Cannot pass raw ampersand '&' to parseText

[xave]

The objective is to give parseText a string "Apples & Oranges" without escaping the ampersand.

Using

...parseText def {psDecodeIllegalCharacters=passAmpersand}
where
passAmpersand :: Int -> Maybe Char
passAmpersand = \case
38 -> Just '&'
_ -> Nothing

does not work because ampersand is not an illegal character, thus failing to trigger the psDecodeIllegalCharacters.

A potential workaround is to parse my string "Apples & Oranges" and replace it with something outside of the range such as &#[0-9]+; as in the docs for psDecodeIllegalCharacters.

1. I am unsure of how to represent &#[0-9]+; as an Int, where the output would be turned into Just '&'
2. It would be ideal to just say raw ampersand is fine instead of (1).

[k0ral]

Unless I am mistaken, parseText is designed to consume valid XML data, which the string Apples & Oranges is not. See specification:

The ampersand character (&) and the left angle bracket (<) MUST NOT appear in their literal form, except when used as markup delimiters, or within a comment, a processing instruction, or a CDATA section. If they are needed elsewhere, they MUST be escaped using either numeric character references or the strings " & " and " < " respectively.

It seems to me that what you want is a pre-processing function that escapes & (and other special characters) before feeding the string to parseText. Example implementations (not type-checked, not tested):

• specific to &, less efficient:

  import Data.Text (Text, replace) 
  
  escape :: Text -> Text
  escape = replace "&" "&"

• applicable to all XML special characters, more efficient:

  import Data.Default (def)
  import Data.XML.Types (Content(..), Event(..))
  import Text.XML.Stream.Render (renderText)
  
  escape :: (PrimMonad m, MonadThrow m) => Text -> ConduitT () Text m ()
  escape text = yield (EventContent $ ContentText text) .| renderText def

Does that fulfill your need ?

[xave]

It does. Thank you very much!