Proposal (Trac ticket #3671): Add takeRec, genericTakeRec and
spanRec to Data.List
Philip K.F. Hölzenspies
p.k.f.holzenspies at utwente.nl
Tue Nov 17 10:11:21 EST 2009
On Tue, 2009-11-17 at 14:35 +0000, Ian Lynagh wrote:
> > http://hackage.haskell.org/trac/ghc/ticket/3671
> I don't think spanRec does what you want:
> take 10 $ spanRec (< 3) [1,2,3,4,5,6,1,2,3,4,5,6]
> I have also defined your "takeRec" a number of times in the past, but I
> have called it something like "splitAts". I've also defined functions
> called "breaks". I think the 's' suffix is more consistent with "tails",
> "inits" etc.
Dear Ian, et al.
I posted the proposal too quickly. Two corrections and *still* missed
it. I should have dug up my PreludeEx from somewhere. You are right
about the mistake, though. Also, I also usually use the 's' suffix, but
thought it could possibly be considered too invasive in the namespace.
If no one objects to +s names, then I actually prefer it.
When also including spans antonym 'breaks', they can actually be defined
in mutually recursive fashion.
Thus, the new proposal:
splitAts :: Int -> [a] -> [[a]]
splitAts = genericSplitAts
genericSplitAts :: (Integral a) => a -> [b] -> [[b]]
genericSplitAts n _ | n <= 0 = 
genericSplitAts _  = 
genericSplitAts i xs = let (hs,ts) = genericSplitAt i xs in hs : genericSplitAts i ts
spans :: (a -> Bool) -> [a] -> [[a]]
spans _  = 
spans p xs = let (hs,ts) = span p xs in hs : breaks p ts
breaks :: (a -> Bool) -> [a] -> [[a]]
breaks _  = 
breaks p xs = let (hs,ts) = break p xs in hs : spans p ts
Am I chalking you up as a +1?
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