[Haskell-cafe] How to take a minimum sub list that only contain certain number of elements of certain type?

[Richard O'Keefe]

On 26/09/2012, at 12:28 PM, Gwern Branwen wrote:

> On Tue, Sep 25, 2012 at 8:17 PM, Richard O'Keefe <ok at cs.otago.ac.nz> wrote:
>> Wrong.  The original poster gave an explicit example
>> in which even elements were *retained* in the output,
>> they just weren't *counted*.
> 
> You are at least the fourth person to email me now to point this out.
> I'm glad I could make four people's day better with the joy of
> correction...
> 
> But I never said it was a full solution - please note that I did
> include the output of the function!

The "tsk tsk" is probably what made people so keen to reply.

> One could consider it a partial solution, however: that gives you the
> _nth_ odd, so if you want a list of numbers up to the _nth_ odd, that
> gives you a stop condition - 'takeWhile =/ nth+1' etc.

That doesn't work either.  Consider the list [1,1,1,1,1].
The element just after the 5th odd number in the list is 1;
takeWhile (/= 1) will thus return [] instead of [1,1,1,1].

|A 2N traverse
> (which laziness might fuse to just 1 traverse, dunno).

Not in this case, for fairly obvious reasons.