I've just recently started learning how to use the enumerator library. I'm designing a utility that parses a file where each line is a JSON object. I've designed it to look like: source enumerator (enumHandle pretty much) -> chunk by lines (enumeratee) -> parse a line into an Object (enumeratee) -> filter objects based on a criteria (enumeratee) -> limit some keys from each object (enumeratee) -> encode the object into a lazy bytestring (enumeratee) -> output the file to stdout (iteratee) I'm having difficulties with the types, particularly composing the enumeratees in the middle. Someone in #haskell said that's a good case for
=> from Control.Monad but that seems to not like it if an enumeratee changes the type between the input and output.
Here's the relevant types: type Object = Map Text Value pipeline :: MonadIO m => (Enumerator ByteString IO ()) -> [Text] -> [Filter] -> Iteratee a m () pipeline s rfs fs = s $$ splitLines >=> parseLine >=> (filterObjects fs) >=> (restrictFields rfs) >=> encoder $$ output splitLines :: Monad m => Enumeratee ByteString ByteString m b parseLine :: Monad m => Enumeratee ByteString Object m b filterObjects :: Monad m => [Filter] -> Enumeratee Object Object m b restrictObjects :: Monad m => [Text] -> Enumeratee Object Object m b encoder :: Monad m => Enumeratee Object LBS.ByteString m b output :: MonadIO m => Iteratee LBS.ByteString m () I'm using >=> because I'd prefer to compose them left-right for readability Here's the error I get Couldn't match expected type `ByteString' with actual type `M.Map Text Value' Expected type: Data.Enumerator.Step ByteString m b1 -> Iteratee ByteString m b2 Actual type: Enumeratee ByteString Object m0 b0 In the first argument of `(>=>)', namely `parseLine' In the second argument of `(>=>)', namely `parseLine >=> (filterObjects fs) >=> (restrictFields rfs) >=> encoder' Any ideas? -- Michael Xavier http://www.michaelxavier.net LinkedIn http://www.linkedin.com/pub/michael-xavier/13/b02/a26