From enumerators to cursors: turning the left fold inside out

[Hal Daume III]

Hi Oleg,

Just thought I'd mention that this is, in fact, my preferred method of
iterating over a file.  It alleviates the pain associated with lazy file
IO, and simultaneously provides a useful abstraction.  I actually have
3*2 functions that I use which look like:

> type Iteratee  iter seed = seed -> iter -> Either seed seed
> hFoldChars  :: FilePath -> Iteratee  Char     seed -> seed -> IO seed
> hFoldLines  :: FilePath -> Iteratee  String   seed -> seed -> IO seed
> hFoldWords  :: FilePath -> Iteratee  [String] seed -> seed -> IO seed

> type IterateeM iter seed = seed -> iter -> IO (Either seed seed)
> hFoldCharsM :: FilePath -> IterateeM Char     seed -> seed -> IO seed
> hFoldLinesM :: FilePath -> IterateeM String   seed -> seed -> IO seed
> hFoldWordsM :: FilePath -> IterateeM [String] seed -> seed -> IO seed

Which perform as expected (hFoldWords(M) can be written in terms of
hFoldLinesM, but I find I use it sufficiently frequently to warrent
having it stand out).  Also, of course, the only ones actually
implemented are the (M) variants; the non-M variants just throw a return
into the Iteratee.

 - Hal

> > hfold_left:: FileName -> Title 
> >              -> Iteratee seed
> >              -> seed      -- the initial seed
> >              -> IO seed
> > type FileName  = String
> > type Title     = String   -- just an identifying string for debug printing
> > type Iteratee seed = seed -> Char -> Either seed seed

-- 
 Hal Daume III                                   | hdaume at isi.edu
 "Arrest this man, he talks in maths."           | www.isi.edu/~hdaume