Re: [Haskell-cafe] List of numbers to list of ranges

On Thu, 23 Dec 2010, Stephen Tetley wrote:

On 23 December 2010 21:12, Stephen Tetley wrote:

...

I'd go with direct recursion for this one - the pattern of consumption and production that generates the answer doesn't seem to neatly match any of the standard recursion combinators (map, unfold, fold, mapAccum, ...) nor exotic ones (skipping streams c.f. the Stream fusion paper, apomorphisms, ...).

Here's a synthesized functional that matches the needed behaviour.

It takes from:

a) apomorphism - has a final flush operation b) skipping streams (the Stream fusion paper) - a skipping Next case, though in this case there is no Done c) unfoldMap - itself a synthetic combination of unfold and map, which is unfolding against a list as well as state (the unfold equivalent of mapAccumL).

Unimaginatively I've called it "lessproductive" as it can produce less than it consumes, though as it has no Done it must consume everything...

data Step st a = Yield a !st | Next !st

This could be seen as "type Step st a = (Maybe a, st)". I have thought about mapping from [Int] to [Maybe (Int, Int)] by mapAccumL, then compressing the result with catMaybes. However we need to append a final pair when the end of the list is reached, which risks a memory leak.