Proposal: Add chop function to Data.List

[Neil Mitchell]

(+1) for chop - it looks useful, and simple. I can see quite a few
places where I've wanted something similar.

Thanks, Neil

On Wed, Dec 15, 2010 at 2:57 AM, Isaac Dupree
<ml at isaac.cedarswampstudios.org> wrote:
>
>> chop :: ([a] -> (b, [a])) -> [a] -> [b]
>> chop _ [] = []
>> chop f as = b : chop f as'
>>   where (b, as') = f as
>
> Let's compare with an existing similar function, unfoldr.  (This is a
> somewhat academic exploration.)
>
> unfoldr :: (c -> Maybe (d, c)) -> c -> [d]
> c=[a],d=b: ([a] -> Maybe (b, [a])) -> [a] -> [b]
>
> chop    :: ([a] -> (b, [a])) -> [a] -> [b]
>
> In unfoldr, termination is signalled after 'f' and before a value is
> emitted.  In unfoldr, termination is signalled before 'f' by an empty list,
> even though the list is passed intact to 'f'.
>
> That seems peculiar.  But it's what we want.  'f' in 'chop' is usually a
> function that picks an 'n' somehow and yields
> (g (take n as), drop n as)
> for f's corresponding 'g',
> but does it efficiently.
> Clearly this is boring for an empty list, as (g [], []) would be a repeating
> constant forever under these conditions.
> 'f' *can* be different though.  It can take into account more of the list
> than is consumed (e.g. tokenizing "this+that" needs to look at the + before
> returning "this"): that's pretty common.  I wonder if there are any common
> examples where the returned list /= (drop n as) for some n, though.
>
> -Isaac
>
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