Christopher Tauss ctauss1 at gmail.com
Tue Sep 21 23:54:51 EDT 2010

```Thanks Richard and the others who responded to my query.

I truly appreciate you taking the time and effort to respond to me (and the

to solve this.

My main reson for looking into Haskell is because I am intruiged that
Haskell does not allow side effects.  I have literally seen code (not my
own, of course!) that is nothing but side effects, and over time such code
becomes very expensive to maintain and extend.  I wonder if functional
programming may be the paradigm of the future just because it is in the
longer run cost effective.

Anyway, thanks again, and Best Regards

Chris Tauss
On Mon, Sep 20, 2010 at 6:11 PM, Richard O'Keefe <ok at cs.otago.ac.nz> wrote:

>
> On Sep 18, 2010, at 7:51 PM, Christopher Tauss wrote:
>
> > Hello Haskell Community -
> >
> > I am a professional programmer with 11 years experience, yet I just do
> not seem to be able to get the hang of even simple things in Haskell.  I am
> trying to write a function that takes a list and returns the last n
> elements.
> >
> > There may be a function which I can just call that does that, but I am
> trying to roll my own just to understand the concept.
> >
> > Let's call the function n_lastn and, given a list  [1,2,3,4,5], I would
> like
> > n_lastn 3 = [3,4,5]
> >
> > Seems like it would be something like:
> >
> > n_lastn:: [a]->Int->[a]
> > n_lastn 1 (xs) = last(xs)
> > n_lastn n (x:xs) = ????
> >
> > The issue is I do not see how you can store the last elements of the
> list.
>
> Part of the reason why you may be having trouble is that you are thinking
> about things like "how you can store the last elements of the list".  I'm
> having trouble imagining what that might mean.
>
> Let's look at this in completely abstract terms.
> A list is a sequence.
> A (finite) sequence has a length, and there is a length function to find
> it.
> We can split a sequence into pieces, and there are take and drop functions
> to do it.
> We have these building blocks:
>
>        length :: [t] -> Int         --length of sequence
>        take   :: Int -> [t] -> [t]  --first n elements
>        drop   :: Int -> [t] -> [t]  --all BUT the first n elements
>
> The only one of these that gives us a suffix of a list is 'drop'.
> So we are going to need
>
>        n_lastn :: Int -> [t] -> [t]
>        n_lastn count list = drop ???? list
>
> What is the first argument of drop going to be?
> drop wants the number to DISCARD.
> You have the number to KEEP.
> The number to discard + the number to keep is the total length.
> So you will discard (length list - count) elements.
> And here we go with a complete answer:
>
>        n_lastn count list = drop (length list - count) list
>
> This is a mathematical definition about (finite) sequences.
> We could write it and reason about it even if there had never been
> any such thing as a computer.  It doesn't actually matter in the
> least HOW a list is stored; this definition will be RIGHT.
>
> Whether it is *efficient* does depend on how a list is stored.
> There are questions like
>  - how often does the sequence have to be traversed
>   (with lists, determining the length involves walking along
>   the whole list until the end is found, although it does not
>   involve looking at the elements)
>  - how much copying is done
>   (with arrays, you'd have to make a new array of count elements
>   and copy the last count elements of list to it, with lists you
>   don't have to copy anything)
>
> I can make this point another way.  n_lastn is a bad name, because
> really, it's just the same as `take` except working from the other
> end.  So we can define
>
>        reverse_take n = reverse . take n . reverse
>        reverse_drop n = reverse . drop n . reverse
>
> "the last n items of a sequence are the first n items of its reversal,
> reversed"
> "all but the last n items of a sequence are all but the first n
> items of its reversal, reversed"
>
> and your n_lastn is just reverse_take.  This definition does three list
> traversals and two copies.
>
> There's a trick I found very useful in Prolog, and that is to exploit
> the homomorphism between lists and natural numbers, where a list can be
> used to represent its own length.
>
> Let's take `take` and `drop`.
>
>        take (n+1) (x:xs) = x : take n xs
>        take _     _      = []
>
>        drop (n+1) (_:xs) = drop n xs
>        drop _     xs     = xs
>
> Instead of passing a natural number as first argument, we'll pass
> a list, and the analogue of (n+1) is then (_:k).
>
>        list_take, list_drop :: [any] -> [t] -> [t]
>
>        list_take (_:k) (x:xs) = x : list_take k xs
>        list_take _     _      = []
>
>        list_drop (_:k) (_:xs) = list_drop k xs
>        list_drop _     xs     = xs
>
> (drop count list) is a list, whose length is the number of elements we
> want to discard from the beginning of list.  So we can define
>
>        reverse_take count list = list_drop (drop count list) list
>
> and this does O(length list) work; ONE list traversal and NO copies.
>
>        reverse_drop count list = list_take (drop count list) list
>
> This code was tested thus:
> *Main> [reverse_take n "abcd" | n <- [0..4]]
> ["","d","cd","bcd","abcd"]
> *Main> [reverse_drop n "abcd" | n <- [0..4]]
> ["abcd","abc","ab","a",""]
>
>
>
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