YamlLight.hs

-----------------------------------------------------------------------------
-- |
-- Module      :  Data.Yaml.YamlLight
-- Copyright   :  Michael Ilseman (c) 2010
-- License     :  BSD-style (see the file LICENSE)
--
-- Maintainer  :  michael <dot> ilseman <at> gmail <dot> com
-- Stability   :  provisional
-- Portability :  portable
--
-- A light-weight wrapper with utility functions around HsSyck

{-# LANGUAGE OverloadedStrings #-}

module Data.Yaml.YamlLight
  ( -- * YamlLight data type
    YamlLight(..)
    -- * YamlLight versions of Syck functions
  , parseYaml, parseYamlFile, parseYamlBytes
    -- * YamlLight utility functions
  , fromYamlNode, lookupYL, lookupYLWith, combineSequencedMaps
    -- ** Extractors
  , unSeq, unMap, unStr
  ) where
  import Control.Applicative
  import Data.Data
  import Data.List
  import Data.Maybe
  import qualified Data.Yaml.Syck as Syck
  import qualified Data.Map as Map
  import qualified Data.ByteString as ByteString

  -- | A Single, lighter-weight representation than that of HsSyck. Note that the YMap is an actual Map from
  --   Data.Map, so behavior with respect to identical keys and ordering of entries will behave as Data.Map
  --   dictates. This behavior is also in compliance with the Yaml spec. If you currently rely on HsSyck's
  --   preservation of ordering, you can also consider representing
  --   such maps as sequences of single entry maps. See the examples of \"Ordered Mappings\" in the Yaml spec: <http://www.yaml.org/spec/1.2/spec.html>.

  data YamlLight = YMap (Map.Map YamlLight YamlLight)
                 | YSeq [YamlLight]
                 | YStr ByteString.ByteString
                 | YNil
    deriving (Show, Ord, Eq)

  convert :: (a -> Syck.YamlNode) -> (a -> YamlLight)
  convert f = fromYamlNode . f

  convertIO :: (a -> IO Syck.YamlNode) -> (a -> IO YamlLight)
  convertIO f yn = fromYamlNode <$> f yn

  -- | Parse a regular Haskell string
  parseYaml :: String -> IO YamlLight
  parseYaml = convertIO Syck.parseYaml

  -- | Given a file name, parse contents of file
  parseYamlFile :: String -> IO YamlLight
  parseYamlFile = convertIO Syck.parseYamlFile

  -- | Parse a ByteString buffer (this is faster)
  parseYamlBytes :: ByteString.ByteString -> IO YamlLight
  parseYamlBytes = convertIO Syck.parseYamlBytes

  -- | Convert a Syck YamlNode to a YamlLight
  fromYamlNode :: Syck.YamlNode -> YamlLight
  fromYamlNode = yamlElemToLight . Syck.n_elem

  yamlElemToLight :: Syck.YamlElem -> YamlLight
  yamlElemToLight (Syck.EMap ms)  = YMap . Map.fromList . map (\(a,b) -> (fromYamlNode a, fromYamlNode b)) $ ms
  yamlElemToLight (Syck.ESeq s)   = YSeq $ map fromYamlNode s
  yamlElemToLight (Syck.EStr buf) = YStr buf
  yamlElemToLight (Syck.ENil)     = YNil

  -- | Lookup the key's corresponding value in a Map. Returns Nothing if the YamlLight is not a map, or if
  -- the key is not found
  lookupYL :: YamlLight -> YamlLight -> Maybe YamlLight
  lookupYL key (YMap m) = Map.lookup key m
  lookupYL _ _          = Nothing

  -- | General form of lookup. Will return the first element that satisfies predicate p, otherwise Nothing
  lookupYLWith :: (YamlLight -> Bool) -> YamlLight -> Maybe YamlLight
  lookupYLWith p (YMap m) = snd <$> (find (p . fst) $ Map.toList m)
  lookupYLWith _ _        = Nothing

  {- | Combine a sequence of maps into a list of (key,value) pairs. The ordering of the result preserves the ordering
     of the sequence, but the ordering of the individual maps is as Data.Map handles it.

    Example:

    @
     - key1: val1
       key2: val2
     - key3: val3
    @

    Would become:

     @
     [(key1,val1),(key2,val2),(key3,val3)]
     @

     where key1 and key2 might be arranged differently as Data.Map would
     arrange them. This does not preserve uniqueness of keys across maps.
     Any items of the sequence that are not maps will be ignored. Returns Nothing if not called on a Sequence
   -}
  combineSequencedMaps :: YamlLight -> Maybe [(YamlLight, YamlLight)]
  combineSequencedMaps (YSeq ys) = Just . concatMap Map.assocs . catMaybes $ map unMap ys
  combineSequencedMaps _         = Nothing

  -- | Get the contents of a sequence
  unSeq :: YamlLight -> Maybe [YamlLight]
  unSeq (YSeq s) = Just s
  unSeq _        = Nothing

  -- | Get the contents of a map
  unMap :: YamlLight -> Maybe (Map.Map YamlLight YamlLight)
  unMap (YMap m) = Just m
  unMap _       = Nothing

  -- | Get the contents of a string
  unStr :: YamlLight -> Maybe ByteString.ByteString
  unStr (YStr s) = Just s
  unStr _       = Nothing