[Haskell-cafe] Please help with double recursion

[Dmitri O.Kondratiev]

On Mon, May 30, 2011 at 11:26 AM, Richard O'Keefe <ok at cs.otago.ac.nz> wrote:

>
> On 28/05/2011, at 11:47 PM, Dmitri O.Kondratiev wrote:
>
> > Hello,
> > I am trying to solve a simple task, but got stuck with double recursion -
> for some reason not all list  elements get processed.
> > Please advice on a simple solution, using plane old recursion :)
> > *** Task:
> > From a sequence of chars build all possible chains where each chain
> consists of chars that can be accessed from one to another.
> > For example, given the sequence:
> > "abcde"
> > In table below chars in a left column can access a list of chars in the
> right column:
> > a | b c d e
> > b | c d e
> > c | d e
> > d | e
>
> You have a set S of characters and a binary relation R ⊆ S × S
> and a chain is a sequence [x0,x1,...] such that
>  x[0] ∈ S
> and for all i > 0,
>  x[i-1] R x[i]
>
> Can a chain be empty?
>
> What constraints on R do you have that lead you to think that
> each chain is finite, or are you expecting infinite chains as well?
> (S = {a}, R = {(a,a)} admits chains of any length, including ones
> that do not terminate.)
>
>
>
Sorry, I missed to specify that char sequences and chains are finite.
Every chain is built from chars that can be accessed from one to another.
Chars are examined proceeding from the beginning of the sequence to its end,
and never in the opposite direction. Going always in one direction and being
bound with sequence end, makes chains finite.
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