[Haskell-cafe] Re: Fwd: Semantics of iteratees, enumerators, enumeratees?

[Heinrich Apfelmus]

Stephen Tetley wrote:
> John Lato wrote:
> 
>> This is how I think of them.  I particularly your description of them as a
>> foldl with a "pause" button.
>> Maybe it would be helpful to consider iteratees along with delimited
>> continuations?
> 
> Aren't they closer - in implementation and by supported operations -
> to resumptions monads?
> 
> See many papers by William Harrison here:
> http://www.cs.missouri.edu/~harrisonwl/abstracts.html

A general method to implement resumption monads, or, in fact, any monad, 
  is given in my "Operational Monad Tutorial":

    http://apfelmus.nfshost.com/articles/operational-monad.html

Here a tiny toy implementation of Iteratees:

    data IterateeI a where
        Symbol :: IterateeI Char
        EOF    :: IterateeI Bool
    type Iteratee = ProgramT IterateeI
    symbol = singleton . Symbol
    eof    = singleton . EOF

    runString :: Monad m => Iteratee m a -> String -> m a
    runString m cs = go cs =<< viewT m
        where
        go _      (Return x)      = return x
        go []     (Symbol :>>= k) = error "Expecting input"
        go (c:cs) (Symbol :>>= k) = runString (k c) cs
        go cs     (EOF    :>>= k) = runString (k $ null cs) cs

        -- an iteratee that counts the number of elements in the input
    count :: Monad m => Iteratee m Int
    count = go 0
        where
        go n = eof >>= \b -> case b of
            True  -> return n
            False -> symbol >> go $! (n+1)


Regards,
Heinrich Apfelmus

--
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