Proposal: adding 'tailDropWhile' to Data.List

[Maciej Marcin Piechotka]

On Wed, 2011-09-28 at 12:58 +0200, Joachim Breitner wrote:
> Hi,
> 
> Am Mittwoch, den 28.09.2011, 19:14 +0900 schrieb Kazu Yamamoto:
> > > If you still want to use this to teach students (maybe because you're
> > > building them up to "but in general you shouldn't do this") then
> > > provide this function for their use elsewhere (either as something to
> > > copy/paste, or an auxiliary library for your class).  But I don't
> > > think it belongs in Data.List.
> > 
> > So, what about 'reverse'? 
> > 
> > Both 'reverse' and 'tailDropWhile' are inefficient.
> > Data.List has 'reverse'. Why not 'tailDropWhile'?
> 
> also, it’s performance is not too bad, and definitely not as bad as
> reverse: If tailDropWhile is implemented in a way that allows list
> fusion (should be possible, I think), and I know that the suffix is not
> large (e.g. stripping at most trailing "\n"), then tailDropWhile should
> be ok to use.
> 
> Greetings,
> Joachim

Using stream-fusion package types [I know too little to know how the ghc
uses stream-fusion - but I think it shows it is possible to implement
this] (not tested):

tailDropWhile :: (a -> Bool) -> Stream a -> Stream a
tailDropWhile f (Stream next0 s01) = Stream next (Right s0, [])
    where next (Right s1, vs) =
            case next0 s1 of
              Done                      -> Done
              Skip s2                   -> Skip (Right s2, vs)
              Yield v s2
                | f v                   -> Skip (Right s2, v:vs)
                | otherwise -> case vs of
                                 []     -> Yield v (Right s2, [])
                                 (x:xs) -> Yield x (Left (s2, v), xs)
          next (Left (s1, v), [])       = Yield v (Right s2)
          next (Left (s1, v), (x:xs))   = Yield v (Left (s1, v), xs)

Regards
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