gistfile1.txt

extensionFromFields :: [(B.ByteString, B.ByteString)] -> Either String URIExtension
extensionFromFields fields = do
  let
    fieldMap = M.fromList fields
    lookup' :: B.ByteString -> Either String B.ByteString
    lookup' k = maybe (Left $ "URI extension field missing: " <> k) pure (M.lookup k fieldMap)

  uriExtCodecName <- lookup' "codec_name"
  uriExtCodecParams <- lookup' "codec_params" >>= parse "codec_params"
  uriExtTailCodecParams <- lookup' "tail_codec_params" >>= parse "tail_codec_params"
  uriExtSize <- read <$> lookup' "size"
  uriExtSegmentSize <- read <$> lookup' "segment_size"
  uriExtNeededShares <- read <$> lookup' "needed_shares"
  uriExtTotalShares <- read <$> lookup' "total_shares"
  uriExtCrypttextHash <- lookup' "crypttext_hash"                                                                               uriExtTotalShares :: Total  uriExtTotalShares 
  uriExtCryptotextRootHash <- lookup' "crypttext_root_hash"                                                                     read :: forall a. Read a => String -> a  read 
  uriExtShareRootHash <- lookup' "share_root_hash"                                                                 lookup' :: ByteString -> Either String ByteString  lookup' 

  pure URIExtension { .. }