-- |
-- special Filter combinators for XmlTree
--
-- Version : $Id: XmlFilterCombinators.hs,v 1.3 2002/12/12 11:59:24 hxml2 Exp $

module XmlFilterCombinators
    ( whenOk
    , whenOkM
    )
where

import XmlTree

infixr 4 `whenOkM`
infixr 4 `whenOk`

-- ------------------------------------------------------------
-- |
-- apply a filter to a tree and check for errors
-- in case of an error return the error tree
-- else continue

whenOk :: XmlFilter -> XmlFilter ->XmlFilter

whenOk f andThen t
    = let
      res = f t
      err = isXError $$ res
      in
      if null err
	 then andThen $$ res
	 else err

-- ------------------------------------------------------------
-- |
-- monadic version

whenOkM :: Monad m => (a -> XmlTrees) -> (XmlTrees -> m XmlTrees) -> a -> m XmlTrees

whenOkM f action t
    = let
      res = f t
      err = isXError $$ res
      in
      if null err
	 then action res
	 else return err

-- ------------------------------------------------------------