Pete Kazmier
I attempted to read Oleg's fold-stream implementation [1] as this sounds quite appealing to me, but I was completely overwhelmed, especially with all of the various type signatures used. It would be great if one of the regular Haskell bloggers (Tom Moertel are you reading this?) might write a blog entry or two interpreting his implementation for those of us starting out in Haskell perhaps by starting out with a non-polymorphic version so as to emphasize the approach.
In the event any other Haskell newbie comes along someday and is just as overwhelmed as I was, I've found this post by Oleg to be a much easier to understand than the above paper because it is not as generic and thus the type signatures are a bit easier on the eyes: http://www.haskell.org/pipermail/haskell/2003-September/012741.html With that said, I have a question regarding Hal's response to the above email in which he states:
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.
What does he mean by the very last sentence? Oleg's version seems more like the non-M versions. What is his implication? Thanks, Pete