text- An efficient packed Unicode text type

Maintainerbos@serpentine.com, rtharper@aftereternity.co.uk, duncan@haskell.org




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.


Error handling types

data UnicodeException Source

An exception type for representing Unicode encoding errors.


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.

type OnError a b = String -> Maybe a -> Maybe bSource

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.

Useful error handling functions

lenientDecode :: OnError Word8 CharSource

Replace an invalid input byte with the Unicode replacement character U+FFFD.

strictDecode :: OnError Word8 CharSource

Throw a UnicodeException if decoding fails.

strictEncode :: OnError Char Word8Source

Throw a UnicodeException if encoding fails.

ignore :: OnError a bSource

Ignore an invalid input, substituting nothing in the output.

replace :: b -> OnError a bSource

Replace an invalid input with a valid output.