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{-# LANGUAGE CPP, DeriveDataTypeable #-}
-- |
-- Module : Data.Text.Encoding.Error
-- Copyright : (c) Bryan O'Sullivan 2009
--
-- License : BSD-style
-- Maintainer : bos@serpentine.com, rtharper@aftereternity.co.uk,
-- duncan@haskell.org
-- Stability : experimental
-- Portability : GHC
--
-- Types and functions for dealing with encoding and decoding errors
-- in Unicode text.
--
-- The standard functions for encoding and decoding text are strict,
-- which is to say that they throw exceptions on invalid input. This
-- is often unhelpful on real world input, so alternative functions
-- exist that accept custom handlers for dealing with invalid inputs.
-- These 'OnError' handlers are normal Haskell functions. You can use
-- one of the presupplied functions in this module, or you can write a
-- custom handler of your own.
module Data.Text.Encoding.Error
(
-- * Error handling types
UnicodeException(..)
, OnError
, OnDecodeError
, OnEncodeError
-- * Useful error handling functions
, lenientDecode
, strictDecode
, strictEncode
, ignore
, replace
) where
#if __GLASGOW_HASKELL__ >= 610
import Control.Exception (Exception, throw)
#else
import Control.Exception.Extensible (Exception, throw)
#endif
import Data.Typeable (Typeable)
import Data.Word (Word8)
import Numeric (showHex)
-- | Function type for handling a coding error. It is supplied with
-- two inputs:
--
-- * A 'String' that describes the error.
--
-- * The input value that caused the error. If the error arose
-- because the end of input was reached or could not be identified
-- precisely, this value will be 'Nothing'.
--
-- If the handler returns a value wrapped with 'Just', that value will
-- be used in the output as the replacement for the invalid input. If
-- it returns 'Nothing', no value will be used in the output.
--
-- Should the handler need to abort processing, it should use 'error'
-- or 'throw' an exception (preferably a 'UnicodeException'). It may
-- use the description provided to construct a more helpful error
-- report.
type OnError a b = String -> Maybe a -> Maybe b
type OnDecodeError = OnError Word8 Char
type OnEncodeError = OnError Char Word8
-- | An exception type for representing Unicode encoding errors.
data UnicodeException =
DecodeError String (Maybe Word8)
-- ^ Could not decode a byte sequence because it was invalid under
-- the given encoding, or ran out of input in mid-decode.
| EncodeError String (Maybe Char)
-- ^ Tried to encode a character that could not be represented
-- under the given encoding, or ran out of input in mid-encode.
deriving (Typeable)
showUnicodeException :: UnicodeException -> String
showUnicodeException (DecodeError desc (Just w))
= "Cannot decode byte '\\x" ++ showHex w ("': " ++ desc)
showUnicodeException (DecodeError desc Nothing)
= "Cannot decode input: " ++ desc
showUnicodeException (EncodeError desc (Just c))
= "Cannot encode character '\\x" ++ showHex (fromEnum c) ("': " ++ desc)
showUnicodeException (EncodeError desc Nothing)
= "Cannot encode input: " ++ desc
instance Show UnicodeException where
show = showUnicodeException
instance Exception UnicodeException
-- | Throw a 'UnicodeException' if decoding fails.
strictDecode :: OnError Word8 Char
strictDecode desc c = throw (DecodeError desc c)
-- | Replace an invalid input byte with the Unicode replacement
-- character U+FFFD.
lenientDecode :: OnError Word8 Char
lenientDecode _ _ = Just '\xfffd'
-- | Throw a 'UnicodeException' if encoding fails.
strictEncode :: OnError Char Word8
strictEncode desc c = throw (EncodeError desc c)
-- | Ignore an invalid input, substituting nothing in the output.
ignore :: OnError a b
ignore _ _ = Nothing
-- | Replace an invalid input with a valid output.
replace :: b -> OnError a b
replace c _ _ = Just c