Thank you! Working implementation is even more than I've expected.
2011/4/28 Felipe Almeida Lessa <felipe.le...@gmail.com> > On Thu, Apr 28, 2011 at 1:10 PM, Felipe Almeida Lessa > <felipe.le...@gmail.com> wrote: > > On Thu, Apr 28, 2011 at 12:09 PM, Felipe Almeida Lessa > > <felipe.le...@gmail.com> wrote: > >> I foresee one problem: what is the leftover of 'manyToOne xs' if each > >> x in xs needs different lengths of input? > >> > >> One possible untested-but-compiling solution: > > [snip] > > > > Like I said, that manyToOne implementation isn't very predictable > > about leftovers. But I guess that if all your iteratees consume the > > same input OR if you don't care about leftovers, then it should be > > okay. > > Sorry for replying to myself again. =) > > I think you can actually give predictable semantics to manyToOne: > namely, the leftovers from the last iteratee are returned. This new > implementation should be better: > > import Data.Monoid (mappend) > import qualified Data.Enumerator as E > > manyToOne :: Monad m => [E.Iteratee a m b] -> E.Iteratee a m [b] > manyToOne is = E.Iteratee $ mapM E.runIteratee is >>= > E.runIteratee . go > where > go [step] = fmap (:[]) (E.returnI step) > go (E.Yield b _ : xs) = fmap (b:) (go xs) > go (E.Error exc : _) = E.returnI (E.Error exc) > go (E.Continue f : xs) = E.continue $ go' (E.Continue f : xs) > go [] = return [] > > go' xs stream = manyToOne $ feed xs > where > feed [E.Yield b s] = [E.yield b (s `mappend` stream)] > feed (E.Continue f : ys) = f stream : feed ys > feed (step : ys) = E.returnI step : feed ys > feed [] = [] > > With the same test as before: > > *Main> E.run $ E.enumList 1 [5 :: Int, 6, 7] E.$$ manyToOne [return 1, > maybe 2 id `fmap` E.head, return 3, maybe 4 id `fmap` (E.head >> > E.head)] >>= \xs -> (,) xs `fmap` E.head > Right ([1,5,3,6],Just 7) > *Main> E.run $ E.enumList 10 [5 :: Int, 6, 7] E.$$ manyToOne [return > 1, maybe 2 id `fmap` E.head, return 3, maybe 4 id `fmap` (E.head >> > E.head)] >>= \xs -> (,) xs `fmap` E.head > Right ([1,5,3,6],Just 7) > > When the last iteratee doesn't consume anything: > > *Main> E.run $ E.enumList 1 [5 :: Int, 6, 7] E.$$ manyToOne [return 1, > maybe 2 id `fmap` E.head, return 3, maybe 4 id `fmap` (E.head >> > E.head), return 10] >>= \xs -> (,) xs `fmap` E.head > Right ([1,5,3,6,10],Just 5) > *Main> E.run $ E.enumList 10 [5 :: Int, 6, 7] E.$$ manyToOne [return > 1, maybe 2 id `fmap` E.head, return 3, maybe 4 id `fmap` (E.head >> > E.head), return 10] >>= \xs -> (,) xs `fmap` E.head > Right ([1,5,3,6,10],Just 5) > > HTH, > > -- > Felipe. >
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