Traversing online directories for data

import Network.HTTP
import Network.URI
import Text.XML.HXT.Core
import Text.HandsomeSoup

type SearchResult = Either SearchResultErr [String]
data SearchResultErr = NoResultsErr 
                     | TooManyResultsErr 
                     | UnknownErr     
                     deriving (Show, Eq)

getDoc query = do  
    rsp <- simpleHTTP $ myRequest query
    html <- getResponseBody rsp
    return $ readString [withParseHTML yes, withWarnings no] html

scanDoc doc = do
    errMsg <- runX $ doc >>> css "h3" //> getText

    case errMsg of 
        [] -> do 
            text <- runX $ doc >>> css "td" //> getText 
            return $ Right text
        "Error: Sizelimit exceeded":_ -> 
            return $ Left TooManyResultsErr
        "Too many matching entries were found":_ -> 
            return $ Left TooManyResultsErr
        "No matching entries were found":_ -> 
            return $ Left NoResultsErr
        _ -> return $ Left UnknownErr

main :: IO ()
main = main' "a"

main' query = do
    print query
    doc <- getDoc query
    searchResult <- scanDoc doc
    print searchResult
    case searchResult of
        Left TooManyResultsErr -> 
            main' (nextDeepQuery query)
        _ -> if (nextQuery query) >= endQuery 
              then print "done!" else main' (nextQuery query)

nextDeepQuery query = query ++ "a"

nextQuery "z" = endQuery
nextQuery query = if last query == 'z'
                  then nextQuery $ init query
                  else init query ++ [succ $ last query]
endQuery = [succ 'z']