[Haskell-beginners] Traversing generic trees

[Bob Ippolito]

Well, you've picked the wrong beginner project :) This isn't really an
algorithm that you're implementing, you're trying to reuse Haskell syntax
to do something that it's not intended to do. Implementing algorithms in
Haskell is a breeze if you are using a sane representation, but you need to
use the right tools.

On Thu, Jul 23, 2015 at 2:11 PM, Ali Baharev <ali.baharev at gmail.com> wrote:

> Dear Bob,
>
> Thanks. OK, so you are saying that there is no easy way of doing it
> with nested tuples either. Well, that's disappointing. :(
>
> The custom parser is violating the third requirement ("without copying
> the data into some other datastructure first").
>
> It is a learning exercise: I am trying to understand how to achieve
> certain things things in Haskell. I deliberately picked this exercise
> because it is very challenging to do it in certain statically typed
> languages, and I was curious how difficult it would be to solve it in
> Haskell.
>
> In my opinion, you can learn a lot about programming languages in
> general by re-implementing algorithms in them and comparing the
> difficulties you encountered on the way.
>
> Best wishes,
>
> Ali
>
> On Thu, Jul 23, 2015 at 9:38 PM, Bob Ippolito <bob at redivi.com> wrote:
> > You should not try and use the built-in lists or tuples to represent a
> tree
> > data structure. That's not what they're good at. You probably could make
> > tuples work and write functions using GHC.Generics to work with it as a
> tree
> > data structure, but it definitely won't be easy or elegant. If you want a
> > more economic syntax for expressing the tree you can provide shorter
> names
> > for Leaf and Node (one char isn't too bad), or else you're stuck with
> using
> > a parser. You may not have to write a parser though, perhaps JSON or YAML
> > would work fine for this. If it needs to go in the source file, there's
> > always TemplateHaskell and QuasiQuotes.
> >
> > On Thu, Jul 23, 2015 at 9:48 AM, Ali Baharev <ali.baharev at gmail.com>
> wrote:
> >>
> >> Dear All,
> >>
> >> I used to work at the University. Whenever a student asked me a
> >> question, I first focused on the question and tried to answer it.
> >> Sometimes the question was tough, maybe because there wasn't any easy
> >> solution to the problem. In those situations I used to say to my
> >> students: “Problem well spotted, unfortunately I cannot give you an
> >> answer / there is no easy solution to this problem.” However, I have
> >> never started questioning the questions of my students, especially
> >> when I could not give them an answer.
> >>
> >> According to the Wiki on beginners at haskell.org: "Here, there is no
> >> such thing as a 'stupid question.'" If you start questioning my
> >> question, then I really do not know where to go...
> >>
> >> You might as well say: The whole question is pointless, just use
> >> Data.Tree and be done with it!
> >>
> >> It is really not part of the original question, and I sooo not want to
> >> get into a discussion over this: It is important to have language
> >> support for helping humans to input data, without a full-blown parser.
> >> Examples would be user-defined implicit type conversions and
> >> user-defined literals in other statically typed languages.
> >>
> >> OK, let's get back to the original question.
> >>
> >> Apparently, there is no easy way to solve it with nested lists.
> >> However, no one has touched the nested tuples so far. How can I write
> >> a toString function that works similarly to mine but destructures
> >> arbitrarily nested tuples?
> >>
> >> Thanks,
> >>
> >> Ali
> >>
> >> On Thu, Jul 23, 2015 at 9:41 AM, Imants Cekusins <imantc at gmail.com>
> wrote:
> >> > .. on the other hand, if facing this problem:
> >> > - import large trees from concise human-readable representation as
> >> > string
> >> >
> >> > , it is probably possible to write a parser which does just that.
> >> >
> >> > After all json parser works somehow ;)
> >> >
> >> > Just saying that you could do this in Haskell, given enough interest.
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