NOT HAVE ANY CONSTRUCTOR FUNCTION called "MessageTree".
The constructor functions are >> Leaf << and >> Node <<.
MessageTree is a *TYPE NAME*.

I am about to be offensive.
I do not want to be offensive.
I want to be helpful.
I believe this question needs to be asked.
But there is a real risk that I will offend you.

Here goes.

Is there *any* programming language you know how to use?

The reason that I ask is that the problems you keep having
aren't really Haskell problems.  They are general programming
 - declaring a name as one kind of thing
   and using it as if it were another
 - declaring a function (like 'insert' or 'Node')
   to have n arguments but trying to define or call
   it with m arguments where m ~= n.
 - not starting with a description of your problem
 - not having a clue what to do when the compiler tells
   you something.  (It's OK not to understand a
   compiler message.  But you should get *some* clue
   from it, like where exactly to look, and you should
   be able to try to vary the program to see how the
   message changes.)

I honestly feel that the difficulties you are reporting here
aren't really Haskell difficulties, just as the difficulties
you were reporting in the Erlang mailing list weren't really
Erlang difficulties, but in both cases, something much more

To restate my possibly offensive question again:

Have you ever had a course of instruction in the use of
any programming language face to face with a competent

Again, I'm trying to be helpful here.  If you run into a
problem and can get help within *minutes*, and have someone
sit down beside you at the keyboard and *show* you how to
find out what a compiler message might mean and what to do
about it, you can learn very quickly.  If you have to wait
hours for responses, you are obviously going to learn much
more slowly.

I sometimes ask my students: "What's the point of coming here?
Why not just buy a textbook?"  Sadly, they sometimes do not
know the answer.  It's "You can ask ME questions, and each other."

It is clear that you are working at trying to learn Haskell.
It is SMART of you to ask questions.
It's just that there are these really basic issues where I
think you could learn a lot faster with face-to-face help.

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