Re: [Haskell-cafe] Iteratee: manyToOne

Thank you! Working implementation is even more than I've expected. 2011/4/28 Felipe Almeida Lessa

On Thu, Apr 28, 2011 at 1:10 PM, Felipe Almeida Lessa wrote:

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On Thu, Apr 28, 2011 at 12:09 PM, Felipe Almeida Lessa wrote:

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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.